Internal soft starting of the PWM output is accomplished by gradually increasing the error amplifier (E/A) output voltage at COMP. When used in current-mode control, this implementation slowly raises the peak switch current each PWM cycle in succession, forcing a controlled start-up. In voltage-mode (duty-cycle) control, this feature continually widens the pulse width.
Soft-start is performed within the UCCx813-x devices by clamping the E/A amplifier output (COMP) to the voltage on an internal soft-start capacitor (CSS), which is charged by a current source. CSS is discharged following an undervoltage lockout transition or if the reference voltage is below a minimum value for normal operation. Additionally, discharge of CSS occurs whenever the overcurrent protection comparator is triggered by a fault. The soft-start clamp circuitry is overridden once CSS charges above the voltage commanded by the error amplifier for normal PWM operation.