LT1508ISW データシートの表示（PDF） - Linear Technology

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LT1508ISW

Power Factor and PWM Controller (Voltage Mode)

Linear Technology 

LT1508

PIN FUNCTIONS (For application help with the PFC portion of this chip, see the LT1248 data sheet)

RSET (Pin 15): A resistor from RSET to GND sets the

oscillator charging current and the maximum multiplier

output current which is used to limit the maximum line

current.

IM(MAX) = 3.75V/RSET

SS1 (Pin 16): Soft Start. SS1 is reset to zero for low VCC.

When VCC rises above the lockout threshold, SS1 is

released to ramp up at a rate set by the internal 12µA

current source and an external capacitor. During this ramp

up, PFC reference voltage is equal to SS1 voltage. After

SS1 rises past 7.5V, reference voltage remains at 7.5V.

VCC (Pin 17): This is the supply for the chip. The LT1508

has two fast gate drivers required to fast charge high

power MOSFET gate capacitances. Good supply bypass-

ing is required consisting of a 0.1µF ceramic capacitor in

parallel with a low ESR electrolytic capacitor (56µF or

higher) in close proximity to IC GND.

PWM SECTION

SS2 (Pin 13): PWM Soft Start. The comparator PWMOK

monitors the OVP pin and releases the SS2 after the PFC

output gets close to the final voltage.

VC (Pin 18): PWM voltage mode control voltage. Normally

connects to the optocoupler amplifier output. A pull-up

current of 50µA flows out of the pin.

ILIM (Pin 19): PWM current sense input with limit set to

1.1V.

GTDR2 (Pin 20): The PWM MOSFET gate driver is a 1.5A

fast totem pole output. It is clamped at 15V. Capacitive

loads like the MOSFET gates may cause overshoot. A gate

series resistor of at least 5Ω will prevent the overshoot.

APPLICATIONS INFORMATION

Voltage Error Amplifier (PFC Section)

The voltage error amplifier has a 100dB DC gain and 3MHz

unity-gain frequency. The output is internally clamped at

13.3V with VCC = 18V. Maximum error amp output voltage

decreases to VCC – 1.5V for VCC less than 12V. The

noninverting input is tied to the 7.5VREF through a diode

and can be pulled down with the SS1 pin. Referring to

Figure 1, VOUT = VREF [(R1 + R2)/R2]. With R1 = 1M and

R2 = 20k, VOUT = 382V. R1 through R4, C1 and C2 form the

compensation for the voltage loop. Gain of the voltage

error amp with the values shown is given by:

) VAOUT

VOUT

=

–

1

+

j

f

1

(j)(f)(6.6) 1 + j f

11

The small-signal gain for the remaining portion of the

voltage loop for frequencies below the current loop band-

width is (see Figure 2):

VOUT =

VIN

VAOUT (5π)(j)(f)(COUT)(VOUT)

(RREF)(PIN)

RS(RIAC + 25k)

C2

0.047µF

REGULATOR OUTPUT

VOUT = 382V

R3

20k

R1

1M

R2

20k

C1

0.47µF

R4

330k

VSENSE

–

+

7.5VREF

OVP

–

VAOUT

ERROR AMP

LT1508

+

1.05VREF

OVERVOLTAGE

COMPARATOR

1508 • F01

Figure 1

With VIN = 120VAC, PIN = 150W, RS = 0.15Ω, RREF = 4k,

RIAC = 1M, VOUT = 382V and COUT = 470µF, VOUT/VAOUT =

85/(j)(f). At very low frequencies, the loop has a – 40dB/

decade slope. Additional zero-pole compensation is added

at 1Hz and 11Hz. The resulting loop gain and phase margin

is shown in Figure 3. The unity-gain bandwidth is low

compared to 120Hz, which results in low distortion and a

high power factor.

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