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# UCC28711: CDD capacitance selection

Part Number: UCC28711
Other Parts Discussed in Thread: TIDA-00173,

When comparing TIDA-00173 reference design guide and design calculator spreadsheet UCC28710-17 Excel Design Tool , different formulas are used for calculating CDD capacitance (at pin VDD). Based on my 2 output design I would use following formulas following two approaches:

1. TIDA-00173 uses:  CDD = (IRUN + 1mA) x [(COUT1xVOUT1)/IOUT1 + (COUT2xVOUT2)/IOUT2] / (VVDD(on)-VVDD(off) - 1V)
2. Excel design tool formula is: CDD = IHV x dtCDDS / VVDD(on)

Please explain different approaches and recommend which formula to use.

• Hi IgorR,

Thank you for reaching out on UCC28711.

The VDD capacitor is sized to supply sufficient IDD(run) current to the device during initial start-up, and also during the charging phase of the main output capacitors. During the charging phase the bias winding on the transformer must supply the bias power. When VDD reaches the VDD(start) threshold, the device consumes IDD(run) for tSTART(del) before the PWM switching commences. Thereafter, the bias current is the device current plus the MOSFET gate current. The VDD capacitor must support this higher level of current until the output is sufficiently charged that the bias winding rail has increased above the VDD(stop) level.

Calculate the required bias capacitance from the total bias charge associated with the device run current during the tSTART(del) phase (Idd(run)*tstart), plus the device run current during the charge phase, plus the primary MOSFET gate charge current during the charge phase (Idd(run)+fsw*Qg)*Vout*Cout/Iout. The time taken for the output charge phase to reach a sufficient level to supply the bias can be calculated from the size of the output capacitor, target output regulation voltage, and the  maximum load current (assuming that the output capacitor has to be charged whilst also supplying full rated load current). So both the equations which you mentioned hold good, but equation (1) mentioned in the datasheet is more comprehensive to understand. So you can proced with equation (1).

Regards,

Harish