ON Semiconductor NCP1377 combines a true current mode modulator and a demagnetization detector which ensures full borderline/critical Conduction Mode in any load/line conditions together with minimum drain voltage switching (Quasi-Resonant operation). Due to its inherent skip cycle capability, the controller enters burst mode as soon as the power demand falls below a predetermined level. As this happens at low peak current, no audible noise can be heard. For NCP1377, an internal 8.0μs timer prevents the free-run frequency to exceed the 150kHz CISPR-22 EMI starting limit while the skip adjustment capability lets the user select the frequency at which the burst foldback takes place. For NCP1377B, the internal timer duration is reduced to 3.0μs to allow operation at higher frequencies.

The transformer core reset detection is done through an auxiliary winding which, brought via a dedicated pin, also enables fast Over-Voltage Protection (OVP). Once an OVP has been detected, the IC permanently latches-off. The 1377 features a sampling time of 4.5μs whereas it is 1.5μs for the B version.

The NCP1377 also features an efficient protective circuitries which, in presence of an overcurrent condition, disables the output pulses and enters a safe burst mode, trying to re-start. Once the default has gone, the device auto-recovers. Finally an internal 1ms soft-start eliminates the traditional startup stress.