January 1999 TOKO, Inc.

Page 5

TK75002

PIN DESCRIPTION

V

CC

PIN

This pin is connected to the supply voltage. The operation

of the IC is enabled when the supply voltage exceeds

4.1 V, which is the upper threshold of the undervoltage

lockout circuit. The operation is disabled when the supply

voltage drops below 3.98 V.

GND PIN

This pin provides ground return for the IC.

DRV PIN

This pin drives the first terminal of the primary winding of

the external pulse transformer using pulse-amplitude

modulation (PAM). The second terminal of the primary

winding is connected to the V

low, an internal NPN transistor (the PAM switch) is turned

on and sinks an increasing current from the supply voltage

through the primary winding. The PAM switch is turned off

when the current reaches approximately 55 mA. The PAM

switch is turned on again when the transformer is reset

through the internal free-wheeling diode.

INV PIN

The INV pin is the inverting input of the error amplifier. The

non-inverting input of the error amplifier is connected to an

internal 1.35 V precision reference source.

COMP PIN

The COMP pin is the output of the error amplifier. The

frequency compensation feedback network is connected

between the COMP and INV pins. Internally, the COMP pin

drives the inverting input of a level shifter stage. The roles

of the level shifter are to change (1) the reference point of

the error signal from the GND pin to the V

to avoid the parasitic zero in the transfer function caused

by feed-forward through the supply) and (2) the signal

polarity (necessary for start-up of the converter system in

case the reference point was changed). The parasitic zero

is discussed in the "Application Information" section.