SLUSD12A October 2017 – February 2018 UCC28780
UCC28780 contains four modes of operation summarized in Table 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 and SBP modes, PWMH and the ZVS tuning loop are disabled, so the converter operates in valley-switching.
|AAM||Adaptive Amplitude Modulation||ACF operation with PWML and PWMH in complementary switching||Enabled||Yes|
|ABM||Adaptive Burst Mode||Variable fBUR > 20 kHz, ACF operation in complementary switching||Enabled||Yes|
|LPM||Low Power Mode||Fix fBUR ≈ 25 kHz, valley-switching||Disabled||No|
|SBP||StandBy Power||Variable fBUR < 25 kHz, valley-switching||Disabled||No|
Figure 23 addresses the critical parameter changes among the four operating modes, where VCST is the peak current threshold compared with the current-sense voltage from CS pin, fSW is the switching frequency of PWML, fBUR is the burst frequency, and NSW is the pulse number of PWML per burst packet. The following section explains the detailed operation of each mode.