Cisco would like to enable the Brownout feature with the UCD90120A. How large does the capacitor have to be on the 3.3V power rail of the power sequencer chip to keep the voltage high enough for writing the failure log to flash memory? Cisco would like to confirm " if it takes 80 ms that means the cap have to be about 5000uf at least, am I right?" appreciate your input .. thanks.
Hi Raj,
The firmware needs 5ms to write the data. At 2.9V, data write starts and at 2.6V data write should be complete. Use enough capacitance to keep the supply above 2.6V so the write can complete.
C = I x 0.005 / 0.3. Current (I) depends on device loading. The datasheet mentions 80mA so C = .08 x 0.005 / 0.3 = 1333uF. At more typical currents (55mA), C should be less than 1000uF.