JAJSHT2C august 2019 – december 2020 UCC28740-Q1
PRODUCTION DATA
An internal high-voltage startup switch, connected to the bulk-capacitor voltage (VBULK) through the HV pin, charges the VDD capacitor. This startup switch functions similarly to a current source providing typically 250 µA to charge the VDD capacitor. When VVDD reaches the 21-V UVLO turnon threshold the controller is enabled, the converter starts switching, and the startup switch turns off.
Often at initial turnon, the output capacitor is in a fully discharged state. The first three switching-cycle current peaks are limited to IPP(min) to monitor for any initial input or output faults with limited power delivery. After these three cycles, if the sampled voltage at VS is less than 1.33 V, the controller operates in a special startup mode. In this mode, the primary current peak amplitude of each switching cycle is limited to approximately 0.63 × IPP(max) and DMAGCC increases from 0.425 to 0.735. These modifications to IPP(max) and DMAGCC during startup allows high-frequency charge-up of the output capacitor to avoid audible noise while the demagnetization voltage is low. Once the sampled VS voltage exceeds 1.38 V, DMAGCC is restored to 0.425 and the primary current peak resumes as IPP(max). While the output capacitor charges, the converter operates in CC mode to maintain a constant output current until the output voltage enters regulation. Thereafter, the controller responds to the condition dictated by the control law. The time to reach output regulation consists of the time the VDD capacitor charges to 21 V plus the time the output capacitor charges.