BQ79656-Q1: CS ADC and MAIN ADC current sense

Yuto Kitamura

Part Number: BQ79656-Q1

Tool/software:

Dear team,

Please help us to understand CS function of BQ79656.

Q1: Would I know the full scale range of CS delta-sigma ADC? Page#9 8.3 mentions it is VCS_RANGE = +/-100mV, but 9.3.2.4 mentions it is +/-125mV.

Q2: About 9.3.2.4, it mentions as shown below screen shot. Does it mean CURRENT_HI/MID/LOW will be below table? And How will it be when it is lower than -125mV (-100mV.)

< -125mV : -125mV (=0x000000) or -75mV
-125mV ~ -100mV: -125mV ~ -100mV
-100mV ~ +100mV: -100mV ~ +100mV (high accuracy)
+100mV ~+125mV: +100mV ~+125mV:
> + 125mV：75mV

Q3: Related to Q1, can BQ79656-Q1 measure current with CS ADC in accuracy within the range of +/-100mV, and relatively lower in -125mV to -100mV and 100mV to 125mV?

Q4: What is the 0x0000 of MAIN_CURRENT_HI/LO? Its LSB is 30.52uV but not sure its absolute value.

Q5: Same for CS ADC and CURRENT_HI/MID/LOW, its 0x000000 would be -125mV? which means 0x000fff is 0V?

Best regards,
Yuto

6 months ago

0 Zachary Balov 6 months ago

TI__Expert 4470 points

Hello,

Thanks for your question. I will get back to you later this week.

Thank you,
Zachary

0 Yuto Kitamura 6 months ago in reply to Zachary Balov

TI__Expert 6505 points

Hi Zachary,

Thanks for comment. Wait for your update.

0 Yuto Kitamura 6 months ago in reply to Yuto Kitamura

TI__Expert 6505 points

Following up here. Kindly please response back once you see this comment.

0 Zachary Balov 6 months ago in reply to Yuto Kitamura

TI__Expert 4470 points

Hi Yuto,

I apologize for the delayed response.

1) Abs max id +/- 125mV, +/- 100mV is the recommended operating range

2) If the range falls outside the abs max limits, the reading is clamped at 75mV (this is a differential measurement)

3) 10.2.1.2.7 in the datasheet describes the use of the 2 ranges

4 and 5) The readings are absolute and in 2s compliment, so the conversion is very straight forward.

Thanks,

Zachary

0 Yuto Kitamura 6 months ago in reply to Zachary Balov

TI__Expert 6505 points

Hi Zachary

1) Clear.

2) Differential measurement still read negative value - right? how will it be incase <-125mV or >125mV, 1xxxxxxxx or 0xxxxxxx?(sorry for not calculating 75mV in binary)

3) Clear.

4) Sorry for missing the word of 2s complement in datasheet. I understood 2s complement is signed binary. So the 7th-bit of MAIN_CURRENT_HI and CURRENT_HI explains the sign(positive=0, negative=1) - Is it right?

Best regards,
Yuto

0 Yuto Kitamura 6 months ago in reply to Yuto Kitamura

TI__Expert 6505 points

Hi Zachary,

would you please double-check my query?

+1 Zachary Balov 6 months ago in reply to Yuto Kitamura

TI__Expert 4470 points

Hi Yuto,

Yes, I apologize.

2s compliment is signed binary, correct. Negative values will start with 1xxxxx.

The datasheet describes how to read these values from the registers clearly, so it is your best resource for this.

Thanks,

Zachary

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BQ79656-Q1: CS ADC and MAIN ADC current sense