SLUSD60 October 2017 UCC256304
PRODUCTION DATA.
The VCC capacitor should be sized based on the total start-up charge required by the system. The start-up charge will mostly be consumed by the gate driver circuit. Thus the total start-up charge can be estimated by the start-up switching frequency, MOSFET gate charge, and the soft-start time.
Assume the total start-up charge required by the system is shown in Equation 77
During PFC and LLC startup phase, the maximum VCC voltage drop allowed is
The minimum VCC capacitor needed:
Choose 110-µF capacitor.
During burst off period, power consumed by the high side gate driver from the HB pin must be drawn from CBOOT and will cause its voltage to decay. At the start of the next burst period there must be sufficient voltage remaining on CBOOT to power the high side gate driver until the conduction period of LO allows it to be replenished from CRVCC. The power consumed by the high side driver during this burst off period will therefore have a direct impact on the size and cost of capacitors that must be connected to CBOOT and RVCC.
Assume the system has a maximum burst off period of 10 ms.
Assume the bootstrap diode has a forward voltage drop of 1 V:
Assume the boot voltage to be always above 8 V to avoid UVLO fault. Then the maximum allowed voltage drop on boot capacitor is:
Boot capacitor can then be sized:
RVCC capacitor needs to be at least 5 times of boot capacitor. In addition, sizing of the RVCC capacitor depends on the stability of RVCC LDO. If load is light on RVCC, smaller capacitors can be used. The larger the load, the larger the capacitor is needed. In a typical system, the RVCC LDO powers the PFC and LLC gate drivers. The plot below shows the worst case RVCC LDO phase margin versus RVCC capacitor for various load currents. RVCC capacitor should be sized based on the figure below.