TL494

Pin 4 Voltage

PWM Output

Oscillator Ramp Voltage

ton

RT

C2

+5 V

0.1 V

Osc

R6

14

5

4

7

Oscillator Ramp

+

Figure 10-5. Soft-Start Circuit

The soft-start circuit allows the pulse width at the output to increase slowly (see Figure 10-5) by applying a

negative slope waveform to the dead-time control input (pin 4).

Initially, capacitor C2 forces the dead-time control input to follow the 5-V regulator, which disables the outputs

(100% dead time). As the capacitor charges through R6, the output pulse width slowly increases until the control

loop takes command. With a resistor ratio of 1:10 for R6 and R7, the voltage at pin 4 after start-up is 0.1 × 5 V, or

0.5 V.

The soft-start time generally is in the range of 25 to 100 clock cycles. If 50 clock cycles at a 20-kHz switching

rate is selected, the soft-start time is:

1

1

t

50 sper clock cycle

f

20kHz

=

=

=

m

(12)

The value of the capacitor then is determined by:

soft

start time

50 s 50 cycles

C2

2.5 F

R6

1 k

-

m ´

=

=

=

m

W

(13)

This helps eliminate any false signals that might be created by the control circuit as power is applied.

TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

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