BQ40Z50-R2: Role of 100pf Cap in Drone BMS Reference Design?

haku15

Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ27520-G4

Hello,

looking at the schematic of the TI Drone BMS Reference Design http://www.ti.com/tool/TIDA-00982 ,

I am not so sure what is the role of C32 (100pf 50V ceramic cap) marked in red below and part of the sense resistor circuit marked in yellow.

That does not appear in BQ40z50-R2 Datasheet or in its TI evaluation board.
I haven't found any explanation in related docs either.

Thanks for your help

Sincerely

over 4 years ago

0 Kang Kang over 4 years ago

TI__Mastermind 24870 points

Hello Haku,

It is a filter for the coulomb counter.

0 haku15 over 4 years ago in reply to Kang Kang

Prodigy 80 points

Hello Kang,

thanks for your help.

Could you please explain more? Why that capacitor is missing in the BQ40z50-R2 datasheet and in its evaluation board?

From http://www.ti.com/lit/gpn/bq40z50-r2 :

Thanks

0 Kang Kang over 4 years ago in reply to haku15

TI__Mastermind 24870 points

Hello,

This is an optional circuit as you can see in some of our other documentation.

0 haku15 over 4 years ago in reply to Kang Kang

Prodigy 80 points

Kang,

could you please write some links to that documentation?

I can't find it and I'd like to learn more.

Thanks

Sincerely

0 Kang Kang over 4 years ago in reply to haku15

TI__Mastermind 24870 points

Hello Haku,

Can you check the bq27520-G4 EVM user's guide?

We don't really have any documentation to show why it is optional, it is in our backlog of content.

0 haku15 over 4 years ago in reply to Kang Kang

Prodigy 80 points

Hello Kang,

thanks again for your help.

I've looked in the bq27520-G4 EVM and found no info.

However I've seen this in the datasheet https://www.ti.com/lit/gpn/bq27520-g4 :

"The filter network at the input to the coulomb counter is intended to improve differential mode rejection of voltage
measured across the sense resistor"

The capacitor is in this case 0.1uF, not 100pF, so I don't know if the same applies to bq40z50-R2.

Is that what you meant, Kang?

Good to hear that you have more documentation coming.

I really appreciate the efforts that TI puts into documentation and training: I've learned a lot from it.

Thanks

0 Kang Kang over 4 years ago in reply to haku15

TI__Mastermind 24870 points

Hello Haku,

Yes, that capacitor is used and acts similar to a low pass filter for the differential network. It is optional because the coulomb counter integrates Qpass and we are only concerned about the integrated charge and not the fast reaction charges. I have seen designs with both capacitor and without capacitor. I would recommend to leave it there if you are not restricted by board space.

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BQ40Z50-R2: Role of 100pf Cap in Drone BMS Reference Design?