Hi,

I'd like to integrate a 10440 LiFePO4 (250mAh, 3.7v max, 2.0v min) into a design that can be charged from USB. I'm committing to LiFePO4 for the design as it seems to be the most environmentally friendly chemistry widely available at the moment.

1. PMICs for Li-Ion are readily available for this application, but the charging voltage for most seem fixed at 4.2v, e.g. https://datasheet.lcsc.com/szlcsc/Shanghai-Belling-BL4054B-42TPRN_C83783.pdf . Can something like this be used with any sort of circuit to drop that voltage? If so, what would that look like? I'm unsure whether the charging curve for Li-Ion and LiFePO4 differ significantly so would welcome advice on this.
2. Failing that, can you recommend a circuit/battery charging IC? Looking for minimal cost, performance not an issue.

Thanks in advance,

Dev

 

I should add that I've searched and read stuff including below -

https://forum.allaboutcircuits.com/threads/lifepo4-simple-charger-circuit.53192/
https://forum.allaboutcircuits.com/threads/lifepo4-battery-charging.102455/
https://www.allaboutcircuits.com/projects/create-an-arduino-controlled-battery-charger/
https://forum.allaboutcircuits.com/threads/lifepo4-charger-circuit.76760/

http://ww1.microchip.com/downloads/en/AppNotes/01276a.pdf

 

Ok, been busy with work and then out sick, but here's where I've got to so far.

The three options:

1. Consonance CN3801

Datasheet: http://www.consonance-elec.com/pdf/datasheet/DSE-CN3801.pdf
Seen in a project: https://community.hiveeyes.org/t/modified-3a-1s-lifepo4-solar-mppt-loader-based-on-cn3801/570
Put out an Alibaba RFQ and got back $0.25/pc/1000MOQ. Example circuit and project both look to have a lot of external components so I'm not thrilled about the overall footprint.

2. Microchip MCP73123

Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22191E.pdf
The appnote at http://ww1.microchip.com/downloads/en/AppNotes/01276a.pdf shows a nice simple example circuit.
LCSC has them at just over $1/unit though, which is a bit pricey for what I'd like to do.

3. Padauk PMS132B

Datasheet: http://www.padauk.com.tw/upload/doc/PMS132, PMS132B datasheet_v103_EN_20181128.pdf
This isn't a dedicated charge chip, just a microcontroller with a 12-bit ADC and PWM. I think I can use this in place of an Arduino in the same way as this article: https://www.allaboutcircuits.com/projects/create-an-arduino-controlled-battery-charger/ .
LCSC has them at $0.10/pc for 5. I figure this would need a regulator on the supply, then constant current and constant voltage charge phases can be implemented using PWM, a MOSFET and a smoothing capacitor. Measure the battery voltage, pick a PWM duty cycle according to a data map representation of the charging curve, charge for a few seconds, measure again.

Any pitfalls to look out for? I'm leaning towards option 3 on price alone.

 

Ok, I decided to implement the CN3801 on the basis that I don't yet fully understand/appreciate the charging cycle of an LiFePO4 and don't have the time or resources to develop and test a fresh uC-based charger.

I've essentially copied their example circuit using selected components for my application. I selected a charge current of 160mA (approx 2/3C for the 250mAh cell I have in mind. According to the datasheet this makes the worst-case ripple current approx 80mA.



Selection basis:

1. Capacitor C1: 10uF electrolytic for low frequency filtering, C5 (Datasheet: C2) 100nF for high frequency filtering. I must confess I don't have an understanding of the physics here so I've just picked capacitors in the type and range the datasheet mentioned.



2. Capacitor C2 (Datasheet: C3): The project above at https://community.hiveeyes.org/t/modified-3a-1s-lifepo4-solar-mppt-loader-based-on-cn3801/570 has this at 22uF so I've adopted that.

3. Inductor: When the battery is "dead", its voltage is 2.0v, which means the largest differential from the 5V USB supply is 3v.
From the second formula below, this means an inductance of 15uH. From the first formula, this gives a ripple current of 0.267A. Does this mean I need an inductor rated for 160mA+267mA = 427mA?




4. R8 and R10 (Datasheet: R3 and R4). I've used R3=16K, R4=3.9K to fix Vmppt = 5V - same as USB supply.