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BQ76952: Extra current on REGIN

Alex LeBlanc
Prodigy 90 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO

I've got a design that is quite similar to the BQ76952 evaluation board. 

We use the REG0 (NPN) pre-regulator and we configure REG1 for 3.3V. REG2 is unused. REG1 is actually only used to enable a switch mode power supply.  

Using bqStudio, I import the default settings, then set the bit to enable REG0, and set the bits to enable REG1 for 3.3V. I can see REG1 turn on, 3.3V as expected. REGIN is ~5.5V as expected. 

However, I'm seeing about ~22mA flowing into REGIN (measured at the resistor that is in series with the NPN). At first I thought it was REG1 getting loaded down by something, so as a test I disabled REG1 and enabled REG2 instead (to make sure REG0 is enabled). 

With REG1 disabled and REG2 enabled, I see REGIN=5.5V, REG1=0.0V, REG2=1.8V (which is what I set it to), but I still have that ~22mA flowing into REGIN.  Since REG2 isn't connected to anything else, that current has to be going somewhere in the chip.

The voltage at BAT is higher than REGIN, so it's not that diode that the datasheet makes reference to.  

Can you think of any reason there would be ~22mA flowing into REGIN? 

Here are screenshots of the registers and data memory, in case that's any help:

  • 0
    TI__Guru* 81034 points

    Hi Alexandre,

    The REGIN current should be equal to the current on the REG1 and REG2 outputs. Can you share your REGIN circuit and show how you are measuring the 22mA?

    Are you observing the same thing on the EVM?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 90 points

    Hi Matt,

    I can't share too much of the schematic, but here's the REGIN circuit:



    The 22mA is measured by the voltage drop across the 220R (which was ~4.9V). The note on the NPN's collector that you see I cropped out just goes out to a 1uF capacitor (1206 100V). 

    I tried shorting out the 220R resistors as well, and the current going into the board (as per my bench supply) appears to be unchanged, REGIN stays at 5.5V, and of course the collector voltage is higher than it was before shorting the resistors (i.e. 4.9V higher). 

    REG1 goes to an enable pin for another supply. REG2 is floating.

    When I disable the regulators, the pre-regulator stops as expected and the current going into the board drops to ~0mA. 

    On a side note, the latest stable version of bqStudio doesn't include fw version running on the BQ76952; I had to download a test version of bqStudio (this test version auto-detected). 

    Thanks,
    Alex

  • 0 in reply to Alex LeBlanc
    TI__Guru* 81034 points

    Hi Alex,

    That is correct, the BQSTUDIO-TEST version is needed for this device. The voltage drop you are seeing across the 220R is very strange. I just tested this on my board and measure 0.1mV with both REG1 and REG2 enabled. The current should match the current load on REG1 + REG2. 

    Is there anything else on your schematic that could be pulling current from REG1? Do you have a 150R resistor anywhere?

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 90 points

    Hi Matt, 

    There isn't anything else on REG1 (I just use it to drive the enable pin on another supply). I had stuffing options to power the board with REG1, but none of that is stuffed. Even when REG1 is disabled, when I enable REG2 (REG0 won't turn on unless either REG1 or REG2 is enabled), I still see the current flowing through (REG2 which is not connected to anything at all and is just floating). It's behaving like the BQ76952 internally has ~250ohms between REGIN and ground. 

    I see in the datasheet that there is a diode between REGIN and BAT, so if BAT was somehow grounded, I suppose you'd have extra current going into REGIN, and there is a 100R series resistor going to the BAT pin, which would limit this current. But I checked across this resistor and it's only ~30mV, so 0.3mA. And BAT is also well above REGIN. 

    It's sounding like something might be broken with this BQ76952. I'm hesitant to try another board considering how difficult it is to get parts/samples (lead times are into 2023 everywhere). 

    Thanks,
    Alex

  • 0 in reply to Alex LeBlanc
    TI__Guru* 81034 points

    Hi Alex,

    I agree it does sound like something is broken on the BQ76952 device. Do you have another device you could replace on the same board?

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 90 points

    We've only got three chips, all of which were pulled from evaluation boards. We haven't been able to get samples anywhere. If I had a few samples I would experiment a bit more. 

  • 0 in reply to Alex LeBlanc
    TI__Guru* 81034 points

    Hi Alex,

    I am going to have someone reach out to you to see if we can get you a few more samples. Sorry to hear you've had to pull parts from EVMs!

    Best regards,

    Matt