7.3.3 Startup Operation

An internal high-voltage startup switch, connected to the bulk-capacitor voltage (V_BULK) 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 V_VDD 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 I_PP(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 × I_PP(max) and D_MAGCC increases from 0.425 to 0.735. These modifications to I_PP(max) and D_MAGCC 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, D_MAGCC is restored to 0.425 and the primary current peak resumes as I_PP(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.