MAX8727ETB+T - Maxim Integrated

General Description

The MAX8727 is a high-performance step-up DC-DC

converter that provides a regulated supply voltage for

active-matrix thin-film transistor (TFT) liquid-crystal dis-

plays (LCDs). The MAX8727 incorporates current-

mode, fixed-frequency, pulse-width modulation (PWM)

circuitry with a built-in n-channel power MOSFET to

achieve high efficiency and fast transient response.

Users can select 640kHz or 1.2MHz operation using a

logic input pin (FREQ). The high switching frequencies

allow the use of ultra-small inductors and low-ESR

ceramic capacitors. The current-mode architecture pro-

vides fast transient response to pulsed loads. A com-

pensation pin (COMP) gives users flexibility in adjusting

loop dynamics. The 30V internal MOSFET can generate

output voltages up to 24V from an input voltage

between 2.6V and 5.5V. Soft-start slowly ramps the input

current and is programmed with an external capacitor.

The MAX8727 is available in a 10-pin thin DFN package.

Applications

Notebook Computer Displays

LCD Monitor Panels

Features

♦90% Efficiency

♦Adjustable Output from VIN to 24V

♦2.6V to 5.5V Input Supply Range

♦Input Supply Undervoltage Lockout

♦Pin-Programmable 640kHz/1.2MHz Switching

Frequency

♦Programmable Soft-Start

♦0.1µA Shutdown Current

♦Small 10-Pin Thin DFN Package

MAX8727

TFT-LCD Step-Up DC-DC Converter

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-3480; Rev 1; 8/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

PART

TEMP RANGE

PIN-PACKAGE

MAX8727ETB

-40°C to +85°C

10 Thin DFN 3mm x 3mm

MAX8727ETB+

-40°C to +85°C

10 Thin DFN 3mm x 3mm

COMP 1

FB 2

GND 4

GND 5

SHDN

FREQ

MAX8727

TOP VIEW

THIN DFN

3mm x 3mm

Pin Configuration

LX LX

GND

FREQ

COMP

SS 1

VOUT

VIN

2.6V TO 5.5V

SHDN

MAX8727

Minimal Operating Circuit

+Denotes lead-free package.

MAX8727

TFT-LCD Step-Up DC-DC Converter

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN = VSHDN = 3V, FREQ = GND, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

LX to GND ..............................................................-0.3V to +26V

IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V

COMP, SS to GND .........................................-0.3V to VIN + 0.3V

LX Switch Maximum Continuous RMS Current .....................2.4A

Continuous Power Dissipation (TA= +70°C)

10-Pin Thin DFN (derate 24.4mW/°C above +70°C) ....1951mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +160°C

Lead Temperature (soldering, 10s) .................................+300°C

PARAMETER CONDITIONS

MIN TYP MAX

UNITS

VOUT < 18V 2.6 5.5

Input Voltage Range 18V < VOUT < 24V 4.0 5.5 V

Output Voltage Range 24 V

IN Undervoltage-Lockout

Threshold VIN rising, typical hysteresis is 50mV

2.20 2.38 2.57

VFB = 1.3V, not switching

0.225 0.440

IN Quiescent Current VFB = 1.0V, switching 2 5 mA

IN Shutdown Current SHDN = GND 0.1

10.0

µA

ERROR AMPLIFIER

FB Regulation Voltage Level to produce VCOMP = 1.24V

1.22 1.24 1.26

FB Input Bias Current VFB = 1.24V 50

125

250 nA

FB Line Regulation Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V

0.05 0.15

%/V

Transconductance

100 200

300 µS

Voltage Gain

700

V/V

Shutdown FB Input Voltage SHDN = GND

0.05 0.10 0.15

OSCILLATOR

FREQ = GND

540 640

740

Frequency FREQ = IN

1000 1220 1500

kHz

Maximum Duty Cycle 87 90 93 %

n-CHANNEL MOSFET

Current Limit VFB = 1V, 75% duty cycle 3.0 3.8 4.6 A

On-Resistance

125

250 Ω

Leakage Current VLX = 24V 30 45 µA

Current-Sense Transresistance

0.11 0.21 0.31

V/A

SOFT-START

Reset Switch Resistance 100 Ω

Charge Current VSS = 1.2V 2.5 4.5 7.5 µA

MAX8727

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VIN = VSHDN = 3V, FREQ = GND, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

CONTROL INPUTS

SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V 0.3 ×

VIN V

SHDN, FREQ Input High Voltage

VIN = 2.6V to 5.5V 0.7 ×

VIN V

SHDN, FREQ Input Hysteresis VIN = 2.6V to 5.5V 0.1 ×

VIN V

FREQ Pulldown Current 2.3 6.0 9.5 µA

SHDN Input Current SHDN = GND

0.001

1µA

ELECTRICAL CHARACTERISTICS

(VIN = VSHDN = 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS

MIN TYP MAX

UNITS

VOUT < 18V 2.6 5.5

Input Voltage Range 18V < VOUT < 24V 4.0 5.5 V

Output Voltage Range 24 V

IN Undervoltage-Lockout

Threshold VIN rising, typical hysteresis is 50mV

2.20 2.57

VFB = 1.3V, not switching

0.44

IN Quiescent Current VFB = 1.0V, switching 5 mA

IN Shutdown Current SHDN = GND 10 µA

ERROR AMPLIFIER

FB Regulation Voltage Level to produce VCOMP = 1.24V

1.215 1.260

FB Input Bias Current VFB = 1.24V

250

FB Line Regulation Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V

0.15

%/V

Transconductance

100 300

µS

Shutdown FB Input Voltage SHDN = GND

0.05 0.15

OSCILLATOR

FREQ = GND

490 770

Frequency FREQ = IN

900 1600

kHz

Maximum Duty Cycle 86 94 %

n-CHANNEL MOSFET

Current Limit VFB = 1V, 75% duty cycle 3.0 5.1 A

On-Resistance

250

mΩ

Current-Sense Transresistance

0.11 0.31

V/A

SOFT-START

Reset Switch Resistance

100

Charge Current VSS = 1.2V 2.5 7.5 µA

MAX8727

TFT-LCD Step-Up DC-DC Converter

4 _______________________________________________________________________________________

Note 1: Specifications to -40°C are guaranteed by design, not production tested.

ELECTRICAL CHARACTERISTICS (continued)

(VIN = VSHDN = 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

CONTROL INPUTS

SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V 0.3 ×

VIN V

SHDN, FREQ Input High Voltage

VIN = 2.6V to 5.5V 0.7 ×

VIN V

EFFICIENCY vs. LOAD CURRENT

(1.2MHz OPERATION)

MAX8727 toc01

LOAD CURRENT (mA)

EFFICIENCY (%)

10010

100

11000

L = 3.6μH

VIN = 5.0V

VIN = 3.3V

EFFICIENCY vs. LOAD CURRENT

(640kHz OPERATION)

MAX8727 toc02

LOAD CURRENT (mA)

EFFICIENCY (%)

10010

100

1 1000

L = 6.8μH

VIN = 5.0V

VIN = 3.3V

LOAD REGULATION

MAX8727 toc03

LOAD CURRENT (mA)

LOAD REGULATION (%)

10010

-1.5

-1.0

-0.5

0.5

-2.0

1 1000

VIN = 5.0V

VIN = 3.3V

SWITCHING FREQUENCY

vs. INPUT VOLTAGE

MAX8727 toc04

INPUT VOLTAGE (V)

SWITCHING FREQUENCY (kHz)

5.04.54.03.53.0

600

700

800

900

1000

1100

1200

1300

1400

500

2.5 5.5

FREQ = IN

FREQ = GND

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX8727 toc05

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

5.04.54.03.53.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.5 5.5

SWITCHING

NONSWITCHING

SUPPLY CURRENT vs. TEMPERATURE

(SWITCHING)

MAX8727 toc06

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

806040200-20

-40 100

VIN = 5.0V

VIN = 3.3V

Typical Operating Characteristics

(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA= +25°C unless otherwise noted.)

MAX8727

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________ 5

SOFT-START

(RLOAD = 30Ω)

MAX8727 toc07

1ms/div

INDUCTOR

CURRENT

1A/div

VOUT

5V/div

LOAD-TRANSIENT RESPONSE

(ILOAD = 50mA TO 550mA)

MAX8727 toc08

1μs/div

INDUCTOR

CURRENT

1A/div

50mA

15V

VOUT

5mV/div

AC-COUPLED

IOUT

500mA/div

PULSED LOAD-TRANSIENT RESPONSE

(ILOAD = 100mA TO 1.1A)

MAX8727 toc09

1μs/div

INDUCTOR

CURRENT

1A/div

0.1A

15V

VOUT

5mV/div

AC-COUPLED

IOUT

1A/div

SWITCHING WAVEFORMS

(ILOAD = 600mA)

MAX8727 toc10

1μs/div

INDUCTOR

CURRENT

1A/div

10V/div

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA= +25°C unless otherwise noted.)

MAX8727

TFT-LCD Step-Up DC-DC Converter

6 _______________________________________________________________________________________

NAME

FUNCTION

COMP

Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop

Compensation section for component selection guidelines.

2FB

Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltage-divider

between the step-up regulator’s output (VOUT) and GND, with the center tap connected to FB. Place the

divider close to the IC and minimize the trace area to reduce noise coupling. Set VOUT according to the

Output Voltage Selection section.

SHDN

Shutdown Control Input. Drive SHDN low to turn off the MAX8727.

GND

Ground. Connect pins 4 and 5 directly together.

GND

Ground. Connect pins 4 and 5 directly together.

6LX

Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and

minimize the trace area for lower EMI. Connect pins 6 and 7 directly together.

7LX

Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and

minimize the trace area for lower EMI. Connect pins 6 and 7 directly together.

8 IN Supply Pin. Bypass IN with a minimum 1µF ceramic capacitor directly to GND.

FREQ

Frequency-Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high, the

frequency is 1.2MHz. This input has a 5µA pulldown current.

10 SS

Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The soft-

start capacitor is charged with a constant current of 4.5µA. Full current limit is reached after t = 2.5 × 105 CSS.

The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start

capacitor is charged to 0.4V, after which soft-start begins.

Pin Description

LX LX

GND

FREQ

COMP

SS 1

VOUT

15V/600mA

VIN

4.5V TO 5.5V

SHDN

MAX8727

10μF

6.3V

10Ω

1μF

33nF

330pF

39pF

3.6μHD1

28.0kΩ

100kΩ

309kΩ

4.7μF

25V

4.7μF

25V

4.7μF

25V

Figure 1 Typical Operating Circuit

Detailed Description

The MAX8727 is a highly efficient power supply that

employs a current-mode, fixed-frequency, pulse-width

modulation (PWM) architecture for fast transient

response and low-noise operation. The device regu-

lates the output voltage through a combination of an

error amplifier, two comparators, and several signal

generators (Figure 2). The error amplifier compares the

signal at FB to 1.24V and varies the COMP output. The

voltage at COMP determines the current trip point each

time the internal MOSFET turns on. As the load

changes, the error amplifier sources or sinks current to

the COMP output to command the inductor peak cur-

rent necessary to service the load. To maintain stability

at high duty cycles, a slope-compensation signal is

summed with the current-sense signal.

At light loads, this architecture allows the MAX8727 to

“skip” cycles to prevent overcharging the output voltage.

In this region of operation, the inductor ramps up to a

peak value of approximately 50mA, discharges to the

output, and waits until another pulse is needed again.

Output Current Capability

The output current capability of the MAX8727 is a func-

tion of current limit, input voltage, operating frequency,

and inductor value. Because of the slope compensa-

tion used to stabilize the feedback loop, the inductor

current limit depends on the duty cycle. The current

limit is determined by the following equation:

ILIM = (1.26 - 0.35 x D) x ILIM_EC

where ILIM_EC is the current limit specified at 75% duty

cycle (see the Electrical Characteristics) and D is the

duty cycle.

The output current capability depends on the current-

limit value and is governed by the following equation:

where ILIM is the current limit calculated above, ηis the

regulator efficiency (85% nominal), and D is the duty

cycle. The duty cycle when operating at the current

limit is:

where VDIODE is the rectifier diode forward voltage and

RON is the on-resistance of the internal MOSFET.

DVVV

VIRV

OUT IN DIODE

OUT LIM ON DIODE

=−+

−× +

II DV

OUT MAX LIM IN

OSC

OUT

()

=−

××

⎡

⎣

⎢

⎤

⎦

⎥

⎥××

05 η

MAX8727

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________ 7

GND

FREQ

COMP

4μA

5μA

ERROR

COMPARATOR

ERROR

AMPLIFIER

SKIP

COMPARATOR

CLOCK

SKIP

BIAS

SHDN

MAX8727

ΣCURRENT

SENSE

CONTROL

AND DRIVER

LOGIC

SOFT-

START

SLOPE

COMPEN-

SATION

OSCILLATOR

∞

1.24V

Figure 2. MAX8727 Functional Diagram

MAX8727

Soft-Start

The MAX8727 can be programmed for soft-start upon

power-up with an external capacitor. When the shutdown

pin is taken high, the soft-start capacitor (CSS) is immedi-

ately charged to 0.4V. Then the capacitor is charged at a

constant current of 4.5µA (typ). During this time, the SS

voltage directly controls the peak inductor current, allow-

ing 0A at VSS = 0.4V to the full current limit at VSS = 1.5V.

The maximum load current is available after the soft-start

is completed. When the SHDN pin is taken low, the soft-

start capacitor is discharged to ground.

Frequency Selection

The MAX8727’s frequency can be user selected to

operate at either 640kHz or 1.2MHz. Connect FREQ to

GND for 640kHz operation. For a 1.2MHz switching fre-

quency, connect FREQ to IN. This allows the use of

small, minimum-height external components while

maintaining low output noise. FREQ has an internal

pulldown, allowing the user the option of leaving FREQ

unconnected for 640kHz operation.

Shutdown

The MAX8727 shuts down to reduce the supply current

to 0.1µA when SHDN is low. In this mode, the internal

reference, error amplifier, comparators, and biasing cir-

cuitry turn off, and the n-channel MOSFET is turned off.

The step-up regulator’s output is connected to IN by

the external inductor and rectifier diode.

Applications Information

Step-up regulators using the MAX8727 can be

iteration. All designs should be prototyped and tested

prior to production. Table 1 provides a list of power

components for the typical applications circuit. Table 2

lists component suppliers.

External-component-value choice is primarily dictated

by the output voltage and the maximum load current,

as well as maximum and minimum input voltages.

Begin by selecting an inductor value. Once L is known,

choose the diode and capacitors.

Inductor Selection

The minimum inductance value, peak current rating, and

series resistance are factors to consider when selecting

the inductor. These factors influence the converter’s effi-

ciency, maximum output load capability, transient-

response time, and output voltage ripple. Physical size

and cost are also important factors to be considered.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance values minimize the cur-

rent ripple and therefore reduce the peak current,

which decreases core losses in the inductor and I2R

losses in the entire power path. However, large induc-

tor values also require more energy storage and more

turns of wire, which increase physical size and can

increase I2R losses in the inductor. Low inductance val-

ues decrease the physical size but increase the current

ripple and peak current. Finding the best inductor

involves choosing the best compromise between circuit

efficiency, inductor size and cost.

TFT-LCD Step-Up DC-DC Converter

8 _______________________________________________________________________________________

DESIGNATION

DESCRIPTION

10µF ±10%, 6.3V X5R ceramic capacitor

(0805)

Murata GRM21BR60J106K

Taiyo Yuden JMK212BJ106KD

C2, C7, C8

4.7µF±20%, 25V X7R ceramic capacitors

(1206)

Murata GRM31CR71E475M

D1 3A, 30V Schottky diode (M-Flat)

Toshiba CMS02

L1 3.6µH ±30% power inductor

Sumida CDRH6D26-3R6NC

Table 1. Component List

SUPPLIER PHONE FAX WEBSITE

Murata 770-436-1300 770-436-3030 www.murata.com

Sanyo 619-661-4143 619-661-1055 www.sanyovideo.com

Sumida 847-545-6700 847-545-6720 www.sumida.com

Taiyo Yuden 800-348-2496 847-925-0899 www.t-yuden.com

Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec

Table 2. Component Suppliers

The equations used here include a constant LIR, which

is the ratio of the inductor peak-to-peak ripple current

to the average DC inductor current at the full load cur-

rent. The best trade-off between inductor size and cir-

cuit efficiency for step-up regulators generally has an

LIR between 0.3 and 0.5. However, depending on the

AC characteristics of the inductor core material and the

ratio of inductor resistance to other power path resis-

tances, the best LIR can shift up or down. If the induc-

tor resistance is relatively high, more ripple can be

accepted to reduce the number of turns required and

increase the wire diameter. If the inductor resistance is

relatively low, increasing inductance to lower the peak

current can decrease losses throughout the power

path. If extremely thin high-resistance inductors are

used, as is common for LCD panel applications, the

best LIR can increase to between 0.5 and 1.0.

Once a physical inductor is chosen, higher and lower

values of the inductor should be evaluated for efficien-

cy improvements in typical operating regions.

Calculate the approximate inductor value using the typ-

ical input voltage (VIN), the maximum output current

(IMAIN(MAX)), the expected efficiency (ηTYP) taken from

an appropriate curve in the Typical Operating

Characteristics, and an estimate of LIR based on the

above discussion:

Choose an available inductor value from an appropriate

inductor family. Calculate the maximum DC input cur-

rent at the minimum input voltage VIN(MIN) using con-

servation of energy and the expected efficiency at that

operating point (ηMIN) taken from an appropriate curve

in the Typical Operating Characteristics:

Calculate the ripple current at that operating point and

the peak current required for the inductor:

The inductor’s saturation current rating and the

MAX8727’s LX current limit (ILIM) should exceed IPEAK,

and the inductor’s DC current rating should exceed

IIN(DC,MAX). For good efficiency, choose an inductor

with less than 0.1Ωseries resistance.

Considering the typical operating circuit, the maximum

load current (IMAIN(MAX)) is 600mA with a 15V output and

a typical input voltage of 5V. Choosing an LIR of 0.35 and

estimating efficiency of 85% at this operating point:

Using the circuit’s minimum input voltage (4.5V) and

estimating efficiency of 85% at that operating point:

The ripple current and the peak current are:

Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharg-

ing of the output capacitance, and the ohmic ripple due

to the capacitor’s equivalent series resistance (ESR):

where IPEAK is the peak inductor current (see the

Inductor Selection section). For ceramic capacitors,

the output voltage ripple is typically dominated by

VRIPPLE(C). The voltage rating and temperature charac-

teristics of the output capacitor must also be considered.

VV V

and

VIR

RIPPLE RIPPLE C RIPPLE ESR

RIPPLE C MAIN

OUT

MAIN IN

MAIN OSC

RIPPLE ESR PEAK ESR COUT

() ( )

()

() ( )

≈−

⎛

⎝

⎜

⎞

⎠

⎟

≈

IVV V

H V MHz

RIPPLE

PEAK

. ( .)

. .

=×−

μ× × ≈

=+ ≈

45 15 45

36 15 12

073

235 073

270

IAV

IN DC MAX(, )

. .

.=×

×≈

06 15

45 085

235

A MHz

. .

.=⎛

⎝

⎜

⎞

⎠

⎟−

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟≈μ

15 5

06 12

085

035

II I

PEAK IN DC MAX RIPPLE

(, )

IVVV

LV f

RIPPLE

IN MIN MAIN IN MIN

MAIN OSC

( )

() ()

=×−

××

IIV

IN DC MAX

MAIN MAX MAIN

IN MIN MIN

(, )

()

=×

×η

I f LIR

MAIN

MAIN IN

MAIN MAX OSC

TYP

()

=⎛

⎝

⎜

⎞

⎠

⎟−

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

2η

MAX8727

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________ 9

MAX8727

Input Capacitor Selection

The input capacitor (CIN) reduces the current peaks

drawn from the input supply and reduces noise injection

into the IC. A 10µF ceramic capacitor is used in the typi-

cal operating circuit (Figure 1) because of the high

source impedance seen in typical lab setups. Actual

applications usually have much lower source imped-

ance since the step-up regulator often runs directly from

the output of another regulated supply. Typically, CIN

can be reduced below the values used in the typical

operating circuit. Ensure a low noise supply at IN by

using adequate CIN. Alternatively, greater voltage varia-

tion can be tolerated on CIN if IN is decoupled from CIN

using an RC lowpass filter (see R3 and C3 in Figure 1).

Rectifier Diode Selection

The MAX8727’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommend-

ed for most applications because of their fast recovery

time and low forward voltage. The diode should be

rated to handle the output voltage and the peak switch

current. Make sure that the diode’s peak current rating

is at least IPEAK calculated in the Inductor Selection

section and that its breakdown voltage exceeds the

output voltage.

Output Voltage Selection

The MAX8727 operates with an adjustable output from

VIN to 24V. Connect a resistive voltage-divider from the

output (VMAIN) to GND with the center tap connected to

FB (see Figure 1). Select R2 in the 10kΩto 50kΩrange.

Calculate R1 with the following equation:

where VFB, the step-up regulator’s feedback set point,

is 1.24V (typ). Place R1 and R2 close to the IC.

Loop Compensation

The voltage feedback loop needs proper compensation

to prevent excessive output ripple and poor efficiency

caused by instability. This is done by connecting a

resistor (RCOMP) and capacitor (CCOMP) in series from

COMP to GND, and another capacitor (CCOMP2) from

COMP to GND. RCOMP is chosen to set the high-fre-

quency integrator gain for fast transient response, while

CCOMP is chosen to set the integrator zero to maintain

loop stability. The second capacitor, CCOMP2, is cho-

sen to cancel the zero introduced by output-capaci-

tance ESR. For optimal performance, choose the com-

ponents using the following equations:

For the ceramic output capacitor, where ESR is small,

CCOMP2 is optional. The best gauge of correct loop

compensation is by inspecting the transient response

of the MAX8727. Adjust RCOMP and CCOMP as neces-

sary to obtain optimal transient performance.

Soft-Start Capacitor

The soft-start capacitor should be large enough that it

does not reach final value before the output has

reached regulation. Calculate CSS to be:

where COUT is the total output capacitance including

any bypass capacitor on the output bus, VOUT is the

maximum output voltage, IINRUSH is the peak inrush

current allowed, IOUT is the maximum output current

during power-up, and VIN is the minimum input voltage.

The load must wait for the soft-start cycle to finish

before drawing a significant amount of load current.

The duration after which the load can begin to draw

maximum load current is:

tMAX = 6.77 x 105x CSS

VVV

VI I V

SS OUT

OUT IN OUT

IN INRUSH OUT OUT

>× × ×

−×

×−×

⎛

⎝

⎜

⎞

⎠

⎟

−

21 10 6

RVV C

CVC

CRLI

COMP IN OUT OUT

MAIN MAX

COMP OUT OUT

MAIN MAX COMP

COMP

ESR MAIN MAX

IN OUT

()

≈×× ×

≈×

××

≈×××

315

0 0036

RR V

MAIN

12 1 =× −

⎛

⎝

⎜

⎞

⎠

⎟

TFT-LCD Step-Up DC-DC Converter

10 ______________________________________________________________________________________

Multiple-Output Power Supply for TFT LCD

Figure 3 shows a power supply for active-matrix TFT-

LCD flat-panel displays. Output-voltage transient perfor-

mance is a function of the load characteristic. Add or

remove output capacitance (and recalculate compensa-

tion-network component values) as necessary to meet

the required transient performance. Regulation perfor-

mance for secondary outputs (V2 and V3) depends on

the load characteristics of all three outputs.

PC Board Layout and Grounding

Careful PC board layout is important for proper operation.

Use the following guidelines for good PC board layout:

1) Minimize the area of high-current loops by placing

the inductor, rectifier diode, and output capacitors

near the input capacitors and near the LX and GND

pins. The high-current input loop goes from the

positive terminal of the input capacitor to the induc-

tor, to the IC’s LX pin, out of GND, and to the input

capacitor’s negative terminal. The high-current out-

put loop is from the positive terminal of the input

capacitor to the inductor, to the rectifier diode (D1),

and to the positive terminal of the output capacitors,

reconnecting between the output capacitor and

input capacitor ground terminals. Connect these

loop components with short, wide connections.

Avoid using vias in the high-current paths. If vias

are unavoidable, use many vias in parallel to

reduce resistance and inductance.

2) Create a power ground island (PGND) consisting of

the input and output capacitor grounds and GND

pins. Connect all of these together with short, wide

traces or a small ground plane. Maximizing the

width of the power ground traces improves efficien-

cy and reduces output voltage ripple and noise

spikes. Create an analog ground plane (AGND)

consisting of the feedback-divider ground connec-

tion, the COMP and SS capacitor ground connec-

tions, and the device’s exposed backside pad.

Connect the AGND and PGND islands by connect-

ing the GND pins directly to the exposed backside

pad. Make no other connections between these

separate ground planes.

MAX8727

TFT-LCD Step-Up DC-DC Converter

______________________________________________________________________________________ 11

LX LX

GND

FREQ

COMP

SS 1

VIN

4.5V TO 5.5V

SHDN

MAX8727

10μF

6.3V

10Ω

1μF

33nF

330pF

39pF

3.6μHD1

28.0kΩ

100kΩ

309kΩ

+28V C9

1μF

0.1μF

-14V

C10

1μF

VOUT

15V/600mA

4.7μF

25V

4.7μF

25V

4.7μF

25V

Figure 3. Multiple-Output TFT-LCD Power Supply

MAX8727

3) Place the feedback voltage-divider-resistors as

close to the FB pin as possible. The divider’s center

trace should be kept short. Placing the resistors far

away causes the FB trace to become an antenna

that can pick up switching noise. Avoid running the

feedback trace near LX.

4) Place the IN pin bypass capacitor as close to the

device as possible. The ground connection of the

IN bypass capacitor should be connected directly

to GND pins with a wide trace.

5) Minimize the length and maximize the width of the

traces between the output capacitors and the load

for best transient responses.

6) Minimize the size of the LX node while keeping it

wide and short. Keep the LX node away from the

feedback node and analog ground. Use DC traces

as a shield if necessary.

Refer to the MAX8727 evaluation kit for an example of

proper board layout.

Chip Information

TRANSISTOR COUNT: 2746

PROCESS: BiCMOS

TFT-LCD Step-Up DC-DC Converter

12 ______________________________________________________________________________________

MAX8727

TFT-LCD Step-Up DC-DC Converter

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,

go to www.maxim-ic.com/packages.)

6, 8, &10L, DFN THIN.EPS

PIN 1

INDEX

AREA

21-0137

PACKAGE OUTLINE, 6,8,10 & 14L,

TDFN, EXPOSED PAD, 3x3x0.80 mm

-DRAWING NOT TO SCALE-

[(N/2)-1] x e

REF.

PIN 1 ID

0.35x0.35

DETAIL A

COMMON DIMENSIONS

SYMBOL MIN. MAX.

A0.70 0.80

D2.90 3.10

E2.90 3.10

A1 0.00 0.05

L0.20 0.40

PKG. CODE ND2 E2 eJEDEC SPEC b[(N/2)-1] x e

PACKAGE VARIATIONS

0.25 MIN.k

A2 0.20 REF.

2.30±0.101.50±0.106T633-1 0.95 BSC MO229 / WEEA 1.90 REF0.40±0.05

1.95 REF0.30±0.05

0.65 BSC

2.30±0.108T833-1

2.00 REF0.25±0.05

0.50 BSC

2.30±0.1010T1033-1

2.40 REF0.20±0.05- - - -

0.40 BSC

1.70±0.10 2.30±0.1014T1433-1

1.50±0.10

MO229 / WEEC

MO229 / WEED-3

0.40 BSC - - - - 0.20±0.05 2.40 REFT1433-2 14 2.30±0.101.70±0.10

T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF

T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF

T833-3 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF

-DRAWING NOT TO SCALE-

G22

21-0137

PACKAGE OUTLINE, 6,8,10 & 14L,

TDFN, EXPOSED PAD, 3x3x0.80 mm

DOWNBONDS

ALLOWED

YES