BQ78350-R1A: bqStudio No Acknowledge From Device

Hi Kevin,

Yes, REGOUT at 1.85 could be in the reset range of the BQ78350, this is likely the problem.  REGOUT should be 2.5V, the 2 probable causes would be low voltage input or excessive load on the output.

Check the voltage to REGSRC, it should be high enough that the regulator can provide 2.5V.  For 13 cells I might expect about 12 - 18V.  The voltage across your R81 should indicate current.  Current from REGOUT should be very low..

Check the load (REGOUT) net for shorts or unexpected loads.  It seems to go to relatively few locations. Solder bridges are likely candidates or small resistor in place of a capacitor.

You might also check the voltage of CAP1 on U2, it should be 3.3V.  Also LEDP, it should be slightly less.  A shorted Q24 could damage U2 or U3 due to excess voltage.

If you have multiple boards try another.  If a couple work the same look for some systematic problem.

In the schematic the bottom of C20 is preferred to connect to GND as shown in the data sheet.  It worked OK in the TIDA and is not the cause of your low voltage. 

Hi WM5295,

Thanks for the quick reply!

I tried out a different prototype board and confirmed that REGSRC is at 17V.

Voltage across R81 was 0.455V.

CAP1 on U2 did read 3.3V.

I removed Q24 to rule that out, as I've seen in a previous post that that worked for someone else.

However, I'm still seeing 1.85V on REGOUT.

I'll check for any solder bridges or loads, but so far I don't see any.

Also, I am applying a charge to PACK+ and PACK- to "wake up" the board.
Is there something else I might have to do to wake it up?

Thanks,

Kevin

Hi Kevin,

CAP1 at 3.3V indicates your boot worked.  TS1 is applied to boot, when REGOUT rises sufficiently Q25 turns off Q22 and TS1.  Some users have asked about the small pulse, it will be as long as needed.  Since it is on and you can't talk to the gauge to shut down you might have to unplug the cells to try again if interested.  

455mV on 1k R81 indicates 455 uA into REGSRC.  It is a lot more than the internal operating circuit load, but not a lot for the external load.  It does not seem like a short since REGOUT should be able to provide 20 mA at full voltage, at least 30 mA short circuit.  You might check the IC pins as well as the board traces, or various points on the net to be sure there is not a high resistance solder joint or trace. 

If there are no board faults it seems the BQ76940 is damaged somehow and unable to provide the proper output voltage.  If you have a well behaved supply you may be able to apply 2.5V on the net to see if the BQ78350-R1 gauge functions.  Don't exceed the abs max or it will be damaged.