Electronics > Beginners
Please review my lipo charger and step down schematics
online1234:
Hello,
I'm a total noob to circuit design. I'm trying to make a pcb that charge lipo battery and step down to 3v to power a keyboard controller.
I follow the schematic of the Adafruit lipo charger and added MCP1700 to step down from 3.7 to 3v.
I'm not sure the placement of the on/off switch, I read it somewhere saying the components consume power when it's not plugged in, is it true?
I'd like to switch off the keyboard when it's not in use but still able to charge the battery.
Any opinion would be appreciated!
mariush:
A keyboard controller will use very very low power when idle.
The on/off switch will be nearly pointless., unless you're planning to also have rgb leds or something more power hungry.
You'll save way more power by using a switching regulator optimized for low current instead of a linear regulator.
With linear regulator, you have 3.7v in, 3v out ... that's 81% efficiency, 20% of battery will be lost as heat.
Here's an example: https://www.digikey.com/product-detail/en/diodes-incorporated/AUR9713AGH/AUR9713AGHDICT-ND/8545833
You'd get up to around 90% efficiency and it has an enable pin (connect to input voltage to turn on, to ground to turn off)
See page 12 for example schematic : https://www.diodes.com/assets/Datasheets/AUR9713.pdf
It may feel scary and difficult for a beginner but they're really not hard .. and being a surface mount part is also not a big problem, you can bend the leads to make more room between leads.
Here's something a bit more expensive, but can be up to 96% efficient: https://www.digikey.com/product-detail/en/on-semiconductor/NCP1521BSNT1G/NCP1521BSNT1GOSCT-ND/1560581
Here's datasheet: https://www.onsemi.com/pub/Collateral/NCP1521B-D.PDF
You have the example circuit on first page (similar to the other chip's circuit) and detailed notes about how to choose components for a particular voltage output in another page in the datasheet.
Peabody:
I don't have a problem with the linear regulator. Switching regulators tend not to be very efficient at very low currents. You don't have a big voltage drop here, and the regulator has a 1.6µA quiescent current, which is pretty good.
You can charge the battery when the switch is turned off, but may not be able to do so safely when it's turned on. You would be drawing current for the regulator and controller through the charger. If that current is too high, the charger will not be able to sense that the battery is fully charged, and may continue charging. That's not safe for lithium batteries, which cannot be trickle-charged. It depends on the current needed for the load, but in the worst case you would have to turn the switch off while charging the battery.
What's needed is a "load sharing" circuit, which both charges the battery and powers the load, independently, when USB is plugged in, so that battery charging can proceed properly to termination. You might see if Adafruit has a charger which uses the MCP73871, which includes the load sharing circuit. Info on load sharing can be found here:
http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf
Kasper:
I agree with Peabody's post.
That's how Adafruit does it on some of their feather boards. Check the Adafruit feather schematics of you want more part numbers than what is provided in that app note.
Nusa:
And when doing efficiency math, remember that what you label as 3.7V is a nominal voltage. Reality is going to be over 4V much of the time, rather plugged in or not.
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