Author Topic: RC filter on battery  (Read 239 times)

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Offline Nikos A.

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RC filter on battery
« on: April 23, 2021, 05:44:39 am »
Hi everyone,

I am looking the BQ29700 battery protector from Texas Instruments
https://www.ti.com/lit/ds/symlink/bq2970.pdf?ts=1619080532853&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ2970%253FkeyMatch%253DBQ2970%2526tisearch%253Dsearch-everything%2526usecase%253DGPN

The datasheet proposes an RC filter on the battery and they right that "An RC filter is required on the BAT for noise, and enables the device to operate during sharp negative transients. The 330-Ω resistor also limits the current during a reverse connection on the system."

They are referred to the noise coming from the charger? What could occur the sharp negative transients? Does the 330ohm resistor act something like reverse voltage protection by limiting the current?



Thanks
Nick
 

Offline Stuart Coyle

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Re: RC filter on battery
« Reply #1 on: April 23, 2021, 08:57:10 am »
They are referring to noise coming from either the charger or load. The filter stops high frequencies from affecting the voltage reading at BAT, which could otherwise cause switching of the cout or dout when you don't really want it. By sharp negative transient, imagine what happens when you connect a load that may take a lot of inrush current, thus dropping the voltage very quickly. The FET would turn off and you would not get a clean startup. The resistor is part of the filter. Not much current ever flows into the BAT terminal as it is just the input to a pair of comparators, which will be very high impedance.
 
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Offline Nikos A.

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Re: RC filter on battery
« Reply #2 on: April 23, 2021, 09:35:41 am »
Thank you for your great answer Stuart Coyle!!

Not much current ever flows into the BAT terminal as it is just the input to a pair of comparators, which will be very high impedance.

So, no additional reverse voltage protection is needed (let's say a MOSFET)?
Something like this:

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