LT1249 查看數據表（PDF） - Linear Technology

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LT1249

Power Factor Controller

Linear Technology 

LT1249

PIN FUNCTIONS

VSENSE (Pin 6): This is the inverting input to the voltage

amplifier.

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

capacitor in parallel with a low ESR electrolytic capacitor,

56µF or higher is required in close proximity to IC GND.

GTDR (Pin 8): The MOSFET gate driver is a 1.5A fast totem

pole output. It is clamped at 15V. Capacitive loads like

MOSFET gates may cause overshoot. A gate series resis-

tor of at least 5Ω will prevent the overshoot.

APPLICATIONS INFORMATION

Error Amplifier

The error amplifier has a 100dB DC gain and 1.5MHz unity-

gain frequency. It is internally clamped at 12V. The nonin-

verting input is tied to the 7.5V reference.

Current Amplifier

The multiplier output current IM flows out of the MOUT pin

through the 4k resistor RMOUT and develops the reference

signal to the current loop that is controlled by the current

amplifier. Current gain is the ratio of RMOUT to line current

sense resistor. The current amplifier is a transconductance

amplifier. Typical gm is 320µmho and gain is 60dB with no

load. The inverting input is internally tied to GND. The

noninverting input is tied to the multiplier output. The

output is internally clamped at 8V. Output resistance is

about 4M; DC loading should be avoided because it will

lower the gain and introduce offset voltage at the inputs

which becomes a false reference signal to the current loop

and can distort line current. Note that in the current

averaging operation, high gain at twice the line frequency

is necessary to minimize line current distortion. Because

CAOUT may need to swing 5V over one line cycle at high line

condition, 11mV will be present at the inputs of the current

amplifier if gain is rolled off to 450 at 120Hz (1nF in series

with 10k at CAOUT). At light load, when (IM)(RMOUT) can be

less than 100mV, lower gain will distort the current loop

reference signal and line current. If signal gain at the

100kHz switching frequency is too high, the system

behaves more like a current mode system and can cause

subharmonic oscillation. Therefore, the current amplifier

should be compensated to have a gain of less than 15 at

100kHz and more than 300 at 120Hz.

Multiplier

The multiplier is a current multiplier with high noise

immunity in a high power switching environment. The

current gain is:

IM = (IAC)(IEA2)/(200µA)2, and

IEA = (VAOUT – 1.5V)/25k

With a square function, because of the lower gain at light

power load, system stability is maintained and line current

distortion caused by the AC ripple fed back to the error

amplifier is minimized. Note that switching ripple on the

high impedance lines could get into the multiplier from the

IAC pin and cause instability. The LT1249 provides an

internal 25k resistor in series with the low impedance

multiplier current input so that only a capacitor from the

IAC pin to GND is needed to filter out the noise. Maximum

multiplier output current is limited to 250µA. Figure 1

shows the multiplier transfer curves.

300

VAOUT = 6.5V

VAOUT = 6V

VAOUT = 5.5V

150

VAOUT = 5V

VAOUT = 4.5V

VAOUT = 4V

VAOUT = 3.5V

VAOUT = 3V

VAOUT = 2.5V

0

VAOUT = 2V

0

250

500

IAC (µA)

1249 G04

Figure 1. Multiplier Current IM vs IAC and VAOUT

6

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