SLVS074I January 1983 – July 2022 TL494
To reduce stress on the switching transistors at the start-up time, the start-up surge that occurs as the output filter capacitor charges must be reduced. The availability of the dead-time control makes implementation of a soft-start circuit relatively simple (see Figure 10-5).
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:
The value of the capacitor then is determined by:
This helps eliminate any false signals that might be created by the control circuit as power is applied.