Voltage drop when using two AP22811B - Page 1
Forum section: electronics / beginners

author: Chillance
author_rank: Contributor
timestamp: November 08, 2020, 03:27:22 am
content: Hi!So, I did the attached design, hoping that would work out where I could control which USB power pin I would use. The idea was to have a simple circuit to just use one USB power line when both were connected. In this case, the VBUS2 would override VBUS1 by setting pin 4 (EN) there on U603 high (which disabled the device), essentially turning that one off. Now, in hindsight, this might have been a bit of overkill, as it might not be much of an issue having both connected to each other? If so, please do let me know and I could get rid of this whole thing.Anyway, my post here is about trying to understand what is happening with the voltage here. Connecting USB and get power on VBUS2 it all works fine. 5V out on pin 1 on U604.However, connecting the other one and getting 5V on VBUS1, I noticed I get about 1V on pin 4 on U603 and pin 5 on U604. Pin 1 on U603 is like 0.5V. And I'm not sure why. It's definitively related to the wiring with the U604, because if I cut the wire where I have the blue "->" arrow thing, it all works great. Can anyone here be so kind and explain why this is happening?Thanks!

author: ledtester
author_rank: Super Contributor
timestamp: November 08, 2020, 08:42:12 am
content: QuoteI presume this scenario is with VBUS2 unconnected, correct?Also, here's the datasheet:https://www.mouser.com/datasheet/2/115/AP22811-1093295.pdfOut of curiosity, what happens if you cut the trace coming out of pin 1 of the lower device U604?For completeness you might test the FLG outputs of both devices - might indicate something.

author: Chillance
author_rank: Contributor
timestamp: November 09, 2020, 10:27:02 am
content: Ah yes, sorry forgot to mention that VBUS2 is unconnected in this scenario with the voltage drop issue. And thanks for providing datasheet.Also, cutting that trace I don't know what would happen yet. I will post here once I decide to do that (because when I do I can't use that connector anymore).I will also measure FLG and post here.

author: ledtester
author_rank: Super Contributor
timestamp: November 09, 2020, 04:10:40 pm
content: I would also perform this experiment:Using a single AP22811B, do not power VUSB but apply 5V to EN. Check the output.You might be powering the device parasitically through the EN pin.

author: Chillance
author_rank: Contributor
timestamp: November 10, 2020, 04:24:00 pm
content: Ok, so FLG is 5V in both cases.

author: Chillance
author_rank: Contributor
timestamp: November 12, 2020, 11:21:51 am
content: Seems like voltage also can go through the device through pin 1 to pin 5, which explains the behavior I'm seeing...

author: ledtester
author_rank: Super Contributor
timestamp: November 12, 2020, 03:54:14 pm
content: Another approach is to use diodes to combine two voltage sources. Schottky diodes can get the voltage drop to around 0.2V.How much current do you intend to draw over USB?Some links:-https://electronics.stackexchange.com/questions/96398/or-ing-power-supplies-diode-or-mosfet/121207-https://www.ti.com/lit/an/snva747b/snva747b.pdf?ts=1605122711710-https://www.daenotes.com/electronics/basic-electronics/diode-in-parallel

author: Chillance
author_rank: Contributor
timestamp: November 13, 2020, 10:17:16 am
content: First of, thanks ledtester for your help.So, I'm thinking the current draw would be max 600-700mA or so... (charging battery of up to 500mA + components).

author: ledtester
author_rank: Super Contributor
timestamp: November 13, 2020, 03:20:38 pm
content: I think the Schottky diode approach will work for you... I'm assuming you are charging a single cell li-ion battery to 4.2V.I would even just try a 1N400x diode to see how it works out.