BQ76940: Current Sensing Problem

Hi, 

For some reason, your image is not displaying in the post. Can you try posting the image again? 

Can I see a schematic of your SRP/SRN pin connections?

Thanks,

Matt

Hi Matt,

Thanks for your quick reply.

Please find the attachment for current sensing circuit.

Thanks & Regards,

Muralikrishna.

9701620426.BQ76940_facing issue.docx

Hi Muralikrishna,

One side of the sense resistor should connect to BAT- and the other side should connect to PACK-. From the document it looks like the SRN side of the sense resistor is not connected to PACK-, but it is connected through a load to BAT+?

Regards,

Matt 

Hi Matt,

We tested as per your suggestion also, but series 100 ohm resistors are open whenever load is connected.

Please find attached schematic image for reference. Unable to understand why sensing resistors are becoming open. When load is connected.

Thanks & Regards,

Muralikrishna.

9701620426.BQ76940_facing issue1.docx

Hi Matt,

As per attachment i am connecting load across pack- and Bat+.

Thanks & regards,

Muralikrishna.

9701620426.

Hi Muralikrishna,

From your schematic, it looks like you have 6 x 1mOhm resistors in parallel. At 1.6A, the voltage measured should be ~266.7 uV. So you should expect a coulomb counter reading of 0xFFFFFFE0 (a value of -32 since each LSB is ~8.44uV). 

If there is zero current (no load), then the value should be close to 0x00000000, but it can vary slightly so you might see 0xFFFFFFFF (-1) or 0x00000001, or a few LSB of variation. 

Have you verified the FETs are enabled (you need to send a command to enable them)?

Best regards,

Matt

Hi Matt,

Thanks for your support. We are able to read sense current, But still we are facing some problem.

When there is no load it shows +0.1A (i.e., charge current). Is there any calibration to be done in software?

Thanks & Regards,

Muralikrishna.

9701620426.

Hi matt,

Sorry, i forgot to update that i changed shunt resistance from 1m ohm to 10m ohm. Now i am connecting 6 no's 10m ohm resistors in parallel.

Thanks & Regards,

Muralikrishna.

9701620426.

Hi Muralikrishna,

In that case, it sounds like you are seeing around 166uV of offset on your board. You can try applying different currents to verify that the measurement is always offset by the same amount. Since this offset is affected by board layout, it can be calibrated by the host MCU. 

I suggest determining the offset by applying zero current and recording the reading. In your case, I think you are reading a value around 20 which calculates to 0.1A. 

Best regards,

Matt

Hi Matt,

We are trying to fix the variation we are getting in current reading.

The following questions may look like silly but still I got struck in there and am unable to sort it out.
Please help me out with below things.
I have some doubts with my configuration.
1. I want to use 120 amps of current in my application, for that I have decided to use 1.6m-ohm shunt resistor by considering the max SRP_SRN values of 200mv. Is this correct?
2. According to datasheet OCD threshold max value is 100mv and OCD delay is limited to 1280ms.
Is there any possibility to change those values?
3. According to datasheet SCD threshold max value is 200mv and SCD delay is limited to 400us.
Is there any possibility to change those values?
4. Can i control the charge and discharge fets through host micro controller? In case of possibility how we can handle the AFE registers?

Thanks & Regards,

Muralikrishna.

9701620426.