Author Topic: lithium battery protection ic questions  (Read 275 times)

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Offline [steven]

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lithium battery protection ic questions
« on: June 13, 2020, 11:43:19 pm »
I'm working on a design that uses a protection IC for a single lithium cell. When looking at some cheap chinese protection IC's I came across the app circuit in the datasheet for the fs312f-g:

Something immediately stuck out to me: the current sense pin (cs) is connected with a 1K resistor to BATT-. This doesn't make sense to me, from my understanding a current sense pin should measure the voltage across a shunt resistor wired in series between the negative terminal of the battery and the source of M1. This would also make more sense if you look at pin CS in the functional diagram of the chip.
Another weird thing is that if the charger voltage across BATT+ and BATT- is above around 4.3v the mosfet pair will isolate the battery from the charger since there is an overvoltage situation. Normally the IC will monitor the charger voltage until it has dropped to the release voltage. But how is the chip supposed to measure this if it doesn't have a reference of  BATT- anymore since the mosfet pair isolated this pin from the chip? (via CS somehow?).

Apparently there are more protection IC's with similar app circuits:
https://www.digikey.com/en/maker/blogs/lithium-ion-cell-protection

I have worked with an MM3722 protection chip from Mitsumi before which has the following app circuit:

This makes a lot more sense to me: pin CS measures the voltage across shunt resistor Rsns and pin V- is used as a reference for the release voltage.

Why does the fs312f-g not work like this?
« Last Edit: June 13, 2020, 11:46:34 pm by [steven] »
 

Offline [steven]

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Re: lithium battery protection ic questions
« Reply #1 on: June 14, 2020, 12:16:26 am »
 |O already found it, the drain source on-resistance is used as a shunt resistor
 

Offline eblc1388

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Re: lithium battery protection ic questions
« Reply #2 on: June 14, 2020, 01:08:55 am »
I'm glad you have found the answer to your first question.

If you examine the circuit I have posted, actual load current passes through the two MOSFETs from the BATT- terminal back to the physical battery -Ve terminal(also GND). Due to the ohmic nature of the conducted MOSFETs M1 and M2, a voltage(Vx) will develop proportional to the current, between the GND and CS pin. The 1K resistor at the CS pin is connected to the input of a voltage comparater, which is high impedance and thus its value does not matter much. It is not a current measuring resistor.

W.r.t. your second question, the IC actually monitors the physical battery voltage between VCC and GND and this voltage is available regardless of the On/Off of the MOSFETs.

Hope these clarification helps.

 
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Offline [steven]

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Re: lithium battery protection ic questions
« Reply #3 on: June 14, 2020, 12:43:12 pm »
Thanks for the explanation, it's all clear to me now.
 


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