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LTC3125

3125fa

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

1.2A Synchronous Step-Up

DC/DC Converter with

Input Current Limit

The LTC

3125 is a high efﬁ ciency, synchronous step-up

DC/DC converter with an accurate programmable average

input current limit. The resistor programmable average

input current limit is 5% accurate at 500mA and is suitable

for a wide variety of applications. In mobile computing,

GSM and GPRS cards demand high current pulses well

beyond the capability of the PC Card and CompactFlash

slots. The LTC3125 in concert with a reservoir capacitor,

keeps the slot power safely within its capabilities providing

a high performance and simple solution.

Synchronous rectiﬁ cation produces high efﬁ ciency while

the 1.6MHz switching frequency minimizes the solution

footprint. The current mode PWM design is internally com-

pensated. Output disconnect allows the load to discharge in

shutdown, while also providing inrush current limiting.

Other features include a <1µA shutdown current, short-

circuit and thermal overload protection. The LTC3125 is

offered in a low proﬁ le 0.75mm × 2mm × 3mm package.

PCMCIA/CompactFlash (3.3V/500mA Max),

4V GSM Pulsed Load

Programmable Average Input Current Limit

5% Input Current Accuracy

200mA to 1000mA Program Range

: 1.8V to 5.5V, V

OUT

: 2V to 5.25V

Supports High Current GSM/GPRS Load Burst

> V

OUT

Operation

1.6MHz Fixed Frequency Operation

Internal Current Sense Resistor

1.2A Peak Current Limit

Up to 93% Efﬁ ciency

Output Disconnect in Shutdown

Soft-Start

Low Quiescent Current Burst Mode

Operation

Available in 2mm × 3mm × 0.75mm DFN Package

GSM/GPRS PCMCIA/CompactFlash PC Card Modems

Wireless Emergency Locators

Portable Radios

Supercap Chargers

L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

2.2µH

LTC3125

GND

1.24M

2200µF

TANT

10µF

CER

OUT

2A PULSED LOAD

3.3V

500mA

536k

3125 TA01a

SHDN

PROG

OUT

44.2k

OFF ON

Efﬁ ciency vs Load Current

LOAD CURRENT (A)

0.001

EFFICIENCY (%)

POWER LOSS (W)

0.01 0.1 1

3125 TA01b

100

0.1

0.01

0.001

OUT

= 4V

= 3.3V

= 2.4V

Summary of content (18 pages)

PAGE 1
LTC3125 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit FEATURES DESCRIPTION n The LTC®3125 is a high efﬁciency, synchronous step-up DC/DC converter with an accurate programmable average input current limit. The resistor programmable average input current limit is 5% accurate at 500mA and is suitable for a wide variety of applications. In mobile computing, GSM and GPRS cards demand high current pulses well beyond the capability of the PC Card and CompactFlash slots.
PAGE 2
LTC3125 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN, VOUT Voltage ......................................... –0.3V to 6V SW Voltage .................................................. –0.3V to 6V SW Voltage < 100ns .................................... –0.3V to 7V All Other Pins ............................................... –0.3V to 6V Operating Junction Temperature Range (Notes 2, 5) ............................................ –40°C to 125°C Junction Temperature .............................
PAGE 3
LTC3125 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted (Note 2). PARAMETER CONDITIONS MIN PROG Current Gain (Note 3) Maximum Duty Cycle VFB = 1.15V l Minimum Duty Cycle VFB = 1.3V l TYP MAX 22.1 l Frequency SHDN Input High 85 kΩ-A/A 92 % 0 1.3 1.6 1.9 1 VSHDN = 1.2V 0.
PAGE 4
LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Efﬁciency vs Load Current, VOUT = 5V No-Load Input Current vs VIN 1 4.0 90 3.5 80 3.0 60 50 40 0.01 30 2.5 IIN (mA) EFFICIENCY (%) 0.1 POWER LOSS (W) 70 VOUT = 4V 2.0 VOUT = 3.8V 1.5 VOUT = 3.3V 1.0 20 0.5 VIN = 4V VIN = 3.3V 10 0 0.001 0.01 0.1 LOAD CURRENT (A) 0.001 0 1.5 1 VOUT = 2.5V 2 2.5 3.5 3 0 –0.5 –1.0 –1.5 –2.0 1.5 2.0 2.5 0 –0.50 –1.00 0 25 50 TEMPERATURE (°C) 75 2.50 0.75 0.50 0.
PAGE 5
LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) 0.450 240 0.400 220 PMOS 0.250 0.200 NMOS 0.150 180 160 140 NMOS 0.100 120 0.050 1.5 100 –50 –30 2 2.5 3.5 3 VOUT (V) 4 4.5 5 0.50 –4 –8 30 50 –10 10 TEMPERATURE (°C) 70 –10 –50 90 0 –0.25 –0.50 –0.75 70 90 0.25 15.5 0 15.0 –0.25 14.5 –0.50 14.0 –0.75 13.5 –1.00 –50 –30 100 75 16.0 30 50 –10 10 TEMPERATURE (°C) 70 90 13.0 1.5 2 2.5 3.5 3 VOUT (V) 4 4.
PAGE 6
LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) VOUT and IIN During Soft-Start Efﬁciency vs VIN 100 ILOAD = 200mA 95 VOUT = 3.8V VOUT 2V/DIV EFFICIENCY (%) 90 SHDN 5V/DIV INPUT CURRENT 200mA/DIV VIN = 3.3V VOUT = 4.5V COUT = 1F L = 2.7μH 2s/DIV 3125 G20 85 80 75 70 65 60 55 50 2 4 3 5 VIN (V) 3125 G21 PIN FUNCTIONS GND (Pin 1, Exposed Pad Pin 9): Ground.
PAGE 7
LTC3125 BLOCK DIAGRAM L1 VIN CIN 4 5 VIN 8 SW CS RSENSE VSEL VBEST + – gm 3 VB PROG WELL-SWITCH ANTI-RING RPROG INPUT CURRENT SENSE AMP VOUT VOUT 7 VSEL – + 6 SHDN SHUTDOWN SD 4M VREF VBG IZERO COMP GATE DRIVE AND ANTI-CROSS CONDUCTION COUT IPK COMP IPK VREF GOOD ICLMP LOGIC OSC THERMAL SHUTDOWN R2 ICLMP COMP IZERO SLOPE COMP + – + – VREF FB 2 + – R1 CLK CLK TSD MODE CONTROL WAKE AVERAGING CIRCUIT VREF IAVG ERROR AMP EXPOSED PAD GND 9 1 SOFT START + gm – V
PAGE 8
LTC3125 OPERATION The LTC3125 provides high efﬁciency, low noise power for applications in portable instrumentation and those with pulsed-load, power-limited requirements such as GSM modems. The LTC3125 directly and accurately controls the average input current. The high efﬁciency of the LTC3125 provides the maximum possible output current to the load without impacting the host.
PAGE 9
LTC3125 OPERATION prevents the inductor current from reversing in polarity, improving efﬁciency at light loads. OSCILLATOR An internal oscillator sets the switching frequency to 1.6MHz. SHUTDOWN Shutdown of the boost converter is accomplished by pulling SHDN below 0.35V and enabled by pulling SHDN above 1V. Note that SHDN can be driven above VIN or VOUT, as long as it is limited to less than the absolute maximum rating.
PAGE 10
LTC3125 APPLICATIONS INFORMATION Burst Mode operation is inhibited during start-up and soft-start and until VOUT is at least 0.38V greater than VIN. GSM and GPRS modems have become a popular wireless data transfer solution for use in notebook PCs and other mobile systems. GSM transmission requires large bursts of current that exceed the maximum peak current speciﬁcations for CompactFlash and PCMCIA bus power. The GSM standard speciﬁes a 577μs, 2A (typical) transmission burst within a 4.6ms period (12.
PAGE 11
LTC3125 APPLICATIONS INFORMATION power losses compared to cheaper powdered iron types, improving efﬁciency. The inductor should have low DCR (DC resistance of the windings) to reduce the I2R power losses, and must be able to support the peak inductor current without saturating. Molded chokes and some chip inductors usually do not have enough core area to support the peak inductor currents of 1.8A seen on the LTC3125. To minimize radiated noise, use a shielded inductor.
PAGE 12
LTC3125 APPLICATIONS INFORMATION AVERAGE INPUT ILIMIT PROGRAMMING RESISTOR SELECTION The input current limit is user programmable by selection of an external resistor, RPROG. It is important to locate the resistor as close to the pin as possible to minimize capacitance and noise pick-up. Resistor tolerance directly affects the current limit accuracy so it must be factored in to the application requirements. Table 3 shows standard resistors for typical current limit values.
PAGE 13
LTC3125 TYPICAL APPLICATIONS PC Card (3.3V/1000mA Maximum) 4.5V Output, GPRS, Class 10 Pulsed Load Waveforms of Input Current, VOUT for Pulsed Load Current 2.7μH* VIN PC CARD VCC 3.3V ±10% 1000mA MAX SW VOUT CS VIN 10μF CER OFF ON VOUT 4.5V, 2A PULSED LOAD (1154μs PW, 4.6ms PERIOD) LTC3125 SHDN PROG FB GND 22.6k 2200μF** s3 55mΩ TANT 2.74M 1M VOUT 100mV/DIV INPUT CURRENT 500mA/DIV 3125 TA04a LOAD CURRENT 2A/DIV *WURTH 7440420027 **VISHAY 592D228X6R3X220H VIN = 3.3V VOUT = 4.5V COUT = 6.
PAGE 14
LTC3125 TYPICAL APPLICATIONS Stacked Supercap Charger 2.2μH* VIN 2.5V TO 5V 500mA MAX SW VOUT CS VIN 10μF CER OFF ON LTC3125 SHDN 2.74M PROG 44.2k VOUT 4.5V + 100k FB GND + 100k 1M 30F** 2.3V 30F** 2.3V 3125 TA06a *TDK VLF4014ST-2R2M1R9 **PANASONIC EECHWOD306 Waveforms of Input Current, VOUT During Charging VOUT 2V/DIV SHDN 5V/DIV LOAD CURRENT 200mA/DIV VIN = 4.5V VOUT = 4.5V COUT_SERIES = 15F L = 2.2μH RPROG = 44.
PAGE 15
LTC3125 TYPICAL APPLICATIONS 3.3V to 5V with Selectable Input Current Limit 2.2μH* SW VOUT CS VIN VIN 3.3V ±10% 10μF CER OFF ON LTC3125 300mA 500mA M1 COUT SHDN 3.2M PROG 44.2k VOUT 5V FB GND 1M 28.7k 3125 TA07a *TDK VLF4014ST-2R2M1R9 Waveforms of Input Current, VOUT for Pulsed Input Current Limit INPUT CURRENT 200mA/DIV M1 GATE DRIVE 5V/DIV VIN = 3.3V VOUT = 5V COUT = 4.4mF L = 2.
PAGE 16
LTC3125 PACKAGE DESCRIPTION DCB Package 8-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1718 Rev A) 0.70 ±0.05 1.35 ±0.05 3.50 ±0.05 1.65 ± 0.05 2.10 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 1.35 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.115 TYP R = 0.05 5 TYP 2.00 ±0.10 (2 SIDES) 0.40 ± 0.10 8 1.35 ±0.10 1.65 ± 0.10 3.00 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 17
LTC3125 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/10 Text change to Description 1 Change to Electrical Characteristics Quiescent Current-Burst 2 Modiﬁcation of Note 2 3 Pin Functions; change to GND (Pin 1), PROG (Pin 3) and VOUT (Pin 7) 6 Replaced Average Input Current Limit section 8 Added Average Input Limit Programming Resistor Selection section 12 Updated Related Parts table 18 3125fa Information furnished by Linear Technology Corporation is believed to be accurate and
PAGE 18
LTC3125 TYPICAL APPLICATION PC Card or CompactFlash (3.3V/500mA Maximum) 4.5V Output, GSM Pulsed Load Waveforms of Input Current, VOUT for Pulsed Load Current VOUT 100mV/DIV 2.2μH* VIN PC CARD VCC 3.3V ±10% 500mA MAX SW VOUT CS VIN 10μF CER OFF ON 44.2k VOUT 4.5V, 2A PULSED LOAD (577μs PW, 4.6ms PERIOD) LTC3125 2200μF** ×2 55mΩ TANT 2.74M SHDN PROG FB GND 1M INPUT CURRENT 200mA/DIV LOAD CURRENT 2A/DIV 3125 TA02a VIN = 3.3V VOUT = 4.5V COUT = 4.4mF L = 2.2μH RPROG = 44.