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BQSTUDIO: CC Gain < 1mOhm

Jeffrey Wright
Expert 2875 points
Part Number: BQSTUDIO
Other Parts Discussed in Thread: BQ78350-R1

Hello;

As per the 78350 TRM, CC gain can be set as:  CC gain = Ical/(ADCcal - offset), bqStudio displays this in units of "mOhms" (despite this equation results in 1/R?)

So assuming Rshunt = 1mOhm, a 100A current would produce 100mV at SRN, resulting in a CC gain of 1000 (which is displayed in Studio as 1/1000 Ohm).

Now, due to the large currents to be measured in my client's application, it uses a 100uOhm (0.1mOhm) shunt R. I therefore want to scale the current by x10 to stretch the measurable range to ~320A. In addition, to maximize use of the available current range on the 920, the shunt voltage is amplified x3.74, allowing measurement of ~267A, beyond which the 920's OLD threshold (100mV) would open the discharge fet.  My understanding is that this could be achieved by setting the CC gain to 0.374mOhm (in bqStudio)...

….but it seems that bqStudio will not allow setting the CC gain to any value below 1mOhm ? 

For example, when I simulate 100A of charge current (SRN = 37.4mV), the 78350 reads ~4431 (0x114F) from the 920.  And with a CC gain of 1mOhm (apparently default in bqStudio), the 78350 assumes this voltage represents a current of only 37.4A (37.4A x 0.001mOhm).

As a result, I need scale the current to fool the 78350 into thinking this is 100A, which again means (my understanding), setting the CC gain to 0.374mOhm (?). But this cannot be done using bqStudio??

What am I missing?

Thanks.

  • 0
    TI__Guru 54715 points

    Hi Jeffrey,

    The device will have a limited range which should be in the TRM.  BQStudio can also place a limit on the range of values it can except for a parameter.  Other than the IPScale which is supported in BQ78350-R2 current scaling is typically done external to the gauge with an understanding between the user and the system what the values mean.  

    The present experts on this part are out of the office this week.  I expect it will be the middle of next wee before we have a better answer.

  • 0 in reply to WM5295
    Expert 2875 points

    Thanks WM5295; 

    I'm now as flummoxed as ever: I've read in the TRM, and been told by TI definitively, that IPSCALE is broken on -R1 firmware, and so I absolutely must update to R2 to use IPSCALE. But now my client tells me that they are using -R1a F/W, and IPSCALE works OK...?? They are able to scale x10 by setting bit-12 in SpecificationInfo(), just as I've been doing with R2 F/W.

    What am I (or they) missing?

  • 0 in reply to Jeffrey Wright
    TI__Guru 54715 points

    Hi Jeffrey,

    Correct, it has been noted several times that IPScale does not work properly in BQ78350-R1.  The post https://e2e.ti.com/support/power-management/f/196/p/512931/1862266 shows some links for manual scaling with gauges.  From the posts I've read on the topic the IPScale bit will set, but the currents are not scaled properly.  The release notes for -R2 indicate the IPScale was fixed, but does not give details on the improvement. 

  • 0 in reply to WM5295
    Expert 2875 points

    Thanks WM5295;

    Since my client insists it works OK, is it possible that the bug was also fixed in the -R1a release (1.04)? They updated to -R2, and then moved to -R1a which Matt indicated fixed the flash corruption bug. Perhaps it did also address the IPSCALE bug???????????

    Instead of an R2, there will be an R1A device released. It is identical to R1 except that the Shutdown routine is more robust to prevent the possibility of flash corruption. The R1A is the latest version of R1 firmware that was released to ti.com back in February. I do not have a schedule yet (I predict ~3 months), but this will give a couple options.

     

    -          Write the R2 firmware in production

    -          Write the R1A (v1.04 firmware) in production and then switch to R1A when the orderable part is available

  • 0 in reply to Jeffrey Wright
    Expert 2875 points

    It looks like my guess may have been right - After flashing 1.04, and using IPSCALE, the 78350 correctly reports the scaled current, and the OCD and OCD alarm thresholds appear to working correctly.

    Please note that, just as with R2 F/W, many of the DF addresses have changed in R1a.