Critique my LiPo charger circuit current-sense Mod - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: November 21, 2020, 01:24:45 pm
content: Hi all,I want to show you some changes I've made to some SFE (Sparkfun Electronics) designs that I've made. If we start off with the Voltage regulation circuit, first off, I've added a Schottky diode D3 with a low-voltage drop (280mv) on the VBUS USB voltage input line. I've also added the ESD protection chip across the data-lines (D+/D-). This change allows for powering the circuit via EXT_PWR (J5) whilst even leaving V_USB plugged in (although not ideal). The will reverse bias the Schottky diode.Moving onto the Lipo charger circuit: notice that I've addedISENSE1 current-sense amplifier, which connects the V_BATT power via D6 to measure the voltage-drop across a 0R1 current-sense resistor R18 by Vishay. This provides an output voltage at I_BATT that can be measured using an ADC on the microcontroller.D6 mentioned above, is connected via a PPTC F1. Example part for F1:https://export.farnell.com/littelfuse/1206l200pr/fuse-resettable-ptc-1206-6vdc/dp/2119630?st=2APPTC F1 is there to limit current drawn from the LiPo when it is powering the circuit; notice it provides power back to V_IN, which by passes the PPTC F2 connected at V_USB on the V-reg.I've also added a BLUE LED. The way this _should_ work, when V_IN is powered, the LiPO is charged and the Yellow LED is on. However, when V_IN is removed (V_USB), the LiPO now provides power back to the circuit. The BLUE LED should now turn on as well as the Yellow LED.I also realised that adding the Schottky diode D3 on the USB connector, now if a LiPo is already connected it will not pass power back through to the USB connector; although D3 is not vital in this case. It's just there to prevent EXT_PWR affecting any USB connected source.Rest of this circuitry has been taken directly from SFE schematics - do share your thoughts and of course, I'm open to advice and suggestions as well as point out any blaring mistakes I may have made, thanks!Cheers, M.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 21, 2020, 04:48:42 pm
content: Usually, you'll want to measure both the charge and discharge current for more accurate charge level estimation. That's easily done by adding a shunt in series with the low side of the battery and amplifying that with a low offset opamp.I don't see any provision to turn off all LEDs for sleep mode.The Schottky diode from the battery drops enough voltage to make the low part of its discharge curve unusable. That can be solved by replacing it with a MOSFET switched off by the USB power.If you're not leading the USB shield to its own ground, tie it to the USB power ground or provide a provision to do so.

author: bsodmike
author_rank: Regular Contributor
timestamp: February 03, 2021, 02:13:36 pm
content: Thanks for the feedback mate. Yes, I've tied the USB shield to ground now, sorry, that was an oversight.

author: bsodmike
author_rank: Regular Contributor
timestamp: February 03, 2021, 02:23:18 pm
content: Quote from: NiHaoMike on November 21, 2020, 04:48:42 pmDo you mean to replace the Schottky diode D6 with a MOSFET?We'll need the battery to turn the MOSFET on when there is no USB supply, and when USB supply is present, then the MOSFET needs to be turned off.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 03, 2021, 03:04:04 pm
content: Quote from: bsodmike on February 03, 2021, 02:23:18 pmP channel FET will do that with no extra parts.