Datasheet

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LTC1872

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATION

Constant Frequency

Current Mode Step-Up

DC/DC Controller in SOT-23

The LTC

1872 is a constant frequency current mode step-

up DC/DC controller providing excellent AC and DC load

and line regulation. The device incorporates an accurate

undervoltage lockout feature that shuts down the LTC1872

when the input voltage falls below 2.0V.

The LTC1872 boasts a ±2.5% output voltage accuracy and

consumes only 270µA of quiescent current. For applica-

tions where efficiency is a prime consideration, the LTC1872

is configured for Burst Mode operation, which enhances

efficiency at low output current.

In shutdown, the device draws a mere 8µA. The high

550kHz constant operating frequency allows the use of a

small external inductor.

The LTC1872 is available in a small footprint 6-lead

SOT-23.

■

Lithium-Ion-Powered Applications

■

Cellular Telephones

■

Wireless Modems

■

Portable Computers

■

Scanners

■

High Efficiency: Over 90%

■

High Output Currents Easily Achieved

■

Wide V

Range: 2.5V to 9.8V

■

OUT

Limited Only by External Components

■

Constant Frequency 550kHz Operation

■

Burst Mode

Operation at Light Load

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Low Quiescent Current: 270µA

■

Shutdown Mode Draws Only 8µA Supply Current

■

±2.5% Reference Accuracy

■

Tiny 6-Lead SOT-23 Package

Efficiency vs Load Current

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

Burst Mode is a trademark of Linear Technology Corporation.

/RUN

LTC1872

147k

422k

80.6k

0.03Ω

4.7µH

220pF

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT

C2: MURATA GRM42-2X5R226K6.3

D1: IR10BQ015

L1: MURATA LQN6C4R7M04

M1: IRLMS2002

R1: DALE 0.25W

GND

1872 TA01

NGATE

SENSE

–

10µF

10V

3.3V

OUT

2× 22µF

6.3V

Figure 1. LTC1872 High Output Current 3.3V to 5V Boost Converter

LOAD CURRENT (mA)

EFFICIENCY (%)

100

10 100 1000

1872 TA01b

= 3.3V

OUT

= 5V

Summary of content (12 pages)

PAGE 1
LTC1872 Constant Frequency Current Mode Step-Up DC/DC Controller in SOT-23 U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: Over 90% High Output Currents Easily Achieved Wide VIN Range: 2.5V to 9.8V VOUT Limited Only by External Components Constant Frequency 550kHz Operation Burst ModeTM Operation at Light Load Current Mode Operation for Excellent Line and Load Transient Response Low Quiescent Current: 270µA Shutdown Mode Draws Only 8µA Supply Current ±2.
PAGE 2
LTC1872 W U U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) Input Supply Voltage (VIN).........................– 0.3V to 10V SENSE –, NGATE Voltages ............ – 0.3V to (VIN + 0.3V) VFB, ITH/RUN Voltages ..............................– 0.3V to 2.4V NGATE Peak Output Current (< 10µs) ....................... 1A Storage Ambient Temperature Range ... – 65°C to 150°C Operating Temperature Range (Note 2) .. – 40°C to 85°C Junction Temperature (Note 3) ...........................
PAGE 3
LTC1872 U W TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature 10 VIN = 4.2V VFB VOLTAGE (mV) 815 810 805 800 795 790 785 2.24 VIN = 4.2V 8 2.20 6 4 2 0 –2 –4 –6 780 5 25 45 65 85 105 125 TEMPERATURE (°C) 5 25 45 65 85 105 125 TEMPERATURE (°C) 2.04 2.00 1.96 1.84 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1872 G02 Maximum Current Sense Trip Voltage vs Duty Cycle 1872 G03 Shutdown Threshold vs Temperature 600 VIN = 4.2V TA = 25°C 120 2.08 1.
PAGE 4
LTC1872 W FUNCTIONAL DIAGRA U VIN SENSE – 5 4 U + ICMP – VIN RS SLOPE COMP OSC NGATE SWITCHING LOGIC AND BLANKING CIRCUIT R Q S 6 – FREQ FOLDBACK BURST CMP + 0.3V SLEEP – + 0.15V OVP + – VREF + 60mV + VREF 0.8V VIN EAMP 0.5µA VFB + – 1 ITH/RUN 3 VIN VIN 0.3V – 0.35V VOLTAGE REFERENCE + SHDN CMP VREF 0.8V – GND SHDN UV 2 UNDERVOLTAGE LOCKOUT 1.
PAGE 5
LTC1872 U OPERATIO (Refer to Functional Diagram) Comparator OVP guards against transient overshoots > 7.5% by turning off the external N-channel power MOSFET and keeping it off until the fault is removed. Burst Mode Operation The LTC1872 enters Burst Mode operation at low load currents. In this mode, the peak current of the inductor is set as if VITH/RUN = 1V (at low duty cycles) even though the voltage at the ITH/RUN pin is at a lower value.
PAGE 6
LTC1872 U W U U APPLICATIONS INFORMATION The basic LTC1872 application circuit is shown in Figure␣ 1. External component selection is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power MOSFET and the output diode D1 is selected followed by CIN(= C1) and COUT(= C2). RSENSE Selection for Output Current RSENSE is chosen based on the required output current.
PAGE 7
LTC1872 U W U U APPLICATIONS INFORMATION Inductor Selection When selecting the inductor, keep in mind that inductor saturation current has to be greater than the current limit set by the current sense resistor. Also, keep in mind that the DC resistance of the inductor will affect the efficiency. Off the shelf inductors are available from Murata, Coilcraft, Toko, Panasonic, Coiltronics and many other suppliers.
PAGE 8
LTC1872 U W U U APPLICATIONS INFORMATION where f is the operating frequency, COUT is the output capacitance and IRIPPLE is the ripple current in the inductor. 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 ESR (size) product of any aluminum electrolytic at a somewhat higher price.
PAGE 9
LTC1872 U W U U APPLICATIONS INFORMATION For most applications, an 80k resistor is suggested for R1. To prevent stray pickup, locate resistors R1 and R2 close to LTC1872. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement.
PAGE 10
LTC1872 U W U U APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1872. These items are illustrated graphically in the layout diagram in Figure 5. Check the following in your layout: 1. The Schottky diode should be closely connected between the output capacitor and the drain of the external MOSFET. 2. The (+) plate of CIN should connect to the sense resistor as closely as possible.
PAGE 11
LTC1872 U TYPICAL APPLICATIO LTC1872 12V/500mA Boost Converter C1 10µF 10V R1 0.033Ω 1 ITH/RUN VIN 5 L1 10µH LTC1872 10k 2 3 220pF GND SENSE – VFB NGATE 4 6 VIN 3V TO 9.8V M1 D1 + C2 47µF 16V VOUT 12V 1.1M 1872 TA02 78.7k C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT C2: AVX TPSE476M016R0150 D1: IR10BQ015 L1: COILTRONICS UP2B-100 M1: Si9804DV R1: DALE 0.25W LTC1872 Three-Cell White LED Driver VIN = 3 AA CELLS ≈ 2.7V TO 4.8V C1 10µF 10V R1 0.
PAGE 12
LTC1872 U TYPICAL APPLICATIO LTC1872 – 2.5V to 3.3V/0.5A Boost Converter R1 0.034Ω 10k 220pF ITH/RUN VIN 5 + 1 L1 4.7µH LTC1872 2.7V to 9.8V Input to 3.3V/1.2A Output SEPIC Converter C2 2× 100µF 10V LTC1872 2 3 GND SENSE – VFB NGATE 4 6 M1 D1 332k VOUT 3.3V 0.5A CC1 220pF RC1 10k 1 3 0.1µF CERAMIC + C1 100µF 10V ITH/RUN VIN 5 L1A LTC1872 2 VIN 2.7V TO 9.8V CIN 10µF 10V, X5R RCS 0.03Ω GND SENSE – VFB NGATE L1B 4 D1 MBRM120 Rf1 252k C01 180µF 4V, SP VOUT 3.3V/1.