UCC28782 contains six modes of operation summarized in Table 8-1. Starting from heavier load, the AAM mode forces PWML and PWMH into complementary switching with ZVS tuning enabled. ABM mode generates a group of PWML and PWMH pulses as a burst packet, and adjusts the burst off-time to regulate the output voltage. At the same time, the burst frequency variation is confined above 20kHz by adjusting the number of PWML and PWMH pulses per packet to mitigate audible noise and reduce burst output ripple. In LPM, SBP1, and SBP2 modes, PWMH and the ZVS tuning loop are disabled, so the converter operates in valley-switching. The survival mode is to maintain V_VDD higher than V_VDD(OFF) in a long burst off time, and also performs the clamping capacitor balancing function to reduce the voltage stress of the secondary-side rectifier.

Figure 8-24 and Figure 8-25 show the critical parameter changes among the five operating modes, where V_CST is the peak current threshold compared with the current-sense voltage from the CS pin, f_SW is the switching frequency of PWML, f_BUR is the burst frequency, and N_SW is the pulse number of PWML cycles per burst packet. Figure 8-24 represents the control mode difference under the two VS-pin voltage ranges, when the IPC-pin voltage is less than 0.9 V or IPC is connected to AGND. Figure 8-25 illustrates the modified control mode, when the IPC-pin voltage setting is higher than 0.9 V. The following section explains the detailed operation of each mode. The VS-pin voltage and IPC-pin voltage effects will also be introduced in the following section.

Figure 8-24 Control Law Over Entire Load Range Based on the V_VS Condition as V_IPC < 0.9 V

Figure 8-25 Control Law Under Different Load Sweep Direction as V_IPC > 0.9 V and V_VS > V_VSLV(UP)