LM3528TME/NOPB - NATIONAL SEMICONDUCTOR

LM3528

LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential

Dimming Stepsand Integrated OLED Power Supply in a 1.2mm - 1.6mm SMD

Package

Literature Number: SNVS513

August 3, 2008

LM3528

High Efficiency, Multi Display LED Driver with 128

Exponential Dimming Steps and Integrated OLED Power

Supply in a 1.2mm × 1.6mm µSMD Package

General Description

The LM3528 current mode boost converter offers two sepa-

rate outputs. The first output (MAIN) is a constant current sink

for driving series white LED’s. The second output (SUB/FB)

is configurable as a constant current sink for series white LED

bias, or as a feedback pin to set a constant output voltage for

powering OLED panels.

As a dual output white LED bias supply, the LM3528 adap-

tively regulates the supply voltage of the LED strings to max-

imize efficiency and insure the current sinks remain in

regulation. The maximum current per output is set via a single

external low power resistor. An I2C compatible interface al-

lows for independent adjustment of the LED current in either

output from 0 to max current in 128 exponential steps. When

configured as a white LED + OLED bias supply the LM3528

can independently and simultaneously drive a string of up to

6 white LED’s and deliver a constant output voltage of up to

21V for OLED panels.

Output over-voltage protection shuts down the device if

VOUT rises above 22V allowing for the use of small sized low

voltage output capacitors. Other features include a dedicated

general purpose I/O (GPIO) and a multi-function pin (HWEN/

PGEN/GPIO) which can be configured as a 32 bit pattern

generator, a hardware enable input, or as a GPIO. When

configured as a pattern generator, an arbitrary pattern is pro-

grammed via the I2C compatible interface and output at

HWEN/PGEN/GPIO for indicator LED flashing or for external

logic control. The LM3528 is offered in a tiny 12-bump µSMD

package and operates over the -40°C to +85°C temperature

range.

Features

■128 Exponential Dimming Steps

■Programmable Auto-Dimming Function

■Up to 90% Efficient

■Low Profile 12 Bump µ-SMD Package (1.2mm x 1.6mm x

0.6mm)

■Integrated OLED Display Power Supply and LED Driver

■Programmable Pattern Generator Output for LED

Indicator Function

■Drives up to 12 LED’s at 20mA

■Drives up to 5 LED’s at 20mA and delivers 18V at 40mA

■1% Accurate Current Matching Between Strings

■Internal Soft-Start Limits Inrush Current

■True Shutdown Isolation for LED’s

■Wide 2.5V to 5.5V Input Voltage Range

■22V Over-Voltage Protection

■1.25MHz Fixed Frequency Operation

■Dedicated Programmable General Purpose I/O

■Active Low Hardware Reset

Applications

■Dual Display LCD Backlighting for Portable Applications

■Large Format LCD Backlighting

■OLED Panel Power Supply

■Display Backlighting with Indicator Light

30020501

Typical Application Circuit

30020563

Typical PCB Layout

LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and

Integrated OLED Power Supply in micro SMD

Connection Diagram

Top View

30020502

12-Bump (1.215mm × 1.615mm × 0.6mm) TMD12AAA

Ordering Information

Order Number Package Type NSC Package Drawing Top Mark Supplied As

LM3528TME 12-Bump µSMD TMD12AAA SE 250 units, Tape-and-Reel, No Lead

LM3528TMX 12-Bump µSMD TMD12AAA SE 3000 units, Tape-and-Reel, No Lead

Pin Descriptions/Functions

Pin Name Function

A1 OVP Over-Voltage Protection Sense Connection. Connect OVP to the positive terminal of the output

capacitor.

A2 MAIN Main Current Sink Input.

A3 SUB/FB Secondary Current Sink Input or 1.21V Feedback Connection for Constant Voltage Output.

B1 GPIO1 Programmable General Purpose I/O.

B2 SCL Serial Clock Input

B3 SET LED Current Setting Connection. Connect a resistor from SET to GND to set the maximum LED

current into MAIN or SUB/FB (when in LED mode), where ILED_MAX = 192×1.244V/RSET.

C1 HWEN/PGEN/

GPIO

Active High Hardware Enable Input. Programmable Pattern Generator Output, and Programmable

General Purpose I/O.

C2 SDA Serial Data Input/Output

C3 IN Input Voltage Connection. Connect IN to the input supply, and bypass to GND with a 1µF ceramic

capacitor.

D1 VIO Logic Voltage Level Input

D2 SW Drain Connection for Internal NMOS Switch

D3 GND Ground

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LM3528

Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN −0.3V to 6V

VSW, VOVP,−0.3V to 25V

VSUB/FB, VMAIN −0.3V to 23V

VSCL, VSDA, VRESET\GPIO, VIO ,

VSET −0.3V to 6V

Continuous Power Dissipation Internally Limited

Junction Temperature (TJ-MAX)+150°C

Storage Temperature Range -65°C to +150°C

Maximum Lead Temperature

(Soldering, 10s)(Note 3) +300°C

ESD Rating(Note 10)

Human Body Model 2.5kV

Operating Ratings (Notes 1, 2)

VIN 2.5V to 5.5V

VSW, VOVP,0V to 23V

VSUB/FB, VMAIN 0V to 21V

Junction Temperature Range

(TJ)(Note 4)

-40°C to +110°C

Ambient Temperature Range

(TA)(Note 5)

-40°C to +85°C

Thermal Properties

Junction to Ambient Thermal

Resistance (θJA)(Note 6)

68°C/W

ESD Caution Notice

National Semiconductor recommends that all integrated cir-

cuits be handled with appropriate ESD precautions. Failure to

observe proper ESD handling techniques can result in dam-

age to the device.

Electrical Characteristics

Specifications in standard type face are for TA = 25°C and those in boldface type apply over the Operating Temperature Range

of TA = −40°C to +85°C. Unless otherwise specified VIN = 3.6V, VIO = 1.8V, VRESET/GPIO = VIN, VSUB/FB = VMAIN = 0.5V, RSET =

12.0kΩ, OLED = ‘0’, ENM = ENS = ‘1’, BSUB = BMAIN = Full Scale.(Notes 2, 7)

Symbol Parameter Conditions Min Typ Max Units

ILED Output Current Regulation

MAIN or SUB/FB Enabled

UNI = ‘0’, or ‘1’,

2.5V < VIN < 5.5V

18.5 20 22

Maximum Current Per

Current Sink RSET = 8.0kΩ 30 mA

ILED-MATCH IMAIN to ISUB/FB Current

Matching

UNI = ‘1’,

2.5V < VIN < 5.5V (Note 11)

0.15 1%

VSET SET Pin Voltage 3.0V < VIN < 5V 1.244 V

ILED/ISET ILED Current to ISET Current

Ratio

192

VREG_CS Regulated Current Sink

Headroom Voltage

500 mV

VREG_OLED VSUB/FB Regulation Voltage

in OLED Mode

2.5V < VIN < 5.5V, OLED = ‘1’ 1.170 1.21 1.237 V

VHR Current Sink Minimum

Headroom Voltage

ILED = 95% of nominal 300 mV

RDSON NMOS Switch On

Resistance

ISW = 100mA 0.43 Ω

ICL NMOS Switch Current Limit 2.5V < VIN < 5.5V 645 770 900 mA

VOVP Output Over-Voltage

Protection

ON Threshold,

2.5V < VIN < 5.5V 20.6 22 23

OFF Threshold,

2.7V < VIN < 5.5V 19.25 20.6 21.5

fSW Switching Frequency 1.0 1.27 1.4 MHz

DMAX Maximum Duty Cycle 90 %

DMIN Minimum Duty Cycle 10 %

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LM3528

Symbol Parameter Conditions Min Typ Max Units

IQQuiescent Current, Device

Not Switching

VMAIN and VSUB/FB >

VREG_CS,

BSUB = BMAIN = 0x00, 2.5V

< VIN < 5.5V

350 390

µA

VSUB/FB > VREG_OLED,

OLED=’1’, ENM=ENS=’0’,

RSET Open,

2.5V < VIN < 5.5V

250 260

ISHDN Shutdown Current ENM = ENS = OLED = '0',

2.5V < VIN < 5.5V 1.8 3µA

HWEN/PGEN/GPIO, GPIO1 Pin Voltage Specifications

VIL Input Logic Low 2.5V < VIN <5.5V, MODE bit

= 0 0.5 V

VIH Input Logic High 2.5V < VIN < 5.5V, MODE bit

= 0 1.1 V

VOL Output Logic Low ILOAD=3mA, MODE bit = 1 400 mV

I2C Compatible Voltage Specifications (SCL, SDA, VIO)

VIO Serial Bus Voltage Level 2.5V < VIN < 5.5V (Note 9) 1.7 VIN V

VIL Input Logic Low 2.5V < VIN < 5.5V 0.36×VIO V

VIH Input Logic High 2.5V < VIN < 5.5V 0.7×VIO V

VOL Output Logic Low ILOAD = 3mA 400 mV

I2C Compatible Timing Specifications (SCL, SDA, VIO, see Figure 1) (Notes 8, 9)

t1SCL Clock Period 2.5 µs

t2Data In Setup Time to SCL

High

100 ns

t3Data Out Stable After SCL

Low

0 ns

t4SDA Low Setup Time to

SCL Low (Start)

100 ns

t5SDA High Hold Time After

SCL High (Stop)

100 ns

Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended

to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: All voltages are with respect to the potential at the GND pin.

Note 3: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer LEvel Chip Scale

Package (AN-1112).

Note 4: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=+150°C (typ.) and disengages at

TJ=+140°C (typ.).

Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be

derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = +105°C), the maximum power

dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the

following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).

Note 6: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the

JEDEC standard JESD51-7. The test board is a 4-layer FR-4 board measuring 114.3mm x 76.2mm x 1.6mm. The ground plane on the board is 113mm x 75mm.

Thickness of copper layers are 71.5µm/35µm/35µm/71.5µm (2oz/1oz/1oz/2oz). Ambient temperature in simulation is 22°C, still air. Power dissipation is 1W. For

more information on these topics, please refer to National Semiconductor Application Note 1112, and JEDEC Standard JESD51-7.

Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but represent the most likely norm.

Note 8: SCL and SDA must be glitch-free in order for proper brightness control to be realized.

Note 9: SCL and SDA signals are referenced to VIO and GND for minimum VIO voltage testing. VIO limits indicate the minimum voltage at VIO at which the part

is operational.

Note 10: The human body model is a 100pF capacitor discharged through 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7).

Note 11: The matching specification between MAIN and SUB is calculated as 100 × ((IMAIN or ISUB) - IAVE) / IAVE. This simplifies out to be

100 × (IMAIN - ISUB)/(IMAIN + ISUB).

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LM3528

Timing Diagram

30020503

FIGURE 1. I2C Timing

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LM3528

Typical Performance Characteristics VIN = 3.6V, LEDs are Nichia (NSSW008C), COUT = 1µF (LED

Mode), COUT = 2.2µF (OLED Mode), CIN = 1µF, L = TDK VLF4012AT-100MR79, (RL = 0.3Ω), RSET = 12.1kΩ, UNI = '1', ILED =

ISUB + IMAIN, TA = +25°C unless otherwise specified.

2x6 LED Efficiency vs ILED

(2 Strings of 6LEDs)

30020508

2x5 LED Efficiency vs ILED

(2 Strings of 5LEDs)

30020509

2x4 LED Efficiency vs ILED

(2 Strings of 4LEDs)

30020510

2x3 LED Efficiency vs ILED

(2 Strings of 3LEDs)

30020511

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LM3528

2x2 LED Efficiency vs ILED

(2 Strings of 2LEDs)

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LED Efficiency vs VIN

(L = TDK VLF3012AT-100MR92, RL = 0.36Ω, ISUB + IMAIN =

40mA)

30020513

18V OLED Efficiency vs IOUT

30020514

12V OLED Efficiency vs IOUT

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LM3528

LED Line Regulation

(UNI = '0')

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OLED Line Regulation

IOLED = 60mA

30020517

OLED Line Regulation

IOLED = 60mA

30020518

OLED Load Regulation

VOLED = 18V

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LM3528

OLED Load Regulation

VOLED = 12V

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Peak Current Limit vs. VIN

30020521

Over Voltage Limit vs. VIN

30020522

Switch On-Resistance vs. VIN

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LM3528

Switching Frequency vs. VIN

30020524

Maximum Duty Cycle vs. VIN

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Shutdown Current vs. VIN

30020526

Switching Supply Current vs. VIN

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LM3528

LED Current Matching vs. CODE (Note 11)

(UNI = '1', RSET = 12kΩ, TA = -40°C to +85°C)

30020528

LED Current Accuracy vs CODE

(RSET = 12kΩ±0.05%)

30020529

LED Current vs CODE

(IMAIN, ISUB, IIDEAL, RSET = 12kΩ±0.05%)

30020530

ILED vs Current Source Headroom Voltage

(VIN = 3V, UNI = '0')

30020531

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LM3528

Start-Up Waveform (LED Mode)

(2 × 5 LEDs, 20mA per string)

30020556

Channel 2: SDA (5V/div)

Channel 1: VOUT (10V/div)

Channel 3: ILED (20mA/div)

Channel 4: IIN (200mA/div)

Time Base: 400µs/div

Start-Up Waveform (OLED Mode)

(VOUT = 18V, IOUT = 60mA)

30020555

Channel 1: SDA (5V/div)

Channel 2: VOUT (10V/div)

Channel 3: IOUT (20mA/div)

Channel 4: IIN (200mA/div)

Time Base: 400µs/div

Load Step (OLED Mode)

(VOUT = 18V, COUT = 2.2µF)

30020553

Channel 2: VOUT (AC Coupled, 500mV/div)

Channel 3: IOUT (20mA/div)

Time Base: 200µs/div

Line Step (LED Mode)

(2 × 5 LEDs, 20mA per String, COUT = 1µF, VIN from 3V to 3.6V)

30020552

Channel 3: ISUB (5mA/div)

Channel 4: IMAIN (5mA/div)

Channel 2: VIN (AC Coupled, 500mV/div)

Time Base: 100µs/div

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LM3528

Line Step (OLED Mode)

(VOUT = 18V, COUT = 2.2µF, VIN from 3V to 3.6V)

30020557

Channel 2: VOUT (AC Coupled, 100mV/div)

Channel 3: VIN (AC Coupled, 500mV/div)

Time Base: 200µs/div

HWEN Functionality

30020551

Channel 4: ISUB (20mA/div)

Channel 3: IMAIN (20mA/div)

Channel 2: HWEN (5V/div)

Time Base: 200ns/div

GPIO1 Functionality

(GPIO1 Configured as OUTPUT, fSCL = 360kHz)

30020558

Channel 2: GPIO (2V/div)

Channel 1: SCL (5V/div)

Channel 1:SDA (5V/div)

Time Base: 40µs/div

Ramp Rate Functionality

(RMP1, RMP0 = '11')

30020554

Channel 1:SDA (2V/div)

Channel 4: ISUB (10mA/div)

Channel 3: ISUB (10mA/div)

Time Base: 400ms/div

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LM3528

Block Diagram

30020533

FIGURE 2. LM3528 Block Diagram

Operation Description

The LM3528 Current Mode PWM boost converter operates

from a 2.7V to 5.5V input and provides two regulated outputs

for White LED and OLED display biasing. The first output,

MAIN, provides a constant current of up to 30mA to bias up

to 6 series white LED’s. The second output, SUB/FB, can be

configured as a current source for up to 6 series white LED’s

at at up to 30mA, or as a feedback voltage pin to regulate a

constant output voltage of up to 21V. When both MAIN and

SUB/FB are configured for white LED bias the current for each

LED string is controlled independently or in unison via an I2C

compatible interface. When MAIN is configured for white LED

bias and SUB/FB is configured as a feedback voltage pin, the

current into MAIN is controlled via the I2C compatible interface

and SUB/FB becomes the middle tap of a resistive divider

used to regulate the output voltage of the boost converter.

The core of the LM3528 is a Current Mode Boost converter.

Operation is as follows. At the start of each switching cycle

the internal oscillator sets the PWM converter. The converter

turns the NMOS switch on, allowing the inductor current to

ramp while the output capacitor supplies power to the white

LED’s and/or OLED panel. The error signal at the output of

the error amplifier is compared against the sensed inductor

current. When the sensed inductor current equals the error

signal, or when the maximum duty cycle is reached, the

NMOS switch turns off causing the external Schottky diode to

pick up the inductor current. This allows the inductor current

to ramp down causing its stored energy to charge the output

capacitor and supply power to the load. At the end of the clock

period the PWM controller is again set and the process re-

peats itself.

ADAPTIVE REGULATION

When biasing dual white led strings (White LED mode) the

LM3528 maximizes efficiency by adaptively regulating the

output voltage. In this configuration the 500mV reference is

connected to the non-inverting input of the error amplifier via

mux S2 (see Figure 2, Block Diagram). The lowest of either

VMAIN or VSUB/FB is then applied to the inverting input of the

error amplifier via mux S1. This ensures that VMAIN and VSUB/

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LM3528

FB are at least 500mV, thus providing enough voltage head-

room at the input to the current sinks for proper current

regulation.

In the instance when there are unequal numbers of LEDs or

unequal currents from string to string, the string with the high-

est voltage will be the regulation point.

UNISON/NON-UNISON MODE

Within White LED mode there are two separate modes of op-

eration, Unison and Non-Unison. Non-Unison mode provides

for independent current regulation, while Unison mode gives

up independent regulation for more accurate matching be-

tween LED strings. When in Non-Unison mode the LED cur-

rents IMAIN and ISUB/FB are independently controlled via

registers BMAIN and BSUB respectively (see Brightness

Registers BMAIN and BSUB section). When in Unison mode

BSUB is disabled and both IMAIN and ISUB/FB are controlled via

BMAIN only.

START-UP

The LM3528 features an internal soft-start, preventing large

inrush currents during start-up that can cause excessive volt-

age ripple on the input. For the typical application circuits

when the device is brought out of shutdown the average input

current ramps from zero to 450mA in approximately 1.2ms.

See Start Up Plots in the Typical Performance Characteris-

tics.

OLED MODE

When the LM3528 is configured for a single White LED bias

+ OLED display bias (OLED mode), the non-inverting input of

the error amplifier is connected to the internal 1.21V reference

via MUX S2. MUX S1 switches SUB/FB to the inverting input

of the error amplifier while disconnecting the internal current

sink at SUB/FB. The voltage at MAIN is not regulated in OLED

mode so when the application requires white LED + OLED

panel biasing, ensure that at least 300mV of headroom is

maintained at MAIN to guarantee proper regulation of IMAIN.

(see the Typical Performance Characteristics for a plot of

ILED vs Current Source Headroom Voltage)

PEAK CURRENT LIMIT

The LM3528’s boost converter has a peak current limit for the

internal power switch of 770mA typical (650mA minimum).

When the peak switch current reaches the current limit the

duty cycle is terminated resulting in a limit on the maximum

output current and thus the maximum output power the

LM3528 can deliver. Calculate the maximum LED current as

a function of VIN, VOUT, L and IPEAK as:

ƒSW = 1.27MHz. Typical values for efficiency and IPEAK can

be found in the efficiency and IPEAK curves in the Typical Per-

formance Characteristics.

OVER VOLTAGE PROTECTION

The LM3528's output voltage (VOUT) is limited on the high end

by the Output Over-Voltage Protection Threshold (VOVP) of

21.2V (min). In White LED mode during output open circuit

conditions the output voltage will rise to the over voltage pro-

tection threshold. When this happens the controller will stop

switching causing VOUT to droop. When the output voltage

drops below 19.7V (min) the device will resume switching. If

the device remains in an over voltage condition the LM3528

will repeat the cycle causing the output to cycle between the

high and low OVP thresholds. See waveform for OVP condi-

tion in the Typical Performance Characteristics.

OUTPUT CURRENT ACCURACY AND CURRENT

MATCHING

The LM3528 provides both precise current accuracy (% error

from ideal value) and accurate current matching between the

MAIN and SUB/FB current sinks. Two modes of operation af-

fect the current matching between IMAIN and ISUB/FB. The first

mode (Non-Unison mode) is set by writing a 0 to bit 2 of the

General Purpose register (UNI bit). Non-Unison mode allows

for independent programming of IMAIN and ISUB/FB via registers

BMAIN and BSUB respectively. In this mode typical matching

between current sinks is 1%.

Writing a 1 to UNI configures the device for Unison mode. In

Unison mode, BSUB is disabled and IMAIN and ISUB/FB are both

controlled via register BMAIN. In this mode typical matching

is 0.15%.

LIGHT LOAD OPERATION

The LM3528 boost converter operates in three modes; con-

tinuous conduction, discontinuous conduction, and skip mode

operation. Under heavy loads when the inductor current does

not reach zero before the end of the switching period the de-

vice switches at a constant frequency. As the output current

decreases and the inductor current reaches zero before the

end of the switching cycle, the device operates in discontin-

uous conduction. At very light loads the LM3528 will enter skip

mode operation causing the switching period to lengthen and

the device to only switch as required to maintain regulation at

the output.

HARDWARE ENABLE/PATTERN GENERATOR/

GENERAL PURPOSE I/O (HWEN/PGEN/GPIO)

HWEN/PGEN/GPIO can be configured for three different

modes of operation; Hardware Enable, Pattern Generation,

and General Purpose I/O. Register HPG at address 0x80

controls the functionality of this pin (see Table 6).

HARDWARE ENABLE (HWEN)

On initial power-up HWEN/PGEN/GPIO defaults to the Hard-

ware Enable (HWEN) state. In this mode HWEN/PGEN/GPIO

is an active high open-drain input enable to the device. When

in HWEN mode HWEN/PGEN/GPIO must be pulled up to at

least 0.7 × VIO to enable the device. In HWEN mode pulling

HWEN/PGEN/GPIO below 0.36 × VIO will shutdown the

LM3528, resetting all registers, and forcing MAIN, SUB/FB,

and SW high impedance. Bit 0 of the HPG register controls

the HWEN function. Writing a ‘0’ to this bit enables the HWEN

mode. Writing a ‘1’ to this bit disables the HWEN mode and

allows selection between the other two modes.

PATTERN GENERATOR (PGEN)

With bit 0 of the HPG register set to 1, HWEN/PGEN/GPIO

can be programmed as an open drain Pattern Generator Out-

put (PGEN). In PGEN mode a 32 bit pattern is output at

HWEN/PGEN/GPIO. This pattern can be programmed to re-

peat itself at 4 different frequencies and 6 different duty

cycles. The arbitrary pattern is programmed into four 8 bit

registers; PGEN0 (address 0x90), PGEN1 (address 0x91),

PGEN2 (address 0x92), and PGEN3 (address 0x93) (see

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LM3528

Figures 12 - 15). Figure 16 details an example of a 32 bit pat-

tern at a specific programmed duty cycle and frequency. A ‘1’

written to the PGEN_ registers forces HWEN/PGEN/GPIO

low. A ‘0’ causes HWEN/PGEN/GPIO to go open drain.

Bits <5:3> in the HPG register have three functions; GPIO

enable, duty cycle select, and pattern latch. Any combination

of these bits other than ‘000’ or ’111’ puts HWEN/PGEN/GPIO

into PGEN mode at the specified duty cycle shown in Table

6. Writing a ‘111’ to bits <5:3> latches the 32 bit pattern pro-

grammed into the 4 pattern generator registers PGEN0,

PGEN1, PGEN2, PGEN3 into the internal shift register. When

bits <5:3> = ‘000’ the PGEN mode is off and HWEN/PGEN/

GPIO is configured as a GPIO.

Bits <7:6> of the HPG register control the pattern frequency.

See Table 6 for the detailed breakdown of each available fre-

quency. Figure 16 details the pattern programming and figure

17 shows the pattern output at HWEN/PGEN/GPIO.

GENERAL PURPOSE I/O (GPIO1)

With bits <5:3> and bit 0 of the HPG register all set to ‘0’

HWEN/PGEN/GPIO functions as an open drain General Pur-

pose I/O. In this mode, bit 1 of the HPG register controls the

logic direction (Input or Output) and bit 2 holds the logic data.

With bit 1 set to ‘0’ HWEN/PGEN/GPIO is configured as an

output. In this mode a ‘0’ written to bit 2 forces HWEN/PGEN/

GPIO to logic low. Likewise, a ‘1’ written to bit 2 will force

HWEN/PGEN/GPIO open drain. When bit 1 is set to ‘1’

HWEN/PGEN/GPIO is configured as a logic input. In this

mode when HWEN/PGEN/GPIO is externally pulled low a ‘0’

is written to bit 2 of the HPG register. Likewise, when HWEN/

PGEN/GPIO is externally pulled high a ‘1’ is written to bit 2 of

the HPG register. Table 6 and Figure 10 detail the bit functions

of the HPG register and their power-on-reset values. Note that

the logic output levels for the GPIO function of this pin are

inverted compared to the PGEN functions. For example, a 1

written to the PGEN registers cause the HWEN/PGEN/GPIO

pin to pull low while a 1 written to the bit 2 of the HPG register

causes the pin to go open drain.

GENERAL PURPOSE I/O (GPIO0)

The GPIO pin is a dedicated General Purpose I/O (open

drain) and is controlled via the GPIO register at address 0x81.

Bit 1 holds the logic data while bit 0 controls the logic direction

(Input or Output). Bits <7:2> are un-used and will always read

back as logic '1'. With bit 0 set to ‘0’ GPIO is configured as an

output. In this mode a ‘0’ written to bit 1 forces GPIO to a logic

low. Likewise, a ‘1’ written to bit 1 will force GPIO to logic high.

When bit 0 is set to ‘1’ GPIO is configured as a logic input. In

this mode when GPIO is externally pulled low a ‘0’ is written

to bit 1 of the GPIO register. Likewise, when GPIO is exter-

nally pulled high a ‘1’ is written to bit 2 of the HPG register.

Table 7 and Figure 11 detail the bit functions and power-on-

reset values of GPIO.

During an initial GPIO write two I2C sequences (Slave I.D,

state of the GPIO pin. The first write configures the GPIO pin

as an output. The second write will change the state of the

GPIO output to the desired logic '1' or '0'.

THERMAL SHUTDOWN

The LM3528 offers a thermal shutdown protection. When the

die temperature reaches +140°C the device will shutdown

and not turn on again until the die temperature falls below

+120°C.

www.national.com 16

LM3528

I2C COMPATIBLE INTERFACE

The LM3528 is controlled via an I2C compatible interface.

START and STOP conditions classify the beginning and the

end of the I2C session. A START condition is defined as SDA

transitioning from HIGH to LOW while SCL is HIGH. A STOP

condition is defined as SDA transitioning from LOW to HIGH

while SCL is HIGH. The I2C master always generates START

and STOP conditions. The I2C bus is considered busy after a

START condition and free after a STOP condition. During da-

ta transmission, the I2C master can generate repeated

START conditions. A START and a repeated START condi-

tions are equivalent function-wise. The data on SDA must be

stable during the HIGH period of the clock signal (SCL). In

other words, the state of SDA can only be changed when SCL

is LOW.

30020537

FIGURE 3. Start and Stop Sequences

I2C COMPATIBLE ADDRESS

The chip address for the LM3528 is 0110110 (36h). After the

START condition, the I2C master sends the 7-bit chip address

followed by a read or write bit (R/W). R/W= 0 indicates a

WRITE and R/W = 1 indicates a READ. The second byte fol-

lowing the chip address selects the register address to which

the data will be written. The third byte contains the data for

the selected register.

30020538

FIGURE 4. Chip Address

TRANSFERRING DATA

Every byte on the SDA line must be eight bits long, with the

most significant bit (MSB) transferred first. Each byte of data

must be followed by an acknowledge bit (ACK). The acknowl-

edge related clock pulse (9th clock pulse) is generated by the

master. The master releases SDA (HIGH) during the 9th clock

pulse. The LM3528 pulls down SDA during the 9th clock

pulse, signifying an acknowledge. An acknowledge is gener-

ated after each byte has been received. Figure 5 is an exam-

ple of a write sequence to the General Purpose register of the

LM3528.

30020539

FIGURE 5. Write Sequence to the LM3528

17 www.national.com

LM3528

There are 4, 8 bit registers within the LM3528 as detailed in Table 1.

TABLE 1. LM3528 Register Descriptions

General Purpose (GP) 0x10 0xC0

Brightness Main (BMAIN) 0xA0 0xE0

Brightness Sub (BSUB) 0xB0 0xE0

HWEN/PGEN/GPIO Control (HPG) 0x80 0XF8

General Purpose I/O Control (GPIO) 0x81 0xFC

Pattern Register 0 (PGEN0) 0x90 0x00

Pattern Register 1 (PGEN1) 0x91 0x00

Pattern Register 2 (PGEN2) 0x92 0x00

Pattern Register 3 (PGEN3) 0x93 0x00

GENERAL PURPOSE REGISTER (GP)

The General Purpose register has four functions. It controls

the on/off state of MAIN and SUB/FB, it selects between Uni-

son or Non-Unison mode, provides for control over the rate of

change of the LED current (see Brightness Rate of Change

Description), and selects between White LED and OLED

mode. Figure 6 and Table 2 describes each bit available with-

in the General Purpose Register. Table 3 summarizes the

output state of the LM3528 for the different combinations of

General Purpose register settings.

30020540

FIGURE 6. General Purpose Register Description

TABLE 2. General Purpose Register Bit Function

Bit Name Function Power-On-Value

0 ENM Enable MAIN. Writing a 1 to this bit enables the main current sink (MAIN).

Writing a 0 to this bit disables the main current sink and forces MAIN high

impedance.

1 ENS Enable SUB/FB. Writing a 1 to this bit enables the secondary current sink (SUB/

FB). Writing a 0 to this bit disables the secondary current sink and forces SUB/

FB high impedance.

2 UNI Unison Mode Select. Writing a 1 to this bit disables the BSUB register and

causes the contents of BMAIN to set the current in both the MAIN and SUB/

FB current sinks. Writing a 0 to this bit allows the current into MAIN and SUB/

FB to be independently controlled via the BMAIN and BSUB registers

respectively.

3 RMP0 Brightness Rate of Change. Bits RMP0 and RMP1 set the rate of change of

the LED current into MAIN and SUB/FB in response to changes in the contents

of registers BMAIN and BSUB (see brightness rate of change description).

4 RMP1 0

5 OLED OLED = 0 places the LM3528 in White LED mode. In this mode both the MAIN

and SUB/FB current sinks are active. The boost converter ensures there is at

least 500mV at VMAIN and VSUB/FB.

OLED = 1 places the LM3528 in OLED mode. In this mode the boost converter

regulates VSUB/FB to 1.244V. VMAIN is unregulated and must be > 400mV for

the MAIN current sink to maintain current regulation.

6 Don't Care These are non-functional read only bits. They will always read back as a 1. 1

www.national.com 18

LM3528

TABLE 3. Operational Truth Table

UNI OLED ENM ENS Result

X 0 0 0 LM3528 Disabled

1 0 1 X MAIN and SUB/FB current sinks enabled. Current levels set by

contents of BMAIN.

1 0 0 X MAIN and SUB/FB Disabled

0 0 0 1 SUB/FB current sink enabled. Current level set by BSUB.

0 0 1 0 MAIN current sink enabled. Current level set by BMAIN.

0 0 1 1 MAIN and SUB/FB current sinks enabled. Current levels set by

contents of BMAIN and BSUB respectively.

X 1 1 X SUB/FB current sink disabled (SUB/FB configured as a feedback

pin). MAIN current sink enabled current level set by BMAIN.

X 1 0 X SUB/FB current sink disabled (SUB/FB configured as a feedback

pin). MAIN current sink disabled.

* ENM ,ENS, or OLED high enables analog circuitry.

BRIGHTNESS REGISTERS (BMAIN and BSUB)

With the UNI bit (General Purpose register) set to 0 (Non-

Unison mode) both brightness registers (BMAIN and BSUB)

independently control the LED currents IMAIN and ISUB/FB re-

spectively. BMAIN and BSUB are both 8 bit, but with only the

7 LSB’s controlling the current. The MSB’s is a don’t care. The

LED current control is designed to approximate an exponen-

tially increasing response of the LED current vs increasing

code in either BMAIN or BSUB (see Figure 9). Program

ILED_MAX by connecting a resistor (RSET) from SET to GND,

where:

With the UNI bit (General Purpose register) set to 1 (Unison

mode), BSUB is disabled and BMAIN sets both IMAIN and ISUB/

FB. This prevents the independent control of IMAIN and ISUB/

FB, however matching between current sinks goes from typi-

cally 1%(with UNI = 0) to typically 0.15% (with UNI = 1). Figure

7 and Figure 8 show the register descriptions for the Bright-

ness MAIN and Brightness SUB registers. Table 4 and Figure

9 show IMAIN and/or ISUB/FB vs. brightness data as a percent-

age of ILED_MAX.

30020542

FIGURE 7. Brightness MAIN Register Description

30020543

FIGURE 8. Brightness SUB Register Description

19 www.national.com

LM3528

TABLE 4. ILED vs. Brightness Register Data

BMAIN or

BSUB

Brightness

Data

% of

ILED_MAX

BMAIN or

BSUB

Brightness

Data

% of ILED_MAX BMAIN or BSUB

Brightness Data

% of ILED_MAX BMAIN or

BSUB

Brightness

Data

% of ILED_MAX

0000000 0.000% 0100000 0.803% 1000000 4.078% 1100000 20.713%

0000001 0.166% 0100001 0.845% 1000001 4.290% 1100001 21.792%

0000010 0.175% 0100010 0.889% 1000010 4.514% 1100010 22.928%

0000011 0.184% 0100011 0.935% 1000011 4.749% 1100011 24.122%

0000100 0.194% 0100100 0.984% 1000100 4.996% 1100100 25.379%

0000101 0.204% 0100101 1.035% 1000101 5.257% 1100101 26.701%

0000110 0.214% 0100110 1.089% 1000110 5.531% 1100110 28.092%

0000111 0.226% 0100111 1.146% 1000111 5.819% 1100111 29.556%

0001000 0.237% 0101000 1.205% 1001000 6.122% 1101000 31.096%

0001001 0.250% 0101001 1.268% 1001001 6.441% 1101001 32.716%

0001010 0.263% 0101010 1.334% 1001010 6.776% 1101010 34.420%

0001011 0.276% 0101011 1.404% 1001011 7.129% 1101011 36.213%

0001100 0.291% 0101100 1.477% 1001100 7.501% 1101100 38.100%

0001101 0.306% 0101101 1.554% 1001101 7.892% 1101101 40.085%

0001110 0.322% 0101110 1.635% 1001110 8.303% 1101110 42.173%

0001111 0.339% 0101111 1.720% 1001111 8.735% 1101111 44.371%

0010000 0.356% 0110000 1.809% 1010000 9.191% 1110000 46.682%

0010001 0.375% 0110001 1.904% 1010001 9.669% 1110001 49.114%

0010010 0.394% 0110010 2.003% 1010010 10.173% 1110010 51.673%

0010011 0.415% 0110011 2.107% 1010011 10.703% 1110011 54.365%

0010100 0.436% 0110100 2.217% 1010100 11.261% 1110100 57.198%

0010101 0.459% 0110101 2.332% 1010101 11.847% 1110101 60.178%

0010110 0.483% 0110110 2.454% 1010110 12.465% 1110110 63.313%

0010111 0.508% 0111011 2.582% 1010111 13.114% 1110111 66.611%

0011000 0.535% 0110111 2.716% 1011000 13.797% 1111000 70.082%

0011001 0.563% 0111000 2.858% 1011001 14.516% 1111001 73.733%

0011010 0.592% 0111001 3.007% 1011010 15.272% 1111010 77.574%

0011011 0.623% 0111010 3.163% 1011011 16.068% 1111011 81.616%

0011100 0.655% 0111011 3.328% 1011100 16.905% 1111100 85.868%

0011101 0.689% 0111100 3.502% 1011101 17.786% 1111101 90.341%

0011110 0.725% 0111101 3.684% 1011110 18.713% 1111110 95.048%

0011111 0.763% 0111111 3.876% 1011111 19.687% 1111111 100.000%

www.national.com 20

LM3528

30020544

FIGURE 9. IMAIN or ISUB vs BMAIN or BSUB Data

BRIGHTNESS RATE OF CHANGE DESCRIPTION

RMP0 and RMP1 control the rate of change of the LED cur-

rent IMAIN and ISUB/FB in response to changes in BMAIN and/

or BSUB. There are 4 user programmable LED current rates

of change settings for the LM3528 (see Table 5).

TABLE 5. Rate of Change Bits

RMP0 RMP1 Change Rate

(tSTEP)

0 0 12.75µs/step

0 1 3.25ms/step

1 0 6.5ms/step

1 1 13ms/step

For example, if RSET = 12.1kΩ then ILED_MAX = 20mA. With

the contents of BMAIN set to 0x7F (IMAIN = 20mA), suppose

the contents of BMAIN are changed to 0x00 resulting in

(IMAIN = 0mA). With RMP0 =1 and RMP1 = 1 (13ms/step),

IMAIN will change from 20mA to 0mA in 127 steps with 13ms

elapsing between steps, excluding the step from 0x7F to

0x7E, resulting in a full scale current change in 1638ms. The

total time to transition from one brightness code to another is:

The following 3 additional examples detail possible scenarios

when using the brightness register in conjunction with the rate

of change bits and the enable bits.

Example 1:

Step 1: Write to BMAIN a value corresponding to IMAIN = 20-

mA.

Step 2: Write 1 to ENM (turning on MAIN)

Step 3: IMAIN ramps to 20mA with a rate set by RMP0 and

RMP1. (RMP0 and RMP1 bits set the duration spent at one

brightness code before incrementing to the next).

Step 4: ENM is set to 0 before 20mA is reached, thus the LED

current fades off at a rate given by RMP0 and RMP1 without

IMAIN going up to 20mA.

Example 2:

Step 1: ENM is 1, and BMAIN has been programmed with

code 0x01. This results in a small current into MAIN.

Step 2: BMAIN is programmed with 0x7F (full scale current).

This causes IMAIN to ramp toward full-scale at the rate select-

ed by RMP0 and RMP1.

Step 3: Before IMAIN reaches full-scale BMAIN is programmed

with 0x30. IMAIN will continue to ramp to full scale.

Step 4: When IMAIN has reached full-scale value it will ramp

down to the current corresponding to 0x30 at a rate set by

RMP0 and RMP1.

Example 3:

Step 1: Write to BMAIN a value corresponding to IMAIN = 20-

mA.

Step 2: Write a 1 to both RMP0 and RMP1.

Step 3: Write 1 to ENM (turning on MAIN).

Step 4: IMAIN ramps toward 20mA with a rate set by RMP0 and

RMP1. (RMP0 and RMP1 bits set the duration spent at one

brightness code before incrementing to the next).

Step 5: After 1.222s IMAIN has ramped to 19.687% of

ILED_MAX (0.19687 × 20mA = 3.9374mA). Simultaneously,

RMP0 and RMP1 are both programmed with 0.

Step 6: IMAIN continues ramping from 3.9374mA to 20mA, but

at a new ramp rate of 12.75µs/step.

21 www.national.com

LM3528

TABLE 6. HPG Register Function

Bits 7 – 6

(PGEN Bit

Period)

Bits 5 - 3 (PGEN

Enable/Disable

and Duty Cycle

Selection)

Bit 2 (GPIO

Data)

Bit 1 (GPIO

Data

Direction)

Bit 0 (HWEN

Control)

Function

X X X X 0 HWEN/PGEN/GPIO is configured as an

active high Hardware Enable Input (HWEN)

00 = 1.6µs/

bit (625kHz)

01 = 26ms/

bit (38Hz)

10 = 52ms/

bit (19Hz)

11 = 105ms/

bit (9.5Hz)

Note 1

001 = 100%

010 = 1/2

011 = 1/3

100 = 1/4

101 = 1/6

110 = 1/12

111 = Latch Pat-

tern Into Shift Reg-

ister

Note 2

X X 1 HWEN/PGEN/GPIO is configured as a

Pattern Generator Output with the frequency

set by bits <7:6> and the duty cycle set by bits

<5:3>. (See Figure 11.)

X 000 GPIO Read

Data

1 1 HWEN/PGEN/GPIO is configured as a GPIO

Input. Read data from bit 2.

X 000 GPIO Write

Data

0 1 HWEN/PGEN/GPIO is configured as a GPIO

Output. A ‘1’ written to bit 2 will force HWEN/

PGEN/GPIO high; a 0 written to bit 2 will force

HWEN/PGEN/GPIO low.

Note 1: This represents the amount of time each programmed

bit will be present at HWEN/PGEN/GPIO. The entire pattern

period will be 32 × Bit Period.

Note 2: This duty cycle indicates the fraction of time the pat-

tern is being output at HWEN/PGEN/GPIO. For example the

1/2 duty cycle (bits <5:3> = 010) will have the 32 bit pattern

output once followed by a dead time (HWEN/PGEN/GPIO

high impedance) equal to 1×’s the pattern period (Deadtime

= 32 × Bit_Period × (1/DutyCycle -1). For the 100% duty cycle

setting the 32 bit pattern will repeat constantly with no dead-

time.

30020546

FIGURE 10. HPG Register Description

www.national.com 22

LM3528

GPIO Register Function

Bits 7 - 2 GPIO Data

(Bit 1)

Data

Direction

(Bit 0)

Function

X X 0 GPIO is

configured as

a GPIO input

with the input

data read

back via bit

[1]. This is the

default power

on state.

X X 1 GPIO is

configured as

a logic output.

The output

logic voltage

is written to bit

[1].

30020564

FIGURE 11. GPIO Register Description

Figures 12 – 15 detail the Pattern Generator Data Registers.

These hold the 32 bit data that is output at HWEN/PGEN/

GPIO in PGEN mode. The data is output LSB first (Bit 0 of

PGEN0) and MSB last (Bit 7 of PGEN3).

30020565

FIGURE 12. PGEN0 Register Description

30020566

FIGURE 13. PGEN1 Register Description

23 www.national.com

LM3528

30020567

FIGURE 14. PGEN2 Register Description

30020568

FIGURE 15. PGEN3 Register Description

Figure 16 shows a write sequence to the pattern generator

programmed to output the waveform in Figure 17. In this ex-

ample HPG register bits <7:6> = 01 (for 26ms/bit) and bits

<5:3> = 010 (for 1/2 duty cycle). The pattern data in registers

(PGEN0 – PGEN2) are all loaded with 0xAC. A ‘1’ will force

the HWEN/PGEN/GPIO output low while a ‘0’ will force

HWEN/PGEN/GPIO open drain. When set for a 26ms/bit pe-

riod the pattern will be output LSB first (PGEN0, bit 0) and

repeat every

When set for ½ duty cycle there will be a dead time (HWEN/

PGEN/GPIO high impedance) between each pattern and

equal to the pattern period. In applications where HWEN/

PGEN/GPIO is used to pull current through an indicator LED

a ‘1’ corresponds to the LED on and a ‘0’ corresponds to the

LED off.

www.national.com 24

LM3528

30020569

FIGURE 16. Pattern Generation Write Sequence

30020570

FIGURE 17. Pattern Generation Output

SHUTDOWN AND OUTPUT ISOLATION

The LM3528 provides a true shutdown for either MAIN or

SUB/FB when configured as a White LED bias supply. Write

a 0 to ENM (bit 1) of the General Purpose register to turn off

the MAIN current sink and force MAIN high impedance. Write

a 0 to ENS (bit 2) of the General Purpose register to turn off

the SUB/FB current sink and force SUB/FB high impedance.

Writing a 1 to ENM or ENS turns on the MAIN and SUB/FB

current sinks respectively. When in shutdown the leakage

current into MAIN or SUB/FB is typically 1.8µA. See Typical

Performance Plots for start-up responses of the LM3528 us-

ing the ENM and ENS bits in White LED and OLED modes.

25 www.national.com

LM3528

Application Information

LED CURRENT SETTING/MAXIMUM LED CURRENT

Connect a resistor (RSET) from SET to GND to program the

maximum LED current (ILED_MAX) into MAIN or SUB/FB. The

RSET to ILED_MAX relationship is:

where SET provides the constant 1.244V output.

OUTPUT VOLTAGE SETTING (OLED MODE)

Connect Feedback resistors from the converters output to

SUB/FB to GND to set the output voltage in OLED mode (see

R1 and R2 in the Typical Application Circuit (OLED Panel

Power Supply). First select R2 < 100kΩ then calculate R1

such that:

In OLED mode the MAIN current sink continues to regulate

the current through MAIN, however, VMAIN is no longer regu-

lated. To avoid dropout and ensure proper current regulation

the application must ensure that VMAIN > 0.3V.

INPUT CAPACITOR SELECTION

Choosing the correct size and type of input capacitor helps

minimize the input voltage ripple caused by the switching of

the LM3528’s boost converter. For continuous inductor cur-

rent operation the input voltage ripple is composed of 2 pri-

mary components, the capacitor discharge (delta VQ) and the

capacitor’s equivalent series resistance (delta VESR). These

ripple components are found by:

In the typical application circuit a 1µF ceramic input capacitor

works well. Since the ESR in ceramic capacitors is typically

less than 5mΩ and the capacitance value is usually small, the

input voltage ripple is primarily due to the capacitive dis-

charge. With larger value capacitors such as tantalum or

aluminum electrolytic the ESR can be greater than 0.5Ω. In

this case the input ripple will primarily be due to the ESR.

OUTPUT CAPACITOR SELECTION

The LM3528’s output capacitor supplies the LED current dur-

ing the boost converters on time. When the switch turns off

the inductor energy is discharged through the diode supplying

power to the LED’s and restoring charge to the output capac-

itor. This causes a sag in the output voltage during the on time

and a rise in the output voltage during the off time. The output

capacitor is therefore chosen to limit the output ripple to an

acceptable level depending on LED or OLED panel current

requirements and input/output voltage differentials. For prop-

er operation ceramic output capacitors ranging from 1µF to

2.2µF are required.

As with the input capacitor, the output voltage ripple is com-

posed of two parts, the ripple due to capacitor discharge (delta

VQ) and the ripple due to the capacitors ESR (delta VESR). For

continuous conduction mode, the ripple components are

found by:

Table 7 lists different manufacturers for various capacitors

and their case sizes that are suitable for use with the LM3528.

When configured as a dual output LED driver a 1µF output

capacitor is adequate. In OLED mode for output voltages

above 12V a 2.2µF output capacitor is required (see Low

Output Voltage Operation (OLED) Section).

www.national.com 26

LM3528

TABLE 7. Recommended Output Capacitors

Manufacturer Part Number Value Case Size Voltage Rating

TDK C1608X5R1E105M 1µF 0603 25V

Murata GRM39X5R105K25D539 1µF 0603 25V

TDK C2012X5R1E225M 2.2µF 0805 25V

Murata GRM219R61E225KA12 2.2µF 0805 25V

INDUCTOR SELECTION

The LM3528 is designed for use with a 10µH inductor, how-

ever 22µH are suitable providing the output capacitor is in-

creased 2×. When selecting the inductor ensure that the

saturation current rating (ISAT) for the chosen inductor is high

enough and the inductor is large enough such that at the

maximum LED current the peak inductor current is less than

the LM3528’s peak switch current limit. This is done by choos-

ing:

Values for IPEAK can be found in the plot of peak current limit

vs. VIN in the Typical Performance Characteristics graphs.

Table 8 shows possible inductors, as well as their corre-

sponding case size and their saturation current ratings.

TABLE 8. Recommended Inductors

Manufactur

Part Number Value Dimensions ISAT DC Resistance

TDK VLF3012AT-100MR49 10µH 2.6mm×2.8mm×1mm 490mA 0.36Ω

Coilcraft LPS3008-103ML 10µH 2.95mm×2.95mm×0.8mm 490mA 0.65Ω

TDK VLF4012AT-100MR79 10µH 3.5mm×3.7mm×1.2mm 800mA 0.3Ω

Coilcraft LPS4012-103ML 10µH 3.9mm×3.9mm×1.1mm 700mA 0.35Ω

TOKO A997AS-100M 10µH 3.8mm×3.8mm×1.8mm 580mA 0.18Ω

DIODE SELECTION

The output diode must have a reverse breakdown voltage

greater than the maximum output voltage. The diodes aver-

age current rating should be high enough to handle the

LM3528’s output current. Additionally, the diodes peak cur-

rent rating must be high enough to handle the peak inductor

current. Schottky diodes are recommended due to their lower

forward voltage drop (0.3V to 0.5V) compared to (0.6V to

0.8V) for PN junction diodes. If a PN junction diode is used,

ensure it is the ultra-fast type (trr < 50ns) to prevent excessive

loss in the rectifier. For Schottky diodes the B05030WS (or

equivalent) work well for most designs. See Table 9 for a list

of other Schottky Diodes with similar performance.

27 www.national.com

LM3528

TABLE 9. Recommended Schottky Diodes

Manufacturer Part Number Package Reverse

Breakdown

Voltage

Average

Current

Rating

On Semiconductor NSR0230P2T5G SOD-923 (0.8mm×0.6mm×0.4mm) 30V 200mA

On Semicondcuctor NSR0230M2T5G SOD-723 (1mm×0.6mm×0.52mm) 30V 200mA

On Semiconductor RB521S30T1 SOD-523 (1.2mm×0.8mm×0.6mm) 30V 200mA

Diodes Inc. SDM20U30 SOD-523 (1.2mm×0.8mm×0.6mm) 30V 200mA

Diodes Inc. B05030WS SOD-323 (1.6mm×1.2mm×1mm) 30V 0.5A

Philips BAT760 SOD-323 (1.6mm×1.2mm×1mm) 20V 1A

OUTPUT CURRENT RANGE (OLED MODE)

The maximum output current the LM3528 can deliver in OLED

mode is limited by 4 factors (assuming continuous conduc-

tion); the peak current limit of 770mA (typical), the inductor

value, the input voltage, and the output voltage. Calculate the

maximum output current (IOUT_MAX) using the following equa-

tion:

For the typical application circuit with VOUT = 18V and assum-

ing 70% efficiency, the maximum output current at VIN = 2.7V

will be approximately 70mA. At 4.2V due to the shorter on

times and lower average input currents the maximum output

current (at 70% efficiency) jumps to approximately 105mA.

Figure 11 shows a plot of IOUT_MAX vs. VIN using the above

equation, assuming 80% efficiency. In reality, factors such as

current limit and efficiency will vary over VIN, temperature, and

component selection. This can cause the actual IOUT_MAX to

be higher or lower.

30020562

FIGURE 18. Typical Maximum Output Current in OLED

Mode (assumed 80% efficiency)

OUTPUT VOLTAGE RANGE (OLED MODE)

The LM3528's output voltage is constrained by 2 factors. On

the low end it is limited by the minimum duty cycle of 10%

(assuming continuous conduction) and on the high end it is

limited by the over voltage protection threshold (VOVP) of 22V

(typical). In order to maintain stability when operating at dif-

ferent output voltages the output capacitor and inductor must

be changed. Refer to Table 10 for different VOUT, COUT, and

L combinations.

TABLE 10. Component Values for Output Voltage

Selection

VOUT COUT L VIN Range

18V 2.2µF 10µH 2.7V to 5.5V

15V 2.2µF 10µH 2.7V to 5.5V

12V 4.7µF 10µH 2.7V to 5.5V

9V 10µF 10µH 2.7V to 5.5V

7V 10µF 4.7µH 2.7V to 5.5V

5V 22µF 4.7µH 2.7V to 4.5V

www.national.com 28

LM3528

APPLICATION CIRCUITS

30020561

FIGURE 19. LED Backlight + OLED Power Supply

LAYOUT CONSIDERATIONS

Refer to AN-1112 for µSMD package soldering

The high switching frequencies and large peak currents in the

LM3528 make the PCB layout a critical part of the design. The

proceeding steps should be followed to ensure stable opera-

tion and proper current source regulation.

1, CIN should be located on the top layer and as close to the

device as possible. The input capacitor supplies the driver

currents during MOSFET switching and can have relatively

large spikes. Connecting the capacitor close to the device will

reduce the inductance between CIN and the LM3528 and

eliminate much of the noise that can disturb the internal ana-

log circuitry.

2, Connect the anode of the Schottky diode as close to the

SW pin as possible. This reduces the inductance between the

internal MOSFET and the diode and minimizes the noise gen-

erated from the discontinuous diode current and the PCB

trace inductance that will add ringing at the SW node and filter

through to VOUT. This is especially important in VOUT mode

when designing for a stable output voltage.

3, COUT should be located on the top layer to minimize the

trace lengths between the diode and PGND. Connect the

positive terminal of the output capacitor (COUT+) as close as

possible to the cathode of the diode. Connect the negative

terminal of the output capacitor (COUT-) as close as possible

to the PGND pin on the LM3528. This minimizes the induc-

tance in series with the output capacitor and reduces the

noise present at VOUT and at the PGND connection. This is

important due to the large di/dt into and out of COUT. The

returns for both CIN and COUT should terminate directly to

the PGND pin.

4, Connect the inductor on the top layer close to the SW pin.

There should be a low impedance connection from the induc-

tor to SW due to the large DC inductor current, and at the

same time the area occupied by the SW node should be small

so as to reduce the capacitive coupling of the high dV/dt

present at SW that can couple into nearby traces.

5, Route the traces for RSET and the feedback divider away

from the SW node to minimize the capacitance between these

nodes that can couple the high dV/dt present at SW into them.

Furthermore, the feedback divider and RSETshould have ded-

icated returns that terminate directly to the PGND pin of the

device. This will minimize any shared current with COUT or

CIN that can lead to instability. Avoide routing the SUB/FB

node close to other traces that can see high dV/dt such as the

I2C pins. The capacitive coupling on the PCB between FB

and these nodes can disturb the output voltage and cause

large voltage spikes at VOUT.

6, Do not connect any external capacitance to the SET pin.

7, Refer to the LM3528 Evaluation Board as a guide for proper

layout.

29 www.national.com

LM3528

Physical Dimensions inches (millimeters) unless otherwise noted

12 Bump µSMD

For Ordering, Refer to Ordering Information Table

NS Package Number TMD12

X1 = 1.215mm (±0.1mm), X2 = 1.615mm (±0.1mm), X3 = 0.6mm

www.national.com 30

LM3528

Notes

31 www.national.com

LM3528

Notes

LM3528 High Efficiency, Multi Display LED Driver with 128 Exponential Dimming Steps and

Integrated OLED Power Supply in micro SMD

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