LM3668SD-2833/NOPB - NATIONAL SEMICONDUCTOR

LM3668

LM3668 1A, High Efficiency Dual Mode Single Inductor Buck-Boost DC/DC

Converter

Literature Number: SNVS449L

LM3668

March 4, 2011

1A, High Efficiency Dual Mode Single Inductor Buck-Boost

DC/DC Converter

General Description

The LM3668 is a synchronous buck-boost DC-DC converter

optimized for powering low voltage circuits from a Li-Ion bat-

tery and input voltage rails between 2.5V and 5.5V. It has the

capability to support up to 1A output current over the output

voltage range. The LM3668 regulates the output voltage over

the complete input voltage range by automatically switching

between buck or boost modes depending on the input volt-

age.

The LM3668 has 2 N-channel MOSFETS and 2 P-channel

MOSFETS arranged in a topology that provides continuous

operation through the buck and boost operating modes.

There is a MODE pin that allows the user to choose between

an intelligent automatic PFM-PWM mode operation and

forced PWM operation. During PWM mode, a fixed-frequency

2.2MHz (typ.) is used. PWM mode drives load up to 1A. Hys-

teretic PFM mode extends the battery life through reduction

of the quiescent current to 45µA (typ.) at light loads during

system standby. Internal synchronous rectification provides

high efficiency. In shutdown mode (Enable pin pulled low) the

device turns off and reduces battery consumption to 0.01µA

(typ.).

The LM3668 is available in a 12-pin LLP package. A high

switching frequency of 2.2MHz (typ.) allows the use of tiny

surface-mount components including a 2.2µH inductor, a

10µF input capacitor, and a 22µF output capacitor.

Features

■45µA typical quiescent current

■For 2.8V/3.3V and 3.0/3.4V versions:

- 1A maximum load current for VIN = 2.8V to 5.5V

- 800mA maximum load current for VIN = 2.7V

- 600mA maximum load current for VIN = 2.5V

■For 4.5/5V

- 1A maximum load current for VIN = 3.9V to 5.5V

- 800mA maximum load current for VIN = 3.4V to 3.8V

- 700mA maximum load current for VIN = 3.0V to 3.3V

- 600mA maximum load current for VIN = 2.7V to 2.9V

■2.2MHz PWM fixed switching frequency (typ.)

■Automatic PFM-PWM Mode or Forced PWM Mode

■Wide Input Voltage Range: 2.5V to 5.5V

■Internal synchronous rectification for high efficiency

■Internal soft start: 600µs Maximum startup time after VIN

settled

■0.01µA typical shutdown current

■Current overload and Thermal shutdown protection

■Frequency Sync Pin: 1.6MHz to 2.7MHz

Applications

■Handset Peripherals

■MP3 players

■Pre-Regulation for linear regulators

■PDAs

■Portable Hard Disk Drives

■WiMax Modems

Typical Applications

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Typical Application Circuit

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Efficiency at 3.3V Output

LM3668 1A, High Efficiency Dual Mode Single Inductor Buck-Boost DC/DC Converter

Functional Block Diagram

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FIGURE 1. Functional Block Diagram

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LM3668

Connection Diagrams and Package Mark Information

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Top View

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Bottom View

Pin Descriptions

Pin # Pin Name Description

1 VOUT Connect to output capacitor.

2 SW2 Switching Node connection to the internal PFET switch (P2) and NFET synchronous rectifier (N2).

3 PGND Power Ground.

4 SW1 Switching Node connection to the internal PFET switch (P1) and NFET synchronous rectifier (N1).

5 PVIN Supply to the power switch, connect to the input capacitor.

6 EN Enable Input. Set this digital input high for normal operation. For shutdown, set low.

7 VDD Signal Supply input. If board layout is not optimum an optional 1µF ceramic capacitor is suggested

as close to this pin as possible.

8 NC No connect. Connect this pin to SGND on PCB layout.

9 SGND Analog and Control Ground.

10 MODE/SYNC Mode = LOW, Automatic Mode. Mode= HI, Forced PWM Mode SYNC = external clock

synchronization from 1.6MHz to 2.7MHz (When SYNC function is used, device is forced in PWM

mode).

11 VSEL Voltage selection pin; ( ie: 2.8V/3.3V option) Logic input low (or GND) = 2.8V and logic high = 3.3V

(or VIN) to set output Voltage.

12 FB Feedback Analog Input. Connect to the output at the output filter.

DAP DAP Die Attach Pad, connect the DAP to SGND on PCB layout to enhance thermal performance. It should

not be used as a primary ground connection.

Ordering Information

Order Number Package NSC Package

Marking Supplied As

LM3668SD - 2833 LLP-12 S017B 1000 units, Tape and Reel

LM3668SDX - 2833 4500 units, Tape and Reel

LM3668SD - 3034 LLP-12 S018B 1000 units, Tape and Reel

LM3668SDX - 3034 4500 units, Tape and Reel

LM3668SD - 4550 LLP-12 S019B 1000 units, Tape and Reel

LM3668SDX - 4550 4500 units, Tape and Reel

Note: As an example, if VOUT option is 3.0V/3.4V, when VSEL = Low, set VOUT to 3V; when VSEL = high, set VOUT = 3.4V. This configuration applies to all voltage

options.

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LM3668

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

PVIN, VDD Pin, SW1, SW2 & VOUT:

Voltage to SGND & PGND

−0.2V to +6.0V

FB, EN ,MODE, SYNC pins: (PGND &

SGND-0.2V) to

(PVIN + 0.2)

PGND to SGND -0.2V to 0.2V

Continuous Power Dissipation

(Note 3)

Internally Limited

Maximum Junction Temperature

(TJ-MAX)+125°C

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

Maximum Lead Temperature

(Soldering, 10 sec)

+260°C

Operating Ratings

Input Voltage Range 2.5V to 5.5V

Recommended Load Current 0mA to 1A

Junction Temperature (TJ) Range −40°C to +125°C

Ambient Temperature (TA) Range

(Note 3)

−40°C to +85°C

Thermal Properties

Junction-to-Ambient Thermal Resistance (θJA), 34°C/W

Leadless Lead frame Package (Note 5)

Electrical Characteristics (Note 6, Note 7) Limits in standard typeface are for TJ = +25°C. Limits in

boldface type apply over the full operating ambient temperature range (−40°C ≤ = TA ≤ +85°C). Unless otherwise noted,

specifications apply to the LM3668. VIN = 3.6V = EN, VOUT = 3.3V. For VOUT = 4.5/5.0V, VIN = 4V.

Symbol Parameter Conditions Min Typ Max Units

VFB Feedback Voltage (Note 7) -3 3%

ILIM Switch Peak Current Limit Open loop(Note 2)1.6 1.85 2.05 A

ISHDN Shutdown Supply Current EN =0V 0.01 1µA

IQ_PFM DC Bias Current in PFM No load, device is not switching

(FB forced higher than

programmed output voltage)

45 60 µA

IQ_PWM DC Bias Current in PWM PWM Mode, No Switching 600 750 µA

RDSON(P) Pin-Pin Resistance for PFET Switches P1 and P2 130 180 mΩ

RDSON(N) Pin-Pin Resistance for NFET Switches N1 and N2 100 150 mΩ

FOSC Internal Oscillator Frequency PWM Mode 1.9 2.2 2.5 MHz

FSYNC Sync Frequency Range VIN = 3.6V 1.6 2.7 MHz

VIH Logic High Input for EN, MODE/

SYNC pins

1.1 V

VIL Logic Low Input for EN, MODES/

SYNC pins

0.4 V

IEN, MODE, SYNC EN, MODES/SYNC pins Input

Current

0.3 1µA

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation

of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions,

see the Electrical Characteristics tables.

Note 2: Electrical Characteristic table reflects open loop data (FB = 0V and current drawn from SW pin ramped up until cycle by cycle current limits is activated).

Closed loop current limit is the peak inductor current measured in the application circuit by increasing output current until output voltage drops by 10%.

Note 3: 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 = 125ºC), 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 4: The Human body model is a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200pF capacitor discharged

directly into each pin. MIL-STD-883 3015.7

Note 5: 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 101.6mm x 76.2mm x 1.6mm. Thickness of the copper layers are 2oz/1oz/1oz/

2oz. The middle layer of the board is 60mm x 60mm. Ambient temperature in simulation is 22°C, still air.

Junction-to-ambient thermal resistance is highly application and board-layout dependent. In applications where high maximum power dissipation exists, special

care must be paid to thermal dissipation issues in board design.

Note 6: All voltage is with respect to SGND.

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

Note 8: CIN and COUT: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. COUT_MIN should not exceed −40% of

suggested value. The preferable choice would be a type and make MLCC that issues −30% over the operating temperature and voltage range.

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LM3668

Typical Performance Characteristics Typical Application Circuit (Figure 1): VIN = 3.6V, L = 2.2µH,

CIN = 10µF, COUT = 22µF (Note 8), TA = 25°C , unless otherwise stated.

Supply Current vs. Temperature (Not switching)

(VOUT = 3.4V)

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Switching Frequency vs. Temperature

(VOUT = 3.4V)

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NFET_RDS (on) vs. Temperature

(VOUT = 3.4V)

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PFET_RDS (on) vs. Temperature

(VOUT = 3.4V)

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ILIMIT vs. Temperature

(VOUT = 3.4V)

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Efficiency at VOUT = 2.8V

(Forced PWM Mode)

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LM3668

Efficiency at VOUT = 2.8V

(Auto Mode)

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Efficiency at VOUT = 3.0V

(Forced PWM Mode)

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Efficiency at VOUT = 3.0V

(Auto Mode)

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Efficiency at VOUT = 3.3V

(Forced PWM Mode)

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Efficiency at VOUT = 3.3V

(Auto Mode)

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Efficiency at VOUT = 3.4V

(Forced PWM Mode)

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LM3668

Efficiency at VOUT = 3.4V

(Auto Mode)

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Efficiency at VOUT = 4.5V

(Forced PWM Mode)

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Efficiency at VOUT = 4.5V

(Auto Mode)

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Efficiency at VOUT = 5.0V

(Forced PWM Mode)

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Efficiency at VOUT = 5.0V

(Auto Mode)

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Line Transient in Buck Mode

( VOUT = 3.4V, Load = 500mA)

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LM3668

Line Transient in Boost Mode

( VOUT = 3.4V, Load = 500mA)

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Line Transient in Buck-Boost Mode

( VOUT = 3.4V, Load = 500mA)

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Load Transient in Buck Mode

(Forced PWM Mode)

VIN = 4.2V, VOUT = 3.4V, Load = 0-500mA

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Load Transient in Boost Operation

(Forced PWM Mode)

VIN = 2.7V, VOUT = 3.4V, Load = 0-500mA

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Load Transient in Buck-Boost Operation

(Forced PWM Mode)

VIN = 3.44V, VOUT = 3.4V, Load = 0-500mA

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Load Transient in Buck Mode

(Forced PWM Mode)

VIN = 4.2V, VOUT = 3.0V, Load = 0-500mA

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LM3668

Load Transient in Boost Mode

(Forced PWM Mode)

VIN = 2.7V, VOUT = 3.0V, Load = 0-500mA

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Load Transient in Buck-Boost Mode

(Forced PWM Mode)

VIN = 3.05V, VOUT = 3.0V, Load = 0-500mA

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Load Transient in Buck Mode

(Auto Mode)

VIN = 4.2V, VOUT = 3.3V, Load = 50-150mA

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Load Transient in Boost Mode

(Auto Mode)

VIN = 2.7V, VOUT = 3.3V, Load = 50-150mA

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Load Transient in Buck-Boost Mode

(Auto Mode)

VIN = 3.6V, VOUT = 3.3V, Load = 50-150mA

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Load Transient in Buck Mode

(Forced PWM Mode)

VIN = 5.5V, VOUT = 5.0V, Load = 0-500mA

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LM3668

Load Transient in Boost Mode

(Forced PWM Mode)

VIN = 3.5V, VOUT = 5.0V, Load = 0-500mA

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Typical Switching Waveform in Boost Mode

(PWM Mode)

VIN = 2.7V, VOUT = 3.0V, Load = 500mA

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Typical Switching Waveform in Buck Mode

(PWM Mode)

VIN = 3.6V, VOUT = 3.0V, Load = 500mA

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Typical Switching Waveformt in Boost Mode

(PFM Mode)

VIN = 2.7V, VOUT = 3.0V, Load = 50mA

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Typical Switching Waveform in Buck Mode

(PFM Mode)

VIN = 3.6V, VOUT = 3.0V, Load = 50mA

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Typical Switching Waveform in Boost Mode

(PWM Mode)

VIN = 3V, VOUT = 3.4V, Load = 500mA

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LM3668

Typical Switching Waveform in Buck Mode

(PWM Mode)

VIN = 4V, VOUT = 3.4V, Load = 500mA

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Typical Switching Waveform in Boost Mode

(PFM Mode)

VIN = 3V, VOUT = 3.4V, Load = 50mA

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Typical Switching Waveform in Buck Mode

(PFM Mode)

VIN = 4V, VOUT = 3.4V, Load = 50mA

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Start up in PWM Mode

(VOUT = 3.4V, Load = 1mA)

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Start up in PWM Mode

(VOUT = 3.4V, Load = 500mA)

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LM3668

Circuit Description

The LM3668, a high-efficiency Buck or Boost DC-DC con-

verter, delivers a constant voltage from either a single Li-Ion

or three cell NIMH/NiCd battery to portable devices such as

mobile phones and PDAs. Using a voltage mode architecture

with synchronous rectification, the LM3668 has the ability to

deliver up to 1A depending on the input voltage, output volt-

age, ambient temperature and the chosen inductor.

In addition, the device incorporates a seamless transition

from buck-to-boost or boost-to-buck mode. The internal error

amplifier continuously monitors the output to determine the

transition from buck-to-boost or boost-to-buck operation. Fig-

ure 2 shows the four switches network used for the buck and

boost operation. Table 1 summarizes the state of the switches

in different modes.

There are three modes of operation depending on the current

required: PWM (Pulse Width Modulation), PFM (Pulse Fre-

quency Modulation), and shutdown. The device operates in

PWM mode at load currents of approximately 80mA or higher

to improve efficiency. Lighter load current causes the device

to automatically switch into PFM mode to reduce current con-

sumption and extend battery life. Shutdown mode turns off

the device, offering the lowest current consumption.

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FIGURE 2. Simplified Diagram of Switches

TABLE 1. State of Switches in Different Modes

Mode Always ON Always

OFF

Switching

Buck SW P2 SW N2 SW P1 & N1

Boost SW P1 SW N1 SW N2 & P2

BUCK OPERATION

When the input voltage is greater than the output voltage, the

device operates in buck mode where switch P2 is always ON

and P1 & N1 control the output . Figure 3 shows the simplified

circuit for buck mode operation.

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FIGURE 3. Simplified Circuit for Buck Operation

BOOST OPERATION

When the input voltage is smaller than the output voltage, the

device enters boost mode operation where P1 is always ON,

while switches N2 & P2 control the output. Figure 4 shows the

simplified circuit for boost mode operation.

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FIGURE 4. Simplified Circuit for Boost Operation

PWM OPERATION

In PWM operation, the output voltage is regulated by switch-

ing at a constant frequency and then modulating the energy

per cycle to control power to the load. In Normal operation,

the internal error amplifier provides an error signal, Vc, from

the feedback voltage and Vref. The error amplifier signal, Vc,

is compared with a voltage, Vcenter, and used to generate

the PWM signals for both Buck & Boost modes. Signal Vcen-

ter is a DC signal which sets the transition point of the buck

and boost modes. Below are three regions of operation:

•Region I: If Vc is less than Vcenter, Buck mode.

•Region II: If Vc and Vcenter are equal, both PMOS

switches (P1, P2) are on and both NMOS switches (N1,

N2) are off. The power passes directly from input to output

via P1 & P2

•Region III: If Vc is greater than Vcenter, Boost mode.

The Buck-Boost operation is avoided, to improve the efficien-

cy across VIN and load range.

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FIGURE 5. PWM Generator Block Diagram

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM3668 uses an internal MOSFET

as a synchronous rectifier to reduce rectifier forward voltage

drop and associated power loss. Synchronous rectification

provides a significant improvement in efficiency whenever the

output voltage is relatively low compare to the voltage drop

across an ordinary rectifier diode.

PFM OPERATION

At very light loads, the converter enters PFM mode and op-

erates with reduced switching frequency and supply current

to maintain high efficiency. The part automatically transitions

into PFM mode when either of two following conditions occur

for a duration of 128 or more clock cycles:

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LM3668

A. The inductor current reaches zero.

B. The peak inductor current drops below the IMODE level,

(Typically IMODE < 45mA + VIN/80 Ω ).

In PFM operation, the compensation circuit in the error am-

plifier is turned off. The error amplifier works as a hysteretic

comparator. The PFM comparator senses the output voltage

via the feedback pin and controls the switching of the output

FETs such that the output voltage ramps between ~0.8% and

~1.6% of the nominal PWM output voltage (). If the output

voltage is below the ‘high’ PFM comparator threshold, the P1

& P2 (Buck mode) or N2 & P1 (Boost mode) power switches

are turned on. It remains on until the output voltage reaches

the ‘high’ PFM threshold or the peak current exceeds the

IPFM level set for PFM mode. The typical peak current in PFM

mode is: IPFM = 220mA

Once the P1 ( Buck mode) or N2 ( Boost mode) power switch

is turned off, the N1 & P2 ( Buck mode) or P1 & P2 (Boost

mode) power switches are turned on until the inductor current

ramps to zero. When the zero inductor current condition is

detected, the N1( Buck mode) or P2 (Boost mode) power

switches are turned off. If the output voltage is below the ‘high’

PFM comparator threshold, the P1 & P2 (Buck mode) or N2

& P1 ( Boost mode) switches are again turned on and the

cycle is repeated until the output reaches the desired level.

Once the output reaches the ‘high’ PFM threshold, the N1 &

P2 (Buck mode) or P1 & P2 (Boost mode) switches are turned

on briefly to ramp the inductor current to zero, then both output

switches are turned off and the part enters an extremely low

power mode. Quiescent supply current during this ‘sleep’

mode is 45µA (typ), which allows the part to achieve high ef-

ficiency under extremely light load conditions.

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FIGURE 6. PFM to PWM Mode Transition

In addition to the auto mode transition, the LM3668 operates

in PFM Buck or PFM Boost based on the following conditions.

There is a small delta (~500mV) known as dv1(~200mV) &

dv2(~300mV) when VOUT_TARGET is very close to VIN where

the LM3668 can be in either Buck or Boost mode. For exam-

ple, when VOUT_TARGET = 3.3V and VIN is between 3.1V &

3.6V, the LM3668 can be in either mode depending on the

VIN vs VOUT_TARGET .

•Region I: If VIN < VOUT_TARGET - dv1, the regulator operates

in Boost mode.

•Region II: If VOUT_TARGET - dv1 < VIN < VOUT_TARGET+

dv2 ,the regulator operates in either Buck or Boost mode.

•Region III: If VIN > VOUT_TARGET + dv2, the regulator

operates in Buck mode.

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LM3668

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FIGURE 7. VOUT vs VIN Transition

In the buck PFM operation, P2 is always turned on and N2 is

always turned off , P1 and N1 power switches are switching.

P1 and N1 are turned off to enter " sleep mode" when the

output voltage reaches the "high" comparator threshold. In

boost PFM operation, P2 and N2 are switching. P1 is turned

on and N1 is turned off when the output voltage is below the

"high" threshold. Unlike in buck mode, all four power switches

are turned off to enter "sleep" mode when the output voltage

reaches the "high" threshold in boost mode. In addition, the

internal current sensing of the IPFM is used to determine the

precise condition to switch over to buck or boost mode via the

PFM generator.

CURRENT LIMIT PROTECTION

The LM3668 has current limit protection to prevent excessive

stress on itself and external components during overload con-

ditions. The internal current limit comparator will disable the

power device at a typical switch peak current limit of 1.85A

(typ.).

UNDERVOLTAGE PROTECTION

The LM3668 has an UVP comparator to turn the power device

off in case the input voltage or battery voltage is too low . The

typical UVP threshold is around 2V.

SHORT CIRCUIT PROTECTION

When the output of the LM3668 is shorted to GND, the current

limit is reduced to about half of the typical current limit value

until the short is removed.

SHUTDOWN

When the EN pin is pulled low, P1 and P2 are off; N1 and N2

are turned on to pull SW1 and SW2 to ground.

THERMAL SHUTDOWN

The LM3668 has an internal thermal shutdown function to

protect the die from excessive temperatures. The thermal

shutdown trip point is typically 150°C; normal operation re-

sumes when the temperature drops below 125°C.

STARTUP

The LM3668 has a soft-start circuit that smooth the output

voltage and ramp current during startup. During startup the

bandgap reference is slowly ramped up and switch current

limit is reduced to half the typical value. Soft start is activated

only if EN goes from logic low to logic high after VIN reaches

2.5V. The startup time thereby depends on the output capac-

itor and load current demanded at startup. It is not recom-

mended to start up the device at full load while in soft-start.

Application Information

SYNC/MODE PIN

If the SYNC/MODE pin is set high, the device is set to operate

at PWM mode only. If SYNC/MODE pin is set low, the device

is set to automatically transition from PFM to PWM or PWM

to PFM depending on the load current. Do not leave this pin

floating. The SYNC/MODE pin can also be driven by an ex-

ternal clock to set the desired switching frequency between

1.6MHz to 2.7MHz.

VSEL PIN

The LM3668 has built in logic for conveniently setting the out-

put voltage, for example if VSEL high, the output is set to 3.3V;

with VSEL low the output is set to 2.8V. It is not recommended

to use this function for dynamically switching between 2.8V

and 3.3V or switching at maximum load.

MAXIMUM CURRENT

The LM3668 is designed to operate up to 1A. For input volt-

ages at 2.5V, the maximum operating current is 600mA and

800mA for 2.7V input voltage. In any mode it is recommended

to avoid starting up the device at minimum input voltage and

maximum load. Special attention must be taken to avoid op-

erating near thermal shutdown when operating in boost mode

at maximum load (1A). A simple calculation can be used to

determine the power dissipation at the operating condition;

PD-MAX = (TJ-MAX-OP – TA-MAX)/θJA. The LM3668 has thermal

resistance θJA = 34°C/W ((Note 3) and (Note 6)), and maxi-

mum operating ambient of 85°C. As a result, the maximum

power dissipation using the above formula is around

1176mW. Refer to dissipation table below for PD-MAX value at

different ambient temperatures.

Dissipation Rating Table

θJA TA ≤ 25°C TA ≤ 60°C TA ≤ 85°C

34°C/W ( 4

layers board

per JEDEC

standard)

2941mW 1912mW 1176mW

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LM3668

INDUCTOR SELECTION

There are two main considerations when choosing an induc-

tor: the inductor should not saturate, and the inductor current

ripple should be small enough to achieve the desired output

voltage ripple. Different saturation current rating specifica-

tions are followed by different manufacturers so attention

must be given to details. Saturation current ratings are typi-

cally specified at 25°C. However, ratings at the maximum

ambient temperature of application should be requested from

the manufacturer. Shielded inductors radiate less noise and

should be preferred.

In the case of the LM3668, there are two modes (Buck &

Boost) of operation that must be consider when selecting an

inductor with appropriate saturation current. The saturation

current should be greater than the sum of the maximum load

current and the worst case average to peak inductor current.

The first equation shows the buck mode operation for worst

case conditions and the second equation for boost condition.

•IRIPPLE: Peak inductor current

•IOUTMAX: Maximum load current

•VIN: Maximum input voltage in application

•L : Min inductor value including worst case tolerances

(30% drop can be considered)

•f : Minimum switching frequency

•VOUT: Output voltage

•D: Duty Cycle for CCM Operation

•VOUT : Output Voltage

•VIN: Input Voltage

Example using above equations:

•VIN = 2.8V to 4V

•VOUT = 3.3V

•IOUT = 500mA

•L = 2.2µH

•F = 2MHz

•Buck: ISAT = 567mA

•Boost: ISAT = 638mA

As a result, the inductor should be selected according to the

highest of the two ISAT values.

A more conservative and recommended approach is to

choose an inductor that has a saturation current rating greater

than the maximum current limit of 2.05A.

A 2.2µH inductor with a saturation current rating of at least

2.05A is recommended for most applications. The inductor’s

resistance should be less than 100mΩ for good efficiency. For

low-cost applications, an unshielded bobbin inductor could be

considered. For noise critical applications, a toroidal or shield-

ed-bobbin inductor should be used. A good practice is to lay

out the board with overlapping footprints of both types for de-

sign flexibility. This allows substitution of a low-noise shielded

inductor, in the event that noise from low-cost bobbin model

is unacceptable.

TABLE 2. Suggest Inductors and Suppliers

Model Vendor Dimension

LxWxH

(mm)

D.C.R

(max)

ISAT

LPS4012-

222L

Coilcraft 4 x 4 x 1.2 100 mΩ2.1A

LPS4018-

222L

Coilcraft 4 x 4 x 1.8 70 mΩ2.5A

1098AS-2

R0M (2µF)

TOKO 3 x 2.8x 1.2 67 mΩ1.8A

( lower

current

application

INPUT CAPACITOR SELECTION

A ceramic input capacitor of at least 10µF, 6.3V is sufficient

for most applications. Place the input capacitor as close as

possible to the PVIN pin of the device. A larger value may be

used for improved input voltage filtering. Use X7R or X5R

types; do not use Y5V. DC bias characteristics of ceramic ca-

pacitors must be considered when selecting case sizes like

0805 or 0603. The input filter capacitor supplies current to

the PFET switch of the LM3668 in the first half of each cycle

and reduces voltage ripple imposed on the input power

source. A ceramic capacitor’s low ESR provides the best

noise filtering of the input voltage spikes due to this rapidly

changing current. For applications where input voltage is 4V

or higher, it is best to use a higher voltage rating capacitor to

eliminate the DC bias affect over capacitance.

OUTPUT CAPACITOR SELECTION

A ceramic output capacitor of 22µF, 6.3V (use 10V or higher

rating for 4.5/5V output option) is sufficient for most applica-

tions. Multilayer ceramic capacitors such as X5R or X7R with

low ESR is a good choice for this as well. These capacitors

provide an ideal balance between small size, cost, reliability

and performance. Do not use Y5V ceramic capacitors as they

have poor dielectric performance over temperature and poor

voltage characteristic for a given value. In other words, ensure

the minimum COUT value does not exceed −40% of the above-

suggested value over the entire range of operating tempera-

ture and bias conditions.

Extra attention is required if a smaller case size capacitor is

used in the application. Smaller case size capacitors typically

have less capacitance for a given bias voltage as compared

to a larger case size capacitor with the same bias voltage.

Please contact the capacitor manufacturer for detail informa-

tion regarding capacitance verses case size. Table 1 lists

several capacitor suppliers.

The output filter capacitor smooths out current flow from the

inductor to the load, helps maintain a steady output voltage

during transient load changes and reduces output voltage

ripple. These capacitors must be selected with sufficient ca-

pacitance and sufficiently low ESR to perform these functions.

15 www.national.com

LM3668

Note that the output voltage ripple is dependent on the induc-

tor current ripple and the equivalent series resistance of the

output capacitor (RESR).

The RESR is frequency dependent (as well as temperature

dependent); make sure the value used for calculations is at

the switching frequency of the part.

TABLE 3. Suggested Capacitors and Suppliers

Model Type Vendor Voltage Rating Case Size

Inch (mm)

10 µF for CIN (For 4.5/5V option, use 10V or higher rating capacitor)

GRM21BR60J106K Ceramic, X5R Murata 6.3V 0805 (2012)

JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)

C2012X5R0J106K Ceramic, X5R TDK 6.3V 0805 (2012)

LMK212 BJ106MG (+/-20%) Ceramic, X5R Taiyon-Yuden 10V 0806(2012)

LMK212 BJ106KG (+/-10%) Ceramic, X5R Taiyon-Yuden 10V 0805(2012)

22 µF for COUT (For 4.5/5V option, use 10V or higher rating capacitor)

JMK212BJ226MG Ceramic, X5R Taiyo-Yuden 6.3V 0805(2012)

LMK212BJ226MG Ceramic, X5R Taiyo-Yuden 10V 0805(2012)

LAYOUT CONSIDERATIONS

As for any high frequency switcher, it is important to place the

external components as close as possible to the IC to maxi-

mize device performance. Below are some layout recommen-

dations:

1) Place input filter and output filter capacitors close to the IC

to minimize copper trace resistance which will directly effect

the overall ripple voltage.

2) Route noise sensitive trace away from noisy power com-

ponents. Separate power GND (Noisy GND) and Signal GND

(quiet GND) and star GND them at a single point on the PCB

preferably close to the device GND pin.

3) Connect the ground pins and filter capacitors together via

a ground plane to prevent switching current circulating

through the ground plane. Additional layout consideration re-

garding the LLP package can be found in Application

AN1187.

www.national.com 16

LM3668

Physical Dimensions inches (millimeters) unless otherwise noted

12–Pin LLP

NS Package Number SDF12A

17 www.national.com

LM3668

Notes

LM3668 1A, High Efficiency Dual Mode Single Inductor Buck-Boost DC/DC Converter

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