SLUSD37E October 2017 – November 2019 UCC28056
A second comparator (Ocp2) with a higher threshold, and shorter blanking time, also monitors the current sense voltage signal. If triggered, this second Ocp2 comparator also terminates the current on-time (TON) duration early. In addition, if the controller triggers the Ocp2 comparator on three consecutive switching cycles, it also triggers a long fault. The long fault halts switching operation and prevents restart for a period TLongFlt. After this delay, the controller proceeds with its normal start-up process. In all transient or mild fault conditions the Ocp1 comparator, with its lower threshold, triggers first and prevents the Ocp2 comparator from acting. The Ocp2 comparator acts only if there is a gross fault such as a shorted output capacitor or bypass diode.
Under some fault conditions, including output overload, inductor current may become continuous because the reset voltage is low. In this case even the relatively short Ocp1 blanking time may allow the inductor current to continue ramping up. The UCC28056 controller addresses this condition by reducing the switching frequency to allow a longer period for the inductor current to ramp down between on-time pulses.
The maximum allowed diode conduction period (TDCHMax) is doubled in the sequence (250 µs, 500 µs, 1000 µs) each time the on-time duration terminates early by either one of the OCP comparators. If there is no ZCD signal to indicate that the inductor current has fallen to zero, then the TDCHMax interval must expire before the next switching cycle so the switching frequency is halved. The TDCHMax period is halved to reverse the sequence each time the on-time period does not terminate early by one of the OCP comparators to restore the switching frequency. If the ZCD signal indicates that inductor current has reached zero, then TDCHMax has no effect and normal operation resumes automatically.