6

LT1508

PFC SECTION

GTDR1 (Pin 1): The PFC MOSFET gate driver is a fast

totem pole output which is clamped at 15V. Capacitive

loads like the MOSFET gates may cause overshoot. A gate

series resistor of at least 5

Ω will prevent the overshoot.

GND2 (Pin 2): Power Ground. High current spikes occur

in this line when either GTDR1 or GTDR2 switches low.

GND1 (Pin 3): Analog Ground.

determines oscillator frequency. The oscillator ramp is 5V

comparator is GND. To set current limit, a resistor divider

that senses and forces the line current to follow the

reference signal that comes from the multiplier by com-

the modulator has zero duty cycle.

amplifier. This pin is clamped at – 0.6V by an ESD protec-

tion diode.

current output and the noninverting input of the current

amplifier. This pin is clamped at – 0.6V and 3V.

multiplier. It is a current input that is biased at 2V to

minimize the crossover dead zone caused by low line

voltage. At the pin, a 25k resistor is in series with the

current input, so that a lowpass RC can be used to filter out

the switching noise coming down from the line with a high

line impedance environment.

amplifier. The output is clamped at 13.5V. When the

output goes below 2.5V, the multiplier output current is

zero.

OVP (Pin 11): This is the input to the overvoltage com-

parator. The threshold is 1.05 times the reference voltage.

When the comparator trips, the multiplier, which is quickly

inhibited, blanks PFC switching to prevent further over-

shoot. This pin is also the input to the PWMOK comparator

that releases the PWM soft start (SS2) after the PFC output

gets close to the final voltage and has a hysteresis of

approximately 150V for 382V PFC output.

circuitry and can source up to 5mA externally.

amplifier.

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

120

100

80

60

40

20

0

–20

–40

–60

–80

–100

1508 G12

–1.0

–0.8

–0.6

–0.4

–0.2

0

–360

–300

–240

–180

–120

– 60

0

60

120

180

240

300

1508 G13

0.8

0.4

0

–0.4

–0.8