I really need to get past the glitch that occurs on PMID output, between the time that VBUS gets disconnected and the Boost Mode is operational. This seems to be at most a 30ms dip from about 5V to about Vbat, then back up to about 5V again. (My scope has been showing about 33ms.) LOL, this timing is by design, but I still call it a glitch on the output! Sorry...
I haven't worked with supercapacitors before, but I'm thinking about adding one. Using the Eaton calculator at https://www.eaton.com/us/en-us/products/electronic-components/supercapacitor-calculator.html I have found a series connection of 2 pieces HV0810-2R7105-R ("string") might solve my problem. I'd like to double check that here on this forum. I've attached the Eaton calculator results.
QUESTION ONE: What happens if I momentarily short the PMID pin to ground? If that's not a problem, then I might connect this supercap string directly to PMID. It has ESR about 0.8 ohms, and so would try to draw 6.25A from PMID if 5V initially, assuming the caps have zero charge (voltage) on them and VBUS was just connected. PMID would droop, of course, but only momentarily. Once the cap string charged half way, that current would only be 3.1A, and then progressively less after that.
Later, when VBUS disconnects, the Eaton calculator suggests the supercap string can supply my needed 800mA for more than 35ms, after which the Boost Mode gets going, both supplying my 800mA and recharging the supercap from at-that-time only slight discharge.
QUESTION TWO: If that initial 6.25A is a problem, I wonder about placing a couple parts between PMID and the supercap string. My load would still be connected directly to PMID. The couple parts would be a 1.4 ohm 1W resistor for charging the supercap, plus in parallel with that a Schottky diode for discharging the supercap with less voltage drop than would be over that 1.4 ohms. I have two diodes in mind, which drop either 0.35V or 0.53V at 800mA. My calculations suggest that the supercap with intervening resistor would draw no more than 3.1A from PMID, even when the supercap is fully discharged. Meanwhile, the supercap with intervening diode would provide at least 3.72V to my load. (That's 4.25V supplied by supercaps at 40ms, per attached pdf, less additional 0.53V diode drop.)
Am I crazy or might this get me past the glitch problem? Thanks very much for assistance.
EatonSuperCapCalculatorResult - Two HV0810-2R7105-R in series.pdf
