Hello folks,
I am running into an issue with my battery protection circuit that I am having trouble making progress on, and I am hoping that some of you can give me new ideas for things to try.
To start off with, my high-level circuit goal is to have a Li-Ion coin cell battery light up 4 (or more) LEDs. The coin cell needs to be recharged (I am using the Microchip MCP73831, which works beautifully) and protected from the standard undervoltage / overcurrent.
The issue that I'm hitting involves the protection circuit not allowing full current. This circuit is based around the Diodes Inc. AP9101C, specifically the AP9101CK6-CQTRG1 with Vm sense = 0.15v (see page 14 on the datasheet, all the way at the bottom). Here is the datasheet
https://www.diodes.com/assets/Datasheets/AP9101C.pdf.
My design goals for the protection circuit are:
- Undervoltage cutoff at 2.8v
- Overcurrent cutoff <= 200ma
- Normal current supply ~35ma, perhaps more if I want to add a few more LEDs
- NOT as concerned about overvoltage, since the charger IC handles this
My circuit is identical to the "Typical Applications Circuit" on page 2 of the datasheet. I'm using all of their recommended values as well for R1, C1, and R2. My load (4 LEDs nominal, or sometimes just a test resistor) is across P+ and P-.
The current that I measure with just my LEDs and battery is 26mA. Then when I hook up the protection circuitry, and short P- to Vbat- to wake it up from power-down mode, I measure a current of only 17mA.To investigate further, I did the following tests (quoted from an email to Diodes Inc):
Here are the measurements when the circuit is identical to the "Typical applications circuit" on page 2 with 100 mOhm RdsON MOSFETs for Q1 and Q2.
With 4 parallel LEDs:
Vdd = 3.6v
Vm = 1.13v
Co = 3.59v
Do = 0.11v
With 10kOhm dummy load:
Vdd = 3.61v
Vm = 2.43v
Co = 1.90v
Do = 0.24v
With 1 MOhm dummy load:
Vdd = 3.61v
Vm = 0.22v
Co = 3.6v
Do = 0.51v
On the face of it, it makes sense that the current is being limited, because the current-sense pin, Vm, has a voltage higher than the overcurrent cutoff value of 0.15v in each case. But what I don't understand is
why this is happening.
From the best of my reasoning, the voltage on the Vm pin is dictated by the current, and the RdsON values of the MOSFETs. If this is true, with my tests I would expect a value for Vm to be, at most:
Vm = 5.2mV = 26mA * 100mOhm/MOSFET * 2 MOSFETs
I've tried 3 different MOSFETs with different RdsON values. Something like 80 mOhm, 100 mOhm, and 1 Ohm. Furthermore, each of these MOSFETs have Vthreshold values < 2.8v (undervoltage cutoff).
Here are the MOSFETs I have tried:
https://www.digikey.com/product-detail/en/on-semiconductor/NTR4003NT1G/NTR4003NT1GOSCT-ND/1967270https://www.digikey.com/product-detail/en/diodes-incorporated/ZXMN3B14FTA/ZXMN3B14FCT-ND/1211599
https://www.digikey.com/product-detail/en/on-semiconductor/NTR4501NT1G/NTR4501NT1GOSCT-ND/687133I can hook up my LEDs and battery to the MOSFETs, and simply tie their gates to Vbat+, and the circuit performs as expected, full LED brightness.
Also, I can take Vm pin and simply tie that to Vbat- and the circuit provides full current and undervoltage protection. This makes sense because Vm = 0v < 0.15v so it thinks there is no current flowing.
I guess I'm just stumped here, because it seems like it should work, but it does not.
Are there some other MOSFET properties that I am neglecting to factor in?
Do ya'll have another battery protection circuit that you've gotten to work that I should look into instead?
I'm doing this on a breadboard, could this be a factor?
Anyway, any help is greatly appreciated. Please let me know what additional information you would like.