January 1999 TOKO, Inc.

Page 11

TK75003

t

OFF

= C

T

x 14 k

Ω.

FIGURE 5: TK75003 IN A FORWARD CONVERTER

WITH CONSTANT OFF-TIME CURRENT-MODE

CONTROL

FIGURE 6: TIMING PIN AND FEEDBACK PIN

VOLTAGES WITH CONSTANT OFF-TIME CURRENT-

MODE CONTROL

TK75003 IN NON-ISOLATED APPLICATIONS

Although the IC was intended for off-line power-supply

applications with the voltage-error amplifier at the isolated

output, it is easy and economical to use the device in non-

isolated applications, too. Figure 7 shows a low-cost boost

power factor corrector controlled by the TK75003. Power

factor correction is achieved by controlling the boost

converter with constant-frequency peak-current control

and exploiting the variation of the allowed peak-current

level caused by the variable duty ratio and the stabilizing

ramp. Figure 8 shows a buck-boost converter with negative

input voltage and positive output voltage, controlled by the

TK75003. In both cases, the voltage-error amplifier is a

TL431 shunt regulator, and a PNP transistor provides

interface between the TL431 and the control IC.

FIGURE 7: TK75003 IN A LOW COST BOOST

POWER FACTOR CORRECTOR

FIGURE 8: NON-ISOLATED NEGATIVE-TO-POSITIVE

CONVERTER

APPLICATION INFORMATION (CONT.)

GND

VCC

CT

FB

DRV

Q1

VAUX

RS

TL431

+

VOUT

VIN

R1

VAUX

OC

CT

3.2 V

1.1 V

0

0.98 V

FEEDBACK VOLTAGE LEVEL

FB

CT

FB

CT

GND

VCC

CT

FB

DRV

TL431

VIN (-)

VOUT (+)

GND

VCC

CT

FB

DRV

TL431

VOUT

+

AC

IN