BQ35100: Current sense resistor and current measurement

Stefano Infante

Part Number: BQ35100

Hello,

on a design I am planning to use BQ35100 as a battery gauge. My battery will be a LiSOCl2, C type. The device current will vary from about 5-6 uA (considering BQ35100 sampling every 8 hours) and about 100 mA.

What sense resistor value do you suggest to use?

According to the formula in 8.2.2.5 of the device's data sheet I should use about 1 mohm; is it ok? Can I measure currents as low as a few microamps with such a resistor?

And is BQ35100 good for current measuring? (I only need an estimate of system power consumption)

Thanks in advance.

Best regards

Stefano

over 5 years ago

0 Andy Liu (BMS) over 5 years ago

TI__Guru 52115 points

Hi Stefano,

According to the equation in the bq35100 datasheet, the sense resistor should be 125mV/100mA = 1.25 ohm.

I would suggest you use a bit lower value so that you can have a margin. For example, 1ohm.

Andy

0 Stefano Infante over 5 years ago in reply to Andy Liu (BMS)

Expert 2320 points

Hi Andy,

you are right, it's 1 ohm and not 1 mohm.

But if I use 1 ohm, will I be able to read currents between a few microamps and 100 mA?

Looking at the data sheet, it seems to me that the current ADC has 16 bits and is signed. So I should only consider 15 bits for positive currents. Hence the ADC resolution should be 125 mA/32768 = 3.8 uA. Is it correct?

Thanks again

Stefano

0 Andy Liu (BMS) over 5 years ago in reply to Stefano Infante

TI__Guru 52115 points

Hi Stefano,

Actually you can find the following ADC resolution in the datasheet.

0 Stefano Infante over 5 years ago in reply to Andy Liu (BMS)

Expert 2320 points

Thanks Andy.

So the granted full resolution I should consider is 13 bits (because current is a signed number for the ADC, and my current is always positive), isn't it?

0 Kang Kang over 5 years ago in reply to Stefano Infante

TI__Mastermind 24870 points

Hello Stefano,

No, it doesn't mean current is always positive. It is going to be bi-drectional.

0 Stefano Infante over 5 years ago in reply to Kang Kang

Expert 2320 points

Hello,

as I am using a primary battery, the sign of the current never changes. Hence, if the ADC is bidirectional, actually I lose 1 bit. I just need to confirm this fact, so that I can do my calculations on the basis of an actual 13 bits ADC resolution.

Thanks

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BQ35100: Current sense resistor and current measurement