As N_SW drops to two in ABM and the condition of f_BUR less than f_BUR(LR) is qualified under two consecutive burst periods, UCC28782 enters into LPM mode and disables PWMH. The purpose of LPM is to provide a soft peak current transition between V_CST(BUR) and V_CST(MIN). LPM fixes N_SW at two and sets f_BUR equal to f_LPM of 25 kHz. In LPM mode, V_CST is controlled to regulate the output voltage. At the start of each burst packet, after RUN pulls high, t_D(RUN-PWML) is used to wake up both the gate driver and UCC28782. With PWMH disabled, the two PWML pulses turn on Q_L 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 UCC28782 enters its low-power wait state. For LPM mode with the SET pin connected to REF, the minimum on-time of PWML can be further reduced to t_ON(MIN), to allow the peak magnetizing current to be reduced below the level limited by t_CSLEB 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 t_CSLEB. When the silicon ACF is used and the SET pin is connected to REF, the high capacitance region of the low-side switch will introduce a higher peak current overshoot than the GaN ACF. With this feature, before f_BUR starts to fall below f_LPM and enters the audible frequency range of SBP mode, the peak current is low enough to limit the magnitude of audible excitation. For the LPM mode with GaN ACF by connecting the SET pin to AGND, the minimum on-time of PWML will still be limited by t_CSLEB.

Figure 8-28 PWM Pattern in LPM