Datasheet

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LT1249

Power Factor Controller

■

Standard 8-Pin Packages

■

High Power Factor Over Wide Load Range

with Line Current Averaging

■

International Operation Without Switches

■

Instantaneous Overvoltage Protection

■

Minimal Line Current Dead Zone

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Typical 250µA Start-Up Supply Current

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Rejects Line Switching Noise

■

Synchronization Capability

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Low Quiescent Current: 9mA

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Fast 1.5A Peak Current Gate Driver

The 8-pin LT

1249 provides active power factor correc-

tion for universal offline power systems with very few

external parts. By using fixed high frequency PWM current

averaging without the need for slope compensation, the

LT1249 achieves far lower line current distortion, with a

smaller magnetic element than systems that use either peak

current detection or zero current switching approach, in

both continuous and discontinuous modes of operation.

The LT1249 uses a multiplier containing a square gain

function from the voltage amplifier to reduce the AC gain

at light output load and thus maintains low line current

distortion and high system stability. The LT1249 also

provides filtering capability to reject line switching noise

which can cause instability when fed into the multiplier.

Line current dead zone is minimized with low bias voltage

at the current input to the multiplier.

The LT1249 provides many protection features including

peak current limiting and overvoltage protection. The

switching frequency is internally set at 100kHz.

While the LT1249 simplifies PFC design with minimal

parts count, the LT1248 provides flexibilities in switching

frequency, overvoltage and current limit.

–

200µA

OUT

7.5V

44µA

22µA

15µA

–

32k

20µA

250µA MAX

7.5V

REF

0.7V

OSC

35pF

SENSE

OUT

MOUT

OUT

16V/10V

MULTIPLIER

RUN

16V

GND

GTDR

1249 BD

SYNC

= 1/3k

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

■

Universal Power Factor Corrected Power Supplies

■

Preregulators up to 1500W

FEATURES

DESCRIPTIO

APPLICATIO S

BLOCK DIAGRA

Summary of content (12 pages)

PAGE 1
LT1249 Power Factor Controller U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The 8-pin LT ®1249 provides active power factor correction for universal offline power systems with very few external parts.
PAGE 2
LT1249 W U U W W W Supply Voltage ....................................................... 27V GTDR Current Continuous ..................................... 0.5A GTDR Output Energy (Per Cycle) ............................. 5µJ IAC Input Current ................................................. 20mA VSENSE Input Voltage ............................................ VMAX MOUT Input Current.............................................. ± 5mA Operating Junction Temperature Range LT1249C ....................
PAGE 3
LT1249 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the operating temperature range, otherwise specifications are at TA = 25°C. Maximum operating voltage (VMAX) = 25V, VCC = 18V, IAC = 100µA, CAOUT = 3.5V, VAOUT = 5V, no load on any outputs, unless otherwise noted.
PAGE 4
LT1249 U W TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature Multiplier Current 7.536 300 VAOUT = 5V VAOUT = 6.5V 7.512 VAOUT = 6V 7.500 VAOUT = 5.5V IM (µA) REFERENCE VOLTAGE (V) 7.524 7.488 7.476 VAOUT = 4.5V 150 VAOUT = 4V 7.464 VAOUT = 3.5V 7.452 VAOUT = 3V 7.440 7.428 –75 –50 –25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (°C) 0 0 VAOUT = 2.5V VAOUT = 2V 500 250 IAC (µA) 1249 G04 1249 G03 18.5 10 7 18.0 TJ = 25°C 17.5 0.9 17.0 0.
PAGE 5
LT1249 U W TYPICAL PERFORMANCE CHARACTERISTICS Transconductance of Current Amplifier Over Temperature MOUT Pin Characteristics 1.2 0.9 1.0 0.8 0.8 0.7 0.6 0.6 0.5 0.4 0.3 0.2 400 125°C 25°C –50°C 350 TRANSCONDUCTANCE (µmho) 1.0 MOUT CURRENT (mA) SYNCHRONIZATION THRESHOLD (V) Synchronization Threshold at CAOUT 0.4 0.2 0 –0.2 –0.4 –0.6 0.1 –25 0 25 50 75 TEMPERATURE (°C) –1.0 125 100 –2.4 0 –1.2 1.2 MOUT VOLTAGE (V) 60 100 0 –50 –25 2.
PAGE 6
LT1249 U U U PIN FUNCTIONS VSENSE (Pin 6): This is the inverting input to the voltage amplifier. capacitor in parallel with a low ESR electrolytic capacitor, 56µF or higher is required in close proximity to IC GND. VCC (Pin 7): This is the supply of the chip. The LT1249 has a very fast gate driver required to fast charge high power MOSFET gate capacitance. High current spikes occur during charging. For good supply bypass, a 0.1µF ceramic GTDR (Pin 8): The MOSFET gate driver is a 1.
PAGE 7
LT1249 U W U U APPLICATIONS INFORMATION Line Current Limiting Maximum voltage across RMOUT is internally limited to 1.1V. Therefore, line current limit is 1.1V divided by the sense resistor RS. With a 0.2Ω sense resistor RS line current limit is 5.5A. As a general rule, RS is chosen according RS = With ts = 30ns, fs = 130kHz, VC = 3V and R2 = 10k, offset voltage shift is ≈5mV. Note that this offset voltage will add slight distortion to line current at light load. CAOUT 1N5712 R2 10k K (1.
PAGE 8
LT1249 U W U U APPLICATIONS INFORMATION The Figure 3 circuit therefore has 382V on VOUT, and an overvoltage level = (VOUT + 44V), or 426V. With a 22µA hysteresis, VOUT then has to drop 22V to 404V before feedback recovers and the switch turns back on. MOUT is a high impedance current output. In the current loop, offset line current is determined by multiplier offset current and input offset voltage of the current amplifier.
PAGE 9
LT1249 U U W U APPLICATIONS INFORMATION VP-P = (2)(0.78A)(7.4Ω) = 11.5V. If less ripple is desired, higher capacitance should be used. The selection of the output capacitor should also be based on the operating ripple current through the capacitor. The ripple current can be divided into three major components. The first is at 120Hz whose RMS value is related to the DC load current as follows: I1RMS ≈ (0.71)(ILOADDC) I2RMS = 0.
PAGE 10
LT1249 U W U U APPLICATIONS INFORMATION voltages above 8V and the 12V limit is only for surge conditions. resistor, the standard LT1249 application will not be affected because the chip is not yet powered. Problems are only created if the VCC pin is powered from some external housekeeping supply that remains powered when bridge power is switched off. In normal operation, the voltage into MOUT does not exceed 1.1V, but under surge conditions, the voltage could temporarily go higher.
PAGE 11
LT1249 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.400* (10.160) MAX 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.300 – 0.325 (7.620 – 8.255) 0.065 (1.651) TYP 0.009 – 0.015 (0.229 – 0.381) ( +0.035 0.325 –0.015 8.255 +0.889 –0.381 0.130 ± 0.005 (3.302 ± 0.127) 0.045 – 0.065 (1.143 – 1.651) ) 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 ± 0.003 0.100 (2.54) BSC (0.457 ± 0.
PAGE 12
LT1249 U TYPICAL APPLICATION MURH860 750µH* + 90V TO 270V VOUT EMI FILTER 6A – + IRF840 0.047µF RS 0.2Ω 0.47µF 20k 100pF 1nF 330k 180µF 1M 10Ω 10k 5 7.5V VAOUT 3 EA VSENSE 1M CAOUT 2 1 †V GND 7 4 IA MULTIPLIER IB I 2I IM = A B 2 200µA MAX 250µA IM VCC + 16V/10V – 15µA 1V RUN + – – CA 32k IAC CC 7.5V VREF RMOUT 4k + – 6 MOUT + gm = 1/3k RUN 0.7V + R + Q S 8 GTDR – M1 – + SYNC OSC – 4.7nF 44µA 22µA 4k 20µA 16V ** 1N5819 35pF * 1.