Datasheet

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LTC3421

3421f

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Handheld Computers

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Cordless Phones

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GPS Receivers

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Battery Backup Supplies

, LTC and LT are registered trademarks of Linear Technology Corporation.

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Synchronous Rectification: Up to 96% Efficiency

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True Output Disconnect

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Inrush Current Limiting

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Very Low Quiescent Current: 12µA

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Up to 1.5A Continuous Output Current

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Fixed Frequency Operation Up to 3MHz

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0.5V to 4.5V Input Range

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2.4V to 5.25V Adjustable Output Voltage

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Guaranteed 1V Start-Up

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Programmable Current Limit

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Programmable Soft-Start

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Synchronizable Oscillator

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Manual or Automatic Burst Mode

Operation

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Low-Battery Comparator

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<1µA Shutdown Current

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1.22V Reference Output Voltage

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Small (4mm × 4mm) Thermally Enhanced QFN

Package

3A, 3MHz Micropower

Synchronous Boost Converter

with Output Disconnect

Burst Mode is a registered trademark of Linear Technology Corporation.

The LTC

3421 is a high efficiency, current mode, fixed

frequency, step-up DC/DC converter with true output dis-

connect and inrush current limiting. The device includes a

0.10Ω N-channel MOSFET switch and a 0.14Ω P-channel

synchronous rectifier. This product has the ability to sim-

ply program the output voltage, switching frequency, cur-

rent limit, soft-start, Burst Mode threshold and loop

compensation with external passive components.

Quiescent current is only 12µA during Burst Mode opera-

tion, maximizing battery life in portable applications. The

oscillator frequency can be programmed up to 3MHz and

can be synchronized to an external clock applied to the

SYNC pin. An open-drain uncommitted low-battery com-

parator is included. The part maintains operation in

applications with a secondary cell powering the output

voltage during shutdown.

Other features include: 1µA shutdown, antiringing control,

thermal limit and reference output.

The LTC3421 is available in a small 4mm × 4mm QFN

package.

SHDN

ENB

REF

LBI

LBO

SYNC

LIM

OUTS

OUT

BURST

221141516

1312111056

SW SW

4.7µH

SS R

GND

LTC3421

0.1µF

470pF

C5*

22µF

C1*

4.7µF

1.8V TO 3V

CELLS

28k

20k

100k

340k

OUT

3.3V

1.2A

200k

3421 TA01

45.3k

*LOCATE COMPONENTS CLOSE TO PINS

C1: TAIYO YUDEN JMK212BJ106MM

PGND PGNDPGND

C5: TAIYO YUDEN JMK325BJ226MM

L1: TDK RLF7030T-4R7M3R4

2-Cell to 3.3V Efficiency

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100 1000

3421 G02

Burst Mode OPERATION

= 3V

OUT

= 3.3V

OSC

= 1MHz

= 2.4V

= 2V

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (16 pages)

PAGE 1
LTC3421 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®3421 is a high efficiency, current mode, fixed frequency, step-up DC/DC converter with true output disconnect and inrush current limiting. The device includes a 0.10Ω N-channel MOSFET switch and a 0.14Ω P-channel synchronous rectifier.
PAGE 2
LTC3421 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) VIN, VOUT, VOUTS Voltage ............................ – 0.3V to 6V BURST, SHDN, SS, ENB, SW, LBO, LBI, SYNC Voltages .......................... – 0.3V to 6V Operating Temperature Range (Notes 2, 5) ............................................. –40°C to 85°C Storage Temperature Range ................. – 65°C to 125°C Lead Temperature (Soldering, 10 sec)..................
PAGE 3
LTC3421 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V, RT = 28k, unless otherwise noted. PARAMETER MIN TYP MAX UNITS Frequency Accuracy ● 0.85 1 1.15 MHz SYNC Input High ● 2.2 SYNC Input Low ● SYNC Input Current ● ENB Input High ● ENB Input Low ● 0.4 V ENB Input Current ● 1 µA SHDN Input High CONDITIONS VOUT = 0V (Initial Start-Up) VOUT > 2.
PAGE 4
LTC3421 U W TYPICAL PERFOR A CE CHARACTERISTICS (TA = 25°C, unless otherwise specified) Single Cell to 3.3V Efficiency VIN = 1.5V Burst Mode OPERATION 90 80 VIN = 3V 80 EFFICIENCY (%) 60 50 40 90 VIN = 2.4V 70 60 50 40 50 40 30 30 20 20 20 VOUT = 3.3V fOSC = 1MHz 0 0.1 VOUT = 3.3V fOSC = 1MHz 10 1 10 100 OUTPUT CURRENT (mA) 0 0.1 1000 VOUT = 5V fOSC = 1MHz 10 1 10 100 OUTPUT CURRENT (mA) 0 0.
PAGE 5
LTC3421 U W TYPICAL PERFOR A CE CHARACTERISTICS (TA = 25°C, unless otherwise specified) Burst Mode Threshold vs RBURST FB Voltage 180 Frequency Accuracy 1.24 1.03 1.23 1.01 100 80 FREQUENCY (MHz) 120 VOLTAGE (V) OUTPUT CURRENT (mA) 160 140 1.22 60 OUT OF BURST 1.21 40 0.99 0.97 20 INTO BURST 1.20 –45 –30 –15 0 70 RBURST (kΩ) 20 120 0 15 30 45 60 TEMPERATURE (°C) Burst Mode Quiescent Current 1.65 RLIM = 105k 0.18 1.60 0.16 1.55 0.14 1.50 1.45 1.
PAGE 6
LTC3421 U U U PI FU CTIO S RT (Pin 5): Connect a resistor to ground to program the oscillator frequency according to the formula: fOSC = 28,100 RT where fOSC is in kHz and RT is in kΩ. SS (Pin 6): Soft-Start Pin. Connect a capacitor from this pin to ground to set the soft-start time according to the formula: t(ms) = CSS(µF) • 320 The nominal soft-start charging current is 2.5µA. The active range of SS is from 0.8V to 1.6V. SYNC (Pin 7): Oscillator Synchronization Pin.
PAGE 7
LTC3421 W BLOCK DIAGRA + 1V TO 4.5V 21 14 SW VIN 15 SW 16 18 VIN SW ANTIRING VIN WELL SWITCH PMOS VDD VOUTS VOUT VOUT VOUT ANTICROSS CONDUCTION NMOS VOUT 2.40V TO 5.25V 17 19 20 + – IZERO AMP ISENSE AMP R1 + CURRENT LIMIT ILIM ILIMIT = 150k/RC1 – 2 VOUT 4 SHDN + 6 CURRENT COMP 1.22V REF 2% FB – 1 + + PWM LOGIC R2 Σ + VC 24 CP I/3000 SLEEP THERMAL REG/SHDN RT Burst Mode CONTROL OFF RZ 1% + UV BURST COMP – 5 RC1 CSS 1.
PAGE 8
LTC3421 U OPERATIO LOW VOLTAGE START-UP Oscillator The LTC3421 includes an independent start-up oscillator designed to start-up at input voltages of 0.85V typical. The frequency and peak current limit during start-up are internally controlled. The device can start-up under some load (see graph of Start-Up Current vs Input Voltage). Softstart and inrush current limiting are provided during startup as well as normal mode. The same soft-start capacitor is used for each operating mode.
PAGE 9
LTC3421 U OPERATIO Zero Current Amplifier The zero current amplifier monitors the inductor current to the output and shuts off the synchronous rectifier once the current is below 50mA typical, preventing negative inductor current. Antiringing Control The antiringing control places a resistor across the inductor to damp the ringing on the SW pin in discontinuous conduction mode. The LCSW ringing (L = inductor, CSW = capacitance on SW pin) is low energy, but can cause EMI radiation. VREF The internal 1.
PAGE 10
LTC3421 U OPERATIO Simplified Diagram of Automatic Burst Mode Control Circuit VCC 1mA – VREF –4% IOUT/3000 UV + SSDONE SSDONE – FB 1 MODE 1 = Burst Mode OPERATION 0 = PWM MODE + 0.9V/ 1.1V 9 BURST RB CB – VREF ±1% ERROR AMP/ SLEEP COMP TO MODULATOR + SLEEP 3421 TA03 CLAMP 0.5V TO 1V 24 VC RCOMP CCOMP The circuit connected to BURST should be able to sink or source up to 2mA. Note that Burst Mode operaton is inhibited during start-up and soft-start. Note that if VIN is above VOUT – 0.
PAGE 11
LTC3421 U OPERATIO disconnect, there must not be any external Schottky diodes connected between the SW pins and VOUT. Note: Board layout is extremely critical to minimize voltage overshoot on the SW pins due to stray inductance. Keep the output filter capacitors as close as possible to the VOUT pins and use very low ESR/ESL ceramic capacitors, tied to a good ground plane. In VOUT > 4.
PAGE 12
LTC3421 U W U U APPLICATIO S I FOR ATIO Output Capacitor Selection Operating Frequency Selection The output voltage ripple has two components to it. The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The maximum ripple due to charge is given by: There are several considerations in selecting the operating frequency of the converter.
PAGE 13
LTC3421 U W U U APPLICATIO S I FOR ATIO volts • seconds of the inductor will reverse during the time current is flowing to the output. Since this mode will dissipate more power in the IC, the maximum output current is limited in order to maintain an acceptable junction temperature. IOUT(MAX) = 125 – TA 40 • ( VIN + 1.5) – VOUT ( ) where TA = ambient temperature. For example at VIN = 4.5V and VOUT = 3.3V, the maximum output current is 370mA.
PAGE 14
LTC3421 U W U U APPLICATIO S I FOR ATIO VOUT + 1.22V ERROR AMP R1 FB – 1 R2 VC 24 CC1 CC2 RZ 3421 F03 Figure 3 U TYPICAL APPLICATIO 5V Applications output will provide a peak efficiency improvement but will negate the output disconnect feature. If output disconnect is required, the Schottky to an active snubber network is suggested as shown in Figure 4. When the output voltage is programmed above 4.
PAGE 15
LTC3421 U PACKAGE DESCRIPTIO UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 ±0.05 4.50 ± 0.05 2.45 ± 0.05 3.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.23 TYP R = 0.115 (4 SIDES) TYP 23 24 0.75 ± 0.05 0.38 ± 0.10 PIN 1 TOP MARK (NOTE 5) 1 2 2.45 ± 0.10 (4-SIDES) (UF24) QFN 0603 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC NOTE: 1.
PAGE 16
LTC3421 U TYPICAL APPLICATIO Single Cell to 3.3V at 500mA with Secondary Cell Backup During Shutdown. LOWBAT and VREF Output are Enabled L1 4.7µH D1 VIN 1V TO 1.5V C1* 4.7µF 2 4 0.1µF 301k 1 CELL PRIMARY CELL 3 23 + 21 SHDN ENB R1 60k 15 + 16 SW SW SW VIN VOUTS VREF VOUT LBI VOUT LOW BAT 22 LBO OUTPUT 7 SYNC 8 ILIM 604k 14 VOUT LTC3421 FB VC BURST SS RT 6 C2 0.1µF *LOCATE COMPONENTS CLOSE TO PINS C1: TAIYO YUDEN JMK212BJ106MM 10 11 12 VOUT 3.