AN4003 View Datasheet(PDF) - Fairchild Semiconductor

Part Name

Description

Manufacturer

AN4003

PC POWER SUPPLY DESIGN WITH KA3511

Fairchild Semiconductor 

RT 7

CT 8

OSCILLATOR

DQ

Output

Drive

Q1

2

PWM

CK Q

Q2

COMP

CONTROL

4

3

1.25V

0.12V

DEAD TIME

CONTROLLER

Figure 3. PWM Control Block

The control signals are external inputs that can be fed into the dead-time control, the error amplifier

inputs, or the feedback input. The dead-time control comparator has an effective 120mV input off-

set which limits the minimum output dead time. Dead time may be imposed on the output by set-

ting the dead time control input to a fixed voltage, ranging between 0V to 3.3V.

The pulse width modulator comparator provides a means for the error amplifier to adjust the output

pulse width from the maximum percent on-time, established by the dead time control input, down

to zero, as the voltage at the feedback pin varies from 0.5V to 3.5V. The error amplifier may be

used to sense power-supply output voltage, and its output is connect to noninverting input of the

pulse width modulator comparator. With this configuration, the amplifier that demands minimum

output on time, dominates control of the loop.

When capacitor CT is discharged, a positive pulse is generated on the output of the dead time

comparator, which clocks the pulse-steering flip-flop and inhibits the output transistors, Q1 and Q2.

The pulse-steering flip-flop directs the modulated pulses to each of the two output transistors

always for push-pull operation. The output frequency is equal to half that of the oscillator.

The KA3511 has an internal 5.0V reference capable of sourcing up to 10mA of load current for

external bias circuits. The reference has an internal accuracy of ±2% with typical thermal drift of

less than 50mV over an operating temperature range of -25°C to 85°C

Rev C, November 1999

9

Share Link: