Datasheet

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LTC3872

3872fc

For more information www.linear.com/LTC3872

No R

SENSE

Current Mode Boost

DC/DC Controller

High Efficiency 3.3V Input, 5V Output Boost Converter

IPRG

GND

NGATE

LTC3872

3872 TA01

RUN/SS

17.4k

11k

34.8k

1nF

10µF

100µF

×2

1.8nF

47pF

1µH

3.3V

OUT

Efficiency and Power Loss vs Load Current

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100 10

0.001

1 100 1000 10000

3872 TA01b

0.1

0.01

Typical applicaTion

FeaTures DescripTion

applicaTions

The LT C

3872 is a constant frequency current mode

boost DC/DC controller that drives an N-channel power

MOSFET and requires very few external components. The

No R

SENSE

architecture eliminates the need for a sense

resistor, improves efficiency and saves board space.

The LTC3872 provides excellent AC and DC load and line

regulation with ±1.5% output voltage accuracy. It incor

porates an undervoltage lockout feature that shuts down

the device when the input voltage falls below 2.3V.

High switching frequency of 550kHz allows the use of a

small inductor. The LTC3872 is available in an 8-lead low

profile (1mm) ThinSOT

package and 8-pin 2mm × 3mm

DFN package.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

No R

SENSE

and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks

are the property of their respective owners.

Telecom Power Supplies

42V Automotive Systems

24V Industrial Controls

IP Phone Power Supplies

No Current Sense Resistor Required

OUT

up to 60V

Constant Frequency 550kHz Operation

Internal Soft-Start and Optional External Soft-Start

Adjustable Current Limit

Pulse Skipping at Light Load

Range: 2.75V to 9.8V

±1.5% Voltage Reference Accuracy

Current Mode Operation for Excellent Line and Load

Transient Response

Low Profile (1mm) SOT-23 and 2mm × 3mm DFN

Packages

Summary of content (22 pages)

PAGE 1
LTC3872 No RSENSE Current Mode Boost DC/DC Controller Features n n n n n n n n n n Description No Current Sense Resistor Required VOUT up to 60V Constant Frequency 550kHz Operation Internal Soft-Start and Optional External Soft-Start Adjustable Current Limit Pulse Skipping at Light Load VIN Range: 2.75V to 9.8V ±1.
PAGE 2
LTC3872 Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN), RUN/SS........... –0.3V to 10V IPRG Voltage.................................. –0.3V to (VIN + 0.3V) VFB, ITH Voltages........................................ –0.3V to 2.4V SW Voltage................................................. –0.3V to 60V Operating Junction Temperature Range (Notes 2, 3)............................................. –40°C to 150°C Storage Temperature Range...................
PAGE 3
LTC3872 Electrical Characteristics The l denotes the specifications which apply over the specified operating temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 4.2V unless otherwise noted. PARAMETER CONDITIONS MIN Input Voltage Range l TYP 2.75 MAX UNITS 9.8 V 250 8 20 400 20 35 µA µA µA Input DC Supply Current Normal Operation Shutdown UVLO Typicals at VIN = 4.2V (Note 4) 2.75V ≤ VIN ≤ 9.
PAGE 4
LTC3872 Typical Performance Characteristics FB Voltage Line Regulation 1.2025 1.24 1.2020 1.23 1.2015 1.22 1.21 1.2005 1.2000 1.19 1.1995 80 100 1.1990 0 1 3 2 3872 G01 Shutdown IQ vs VIN 4 5 6 7 8 9 VIN = 2.5V VIN = 3.3V VIN = 5V 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 RUN VOLTAGE (V) 3872 G03 3872 G02 Shutdown IQ vs Temperature Frequency vs Duty Cycle 600 20 500 10 8 6 4 FREQUENCY (kHz) 15 SHUTDOWN IQ (µA) SHUTDOWN MODE IQ (µA) 1.
PAGE 5
LTC3872 Typical Performance Characteristics Gate Drive Rise and Fall Time vs CLOAD 1.0 1.00 90 0.98 RISE TIME 60 FALL TIME 50 40 30 20 RISING 0.96 0.94 0.92 0.90 0.88 RISING 0.9 RUN THRESHOLDS (V) 70 RUN THRESHOLDS (V) 80 FALLING 0.8 FALLING 0.7 0.6 0.86 10 2000 6000 4000 CLOAD (pF) 8000 10000 0.84 0 2 4 6 VIN (V) 8 10 0.
PAGE 6
LTC3872 Pin Functions (TS8/DD8) IPRG (Pin 1/Pin 4): Current Sense Limit Select Pin. ITH (Pin 2/Pin 3): It serves as the error amplifier compensation point. Nominal voltage range for this pin is 0.7V to 1.9V. VFB (Pin 3/Pin 2): Receives the feedback voltage from an external resistor divider across the output. GND (Pin 4/Pin 1, Exposed Pad Pin 9): Ground. The exposed pad must be soldered to PCB ground for electrical contact and rated thermal performance. VIN (Pin 6/Pin 7): Supply Pin.
PAGE 7
LTC3872 Operation Main Control Loop The LTC3872 is a No RSENSE constant frequency, current mode controller for DC/DC boost, SEPIC and flyback converter applications. The LTC3872 is distinguished from conventional current mode controllers because the current control loop can be closed by sensing the voltage drop across the power MOSFET switch or across a discrete sense resistor, as shown in Figures 1 and 2.
PAGE 8
LTC3872 Applications Information Output Voltage Programming The output voltage is set by a resistor divider according to the following formula:  R2  VO = 1.2V • 1+  R1  The external resistor divider is connected to the output as shown in the Typical Application on the front page, allowing remote voltage sensing. Application Circuits A basic LTC3872 application circuit is shown on the front page of this datasheet.
PAGE 9
LTC3872 Applications Information the inductor value can be determined using the following equation: L= VIN(MIN) ∆IL • f •DMAX where: ∆IL = χ • IO(MAX) 1–DMAX Remember that boost converters are not short-circuit protected. Under a shorted output condition, the inductor current is limited only by the input supply capability.
PAGE 10
LTC3872 Applications Information The relationship between the maximum load current, duty cycle and the RDS(ON) of the power MOSFET is: RDS(ON) ≤ VSENSE(MAX) • 1– DMAX 1+ χ •I O(MAX) • ρT  2 VSENSE(MAX) is the maximum voltage drop across the power MOSFET. VSENSE(MAX) is typically 270mV, 170mV and 100mV. It is reduced with increasing duty cycle as shown in Figure 3. The rT term accounts for the temperature coefficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C.
PAGE 11
LTC3872 Applications Information Calculating Power MOSFET Switching and Conduction Losses and Junction Temperatures In order to calculate the junction temperature of the power MOSFET, the power dissipated by the device must be known. This power dissipation is a function of the duty cycle, the load current and the junction temperature itself (due to the positive temperature coefficient of its RDS(ON)). As a result, some iterative calculation is normally required to determine a reasonably accurate value.
PAGE 12
LTC3872 Applications Information For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the following equation: ESRCOUT ≤ 0.01• VO IIN(PEAK) Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance throughhole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest product of ESR and size of any aluminum electrolytic, at a somewhat higher price.
PAGE 13
LTC3872 Applications Information both available in surface mount packages. In the case of tantalum, it is critical that the capacitors have been surge tested for use in switching power supplies. An excellent choice is AVX TPS series of surface mount tantalum. Also, ceramic capacitors are now available with extremely low ESR, ESL and high ripple current ratings.
PAGE 14
LTC3872 Applications Information output current. Assuming an efficiency of 90%, this sense resistor power dissipation represents 1.3% of the overall input power. In other words, for this application, the use of VDS sensing would increase the efficiency by approximately 1.3%. For more details regarding the various terms in these equations, please refer to the section Boost Converter: Power MOSFET Selection. 3. The losses in the inductor are simply the DC input current squared times the winding resistance.
PAGE 15
LTC3872 Applications Information And so the inductor value is: L= VIN(MIN) ∆IL • f •DMAX = 3.3V • 0.39 = 1.8µH 1.3A • 550kHz The component chosen is a 2.2µH inductor made by Sumida (part number CEP125-H 1ROMH). 3. Assuming a MOSFET junction temperature of 125°C, the room temperature MOSFET RDS(ON) should be less than: RDS(ON) ≤ VSENSE(MAX) • = 0.175V • 1–DMAX 6.
PAGE 16
LTC3872 typical applications High Efficiency 3.3V Input, 12V Output Boost Converter 4.7M 0.1µF 2.2nF 23.2k ITH 100pF RUN/SS VIN L1 2.2µH IPRG LTC3872 GND VFB 11.8k 1% VIN 3.3V CIN 10µF SW NGATE M1 107k 1% PDS1040 + COUT1 22µF ×2 COUT2 120µF VOUT 12V 1.5A 3872 F09 COUT1: TAIYO YUDEN TMK325B7226MM L1: COILTRONICS DR125-2R2 M1: VISHAY Si4408DY VOUT 12V AC-COUPLED IL 5A/DIV ILOAD 1A/DIV STEP FROM 500mA TO 1.5A 100µs/DIV 3872 F10 3872fc 16 For more information www.linear.
PAGE 17
LTC3872 typical applications High Efficiency 5V Input, 12V Output Boost Converter 4.7M ILOAD 500mA/DIV STEP FROM 100mA TO 600mA 1nF 2.2nF 11k ITH 100pF RUN/SS VIN IPRG L1 3.3µH LTC3872 GND VIN 5V ILOAD 5A/DIV SW VFB 11.8k 1% CIN 10µF M1 NGATE SBM835L COUT1 22µF ×2 107k 1% COUT1: TAIYO YUDEN TMK325B7226MM L1: TOKO D124C 892NAS-3R3M M1: IRF3717 + COUT2 68µF VOUT 12V 2A VOUT 3872 TA03b 500µs/DIV 3872 TA03a High Efficiency 5V Input, 24V Output Boost Converter 4.7M 0.068µF 1nF 52.
PAGE 18
LTC3872 typical applications High Efficiency 5V Input, 48V Output Boost Converter 1M 0.33µF 2.2nF 63.4k 1% ITH VIN RUN/SS VIN IPRG L1 10µH LTC3872 GND VIN 5V SW VFB 12.1k 1% CIN 10µF NGATE M1 D1 475k 1% COUT1 2.2µF ×3 COUT2 68µF VOUT 48V 0.5A + 3872 TA05a COUT1: NIPPON CHEMI-CON KTS101B225M43N D1: DIODES INC.
PAGE 19
LTC3872 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702 Rev B) 0.61 ±0.05 (2 SIDES) 3.00 ±0.10 (2 SIDES) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.20 ±0.05 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF R = 0.115 TYP 5 R = 0.05 TYP 0.40 ± 0.10 8 2.00 ±0.10 (2 SIDES) 0.56 ± 0.05 (2 SIDES) 0.75 ±0.05 0 – 0.
PAGE 20
LTC3872 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. TS8 Package 8-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1637 Rev A) 0.40 MAX 2.90 BSC (NOTE 4) 0.65 REF 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.22 – 0.36 8 PLCS (NOTE 3) 0.65 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) 1.
PAGE 21
LTC3872 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 3/11 Added I-Grade and H-Grade parts. Changes reflected throughout the data sheet. C 11/13 1 - 22 FOSC normal operation: changed VFB from 1.2V to 1.0V 3 Changed Input Supply Current from 10µA to 8µA 7 Updated MFG part number on Application schematics 16, 17, 18, 22 3872fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
PAGE 22
LTC3872 Typical Application 3.3V Input, 5V/2A Output Boost Converter 47pF 1M 1nF 1.8nF 17.4k ITH VIN RUN/SS VIN IPRG L1 1µH LTC3872 GND VFB 11k 1% CIN 10µF VIN 3.3V SW NGATE M1 34.8k 1% D1 VOUT 5V 2A COUT 100µF ×2 3872 TA02 D1: DIODES INC.