BQ79631-Q1: Negative Differential Readings on Link+/Link– and Charge+/Charge–

Tool/software:

Hi ,

I’ve designed a board using the BQ79631 with the schematic exactly as shown in the datasheet. The device powers up fine, and I’m able to read correct values for HV+ (Battery +) and Fuse+. Below are my measured differential readings (in volts):

VS1 = 0.012 V (Dev 1 Cell 1) → Floating

Battery+:

• VS2 = 4.978 V (Dev 1 Cell 2) → VS2 / TSREF

• VS3 = 2.416 V (Dev 1 Cell 3) → VS3 (HV +)

Link+/Link–:

• VS4 = –4.115 V (Dev 1 Cell 4)

• VS5 = 1.693 V (Dev 1 Cell 5) → VS5 / TSREF

• VS6 = –0.017 V (Dev 1 Cell 6)

Charge+/Charge–:

• VS7 = –1.684 V (Dev 1 Cell 7)

• VS8 = 1.699 V (Dev 1 Cell 8) → VS8 / TSREF

• VS9 = –0.013 V (Dev 1 Cell 9)

Fuse+:

• VS10 = 0.012 V (Dev 1 Cell 10) → VS10 / TSREF

• VS11 = 2.440 V (Dev 1 Cell 11) → VS11 (Fuse +)

Unused:

• VS12 = 4.556 V

• VS13 = –0.000 V

• VS14 = –0.001 V

• VS15 = –0.001 V

• VS16 = –0.001 V

According to the datasheet, for a correct differential measurement:

• VSn – VSn-1 should be between –1 V and 5 V

• Also, VSn > VSn-1

However, with the datasheet-recommended resistor ladder configuration, I can’t see how this is possible — especially since VS2 is referenced to TSREF (as are VS5, VS8, and VS10). This makes it impossible to maintain VS10 > VS8 > VS5 > VS2 in practice.

Because I’m seeing negative readings on some channels (particularly for Charge+/Charge– and Link+/Link–), I’m concerned that:

1. The IC could be damaged over time.

2. It may never be able to measure these points correctly.

I’ve attached my resistor ladder schematic for reference. Any suggestions or clarifications would be appreciated.

Thanks in advance!