JAJSE40A October 2017 – February 2018 UCC28780
As NSW drops to two in ABM and the condition of fBUR less than fBUR(LR) is qualified under two consecutive burst periods, UCC28780 enters into LPM mode and disables PWMH. The purpose of LPM is to provide a soft peak current transition between VCST(BUR) and VCST(MIN). LPM fixes NSW at two and sets fBUR equal to fLPM of 25 kHz. In LPM mode, VCST is controlled to regulate the output voltage. At the start of each burst packet, after RUN pulls high, tD(RUN-PWML) is used to wake up both the gate driver and UCC28780. With PWMH disabled, the two PWML pulses turn on QL close to valley-switching by sensing ZCD. When ZCD is detected again at the end of the second pulse, the RUN pin goes low and the UCC28780 enters its low-power wait state. In LPM mode, the minimum on-time of PWML can be further reduced to tON(MIN), to allow the peak magnetizing current to be reduced beyond the level limited by tCSLEB of the peak current loop. In this condition, operation of the LPM control loop is changed from a current-mode control to a voltage-mode control, so the on-time adjustment of PWML is not limited to tCSLEB. With this feature, before fBUR starts to fall below fLPM and enters the audible frequency range of SBP mode, the peak current is low enough to limit the magnitude of audible excitation.