The current limit portion of the circuit is shown in Figure 6-22.
Resistors Rl and R2 set a reference of about 1V on the inverting input of the current limit amplifier. Resistor Rll , in series with the load, applies 1V to the non-
Soft-Start and Dead Time
To reduce stress on the switching transistors at startup, the startup surge which occurs as the output filter capacitor charges must he reduced. The availability of the dead-time control makes implementation of a soft-start circuit, as shown in Figure 6-23, relatively simple.
The “soft-start” circuit allows the pulse width at the output to increase slowly, as shown in Figure 6-23, 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 reference 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 startup will be 0.1 x 5 V or 0.5 V.
The soft-start time is generally in the range of 25 to 100 clock cycles. If we select 50 clock cycles at a 20 kHz switching rate, the soft start time is calculated as follows:
This helps to eliminate any false signals which might be created by the control circuit as power is applied.