9 Detailed Description

9.1 Overview

The design of the TL494 not only incorporates the primary building blocks required to control a switching power

supply, but also addresses many basic problems and reduces the amount of additional circuitry required in

the total design. The TL494 is a fixed-frequency pulse-width-modulation (PWM) control circuit. Modulation of

output pulses is accomplished by comparing the sawtooth waveform created by the internal oscillator on the

timing capacitor (CT) to either of two control signals. The output stage is enabled during the time when the

sawtooth voltage is greater than the voltage control signals. As the control signal increases, the time during

which the sawtooth input is greater decreases; therefore, the output pulse duration decreases. A pulse-steering

flip-flop alternately directs the modulated pulse to each of the two output transistors. For more information on the

operation of the TL494, see the application notes located on ti.com.

9.2 Functional Block Diagram

GND

VCC

Reference

Regulator

C1

Pulse-Steering

Flip-Flop

C1

1D

DTC

CT

RT

PWM

Comparator

+

−

Error Amplifier 1

≈0.1 V

Dead-Time Control

Comparator

Oscillator

OUTPUT CTRL

(see Function Table)

0.7 mA

E1

C2

E2

+

−

Error Amplifier 2

1IN+

1IN−

2IN+

2IN−

FEEDBACK

REF

6

5

4

1

2

16

15

3

13

8

9

11

10

12

14

7

Q1

Q2

≈0.7 V

9.3 Feature Description

9.3.1 5-V Reference Regulator

The TL494 internal 5-V reference regulator output is the REF pin. In addition to providing a stable reference,

it acts as a preregulator and establishes a stable supply from which the output-control logic, pulse-steering

flip-flop, oscillator, dead-time control comparator, and PWM comparator are powered. The regulator employs a

band-gap circuit as its primary reference to maintain thermal stability of less than 100-mV variation over the

operating free-air temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal

reference and preregulator; 10 mA of load current is available for additional bias circuits. The reference is

internally programmed to an initial accuracy of ±5% and maintains a stability of less than 25-mV variation over

an input voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the

input and tracks it.

9.3.2 Oscillator

The oscillator provides a positive sawtooth waveform to the dead-time and PWM comparators for comparison to

the various control signals.

TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

www.ti.com

10

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