Datasheet

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LTC1872B

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATION

Constant Frequency

Current Mode Step-Up

DC/DC Controller in ThinSOT

The LTC

1872B is a constant frequency current mode

step-up DC/DC controller providing excellent AC and DC

load and line regulation. The device incorporates an accu-

rate undervoltage lockout feature that shuts down the

LTC1872B when the input voltage falls below 2.0V.

The LTC1872B provides a ±2.5% output voltage accuracy

and consumes only 270µA of quiescent current. In shut-

down, the device draws a mere 8µA.

High constant operating frequency of 550kHz allows the

use of a small external inductor. The constant frequency

operation is maintained down to very light loads, resulting

in less low frequency noise generation over a wide load

current range.

The LTC1872B is available in a 6-lead low profile (1mm)

ThinSOT package. For a Burst Mode operation enabled

version of the LTC1872B, please refer to the LTC1872 data

sheet.

■

Optical Communications

■

Lithium-Ion-Powered Applications

■

Cellular Telephones

■

Wireless Devices

■

Portable Computers

■

Scanners

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Burst Mode

Operation Disabled for Lower Output

Ripple at Light Loads

■

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

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Shutdown Mode Draws Only 8µA Supply Current

■

Low Profile (1mm) ThinSOT

Package

Typical Efficiency vs Load Current*

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

Burst Mode and ThinSOT are trademarks of Linear Technology Corporation.

/RUN

LTC1872B

147k

422k

80.6k

0.03Ω

4.7µH

220pF

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT

C2: MURATA GRM42-2X5R106K010AL

D1: IR10BQ015

L1: MURATA LQN6C4R7M04

M1: Si2302DS

R1: DALE 0.25W

GND

1872B TA01

NGATE

SENSE

–

10µF

10V

3.3V

OUT

4× 10µF

10V

Figure 1. LTC1872B High Output Current 3.3V to 5V Boost Converter

1 10 100 1000

LOAD CURRENT (mA)

EFFICIENCY (%)

100

1872B TA01b

= 3.3V

OUT

= 5V

*Output ripple waveforms for the circuit of Figure 1 appear in Figure 2.

Summary of content (12 pages)

PAGE 1
LTC1872B Constant Frequency Current Mode Step-Up DC/DC Controller in ThinSOT U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO Burst ModeTM Operation Disabled for Lower Output Ripple at Light Loads High Efficiency: Over 90% High Output Currents Easily Achieved Wide VIN Range: 2.5V to 9.
PAGE 2
LTC1872B 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
LTC1872B 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) 1872B G02 Maximum Current Sense Trip Voltage vs Duty Cycle 1872B G03 Shutdown Threshold vs Temperature 600 VIN = 4.2V TA = 25°C 120 2.08 1.
PAGE 4
LTC1872B W FUNCTIONAL DIAGRA U VIN SENSE – 5 4 U 15mV + ICMP – VIN RS SLOPE COMP OSC NGATE SWITCHING LOGIC AND BLANKING CIRCUIT R Q S 6 – FREQ FOLDBACK OVP 0.3V + EAMP + – VREF + 60mV + VREF 0.8V 0.5µA VFB + – 1 ITH/RUN 3 VIN VIN – 0.35V VOLTAGE REFERENCE + SHDN CMP VREF 0.8V – GND SHDN UV 2 UNDERVOLTAGE LOCKOUT 1.2V 1872B FD U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1872B is a constant frequency current mode switching regulator.
PAGE 5
LTC1872B U OPERATIO (Refer to Functional Diagram) Figure 2 illustrates this result for the circuit of Figure 1 using both an LTC1872 in Burst Mode operation and an LTC1872B (non-Burst Mode operation). At an output current of 50mA, the Burst Mode operation part exhibits an output ripple of approximately 80mVP-P, whereas the non-Burst Mode operation part has an output ripple of ≈45mVP-P. At lower output current levels, the improvement is even greater.
PAGE 6
LTC1872B U OPERATIO (Refer to Functional Diagram) Short-Circuit Protection Since the power switch in a boost converter is not in series with the power path from input to load, turning off the switch provides no protection from a short-circuit at the output. External means such as a fuse in series with the boost inductor must be employed to handle this fault condition. U W U U APPLICATIONS INFORMATION The basic LTC1872B application circuit is shown in Figure␣ 1.
PAGE 7
LTC1872B U W U U APPLICATIONS INFORMATION This implies a minimum inductance of:  VOUT + VD − VIN  VIN LMIN =    0.03   VOUT + VD  f   RSENSE  A smaller value than L MIN could be used in the circuit; however, the inductor current will not be continuous during burst periods. 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.
PAGE 8
LTC1872B U W U U APPLICATIONS INFORMATION 1 2 2    1  ESR2 +      2π fC OUT     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
LTC1872B U W U U APPLICATIONS INFORMATION 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. Efficiency can be expressed as: Efficiency = 100% – (η1 + η2 + η3 + ...) where η1, η2, etc. are the individual losses as a percentage of input power.
PAGE 10
LTC1872B U U W U APPLICATIONS INFORMATION 1 ITH/RUN NGATE VIN 6 LTC1872B 2 RITH VIN VFB 4 SENSE – 0.1µF 3 CITH + M1 L1 RS 5 GND CIN D1 VOUT + R2 COUT R1 BOLD LINES INDICATE HIGH CURRENT PATHS 1872B F06 Figure 6. LTC1872B Layout Diagram (See PC Board Layout Checklist) U TYPICAL APPLICATIO S LTC1872B 3-Cell White LED Driver VIN = 3 AA CELLS ≈ 2.7V TO 4.8V C1 10µF 10V R1 0.27Ω AA AA 1 10k 220pF ITH/RUN VIN 5 LTC1872B 2 3 GND VFB SENSE – NGATE L1 150µH VOUT ≈ 28.
PAGE 11
LTC1872B U TYPICAL APPLICATIO S LTC1872B 12V/500mA Boost Converter C1 10µF 10V R1 0.033Ω 1 ITH/RUN 5 VIN L1 10µH LTC1872B 10k 2 3 220pF GND VFB 4 SENSE – 6 NGATE VIN 3V TO 9.8V M1 D1 + C2 47µF 16V VOUT 12V 1.1M 1872B TA02 78.7k C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT C2: AVX TPSE476M016R0150 D1: IR10BQ015 L1: COILTRONICS UP2B-100 M1: Si9804DV R1: DALE 0.25W U PACKAGE DESCRIPTION S6 Package 6-Lead Plastic SOT-23 (Reference LTC DWG # 05-08-1634) (Reference LTC DWG # 05-08-1636) 2.
PAGE 12
LTC1872B U TYPICAL APPLICATIO S LTC1872B – 2.5V to 3.3V/0.5A Boost Converter R1 0.034Ω 2 3 220pF + L1 4.7µH C2 2× 100µF 10V LTC1872B 10k 0.1µF CERAMIC VIN ITH/RUN 5 + 1 GND VFB SENSE – NGATE VOUT 3.3V 0.5A 4 6 M1 D1 C1 100µF 10V 332k U1 80.6k VIN –2.5V 180k 1872B TA03 C1, C2: AVX TPSE107M010R0100 D1: MOTOROLA MBR2045CT L1: COILTRONICS UP2B-4R7 M1: Si9804DV R1: DALE 0.25W U1: PANASONIC 2SB709A LTC1872B 2.7V to 9.8V Input to 3.3V/1.