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Original question:

BQ76940: BQ76940 VC5X issue

BQ76940: What is the purpose of R58 and R59 on BQ76940EVM ?

Bkk
Prodigy 145 points
Part Number: BQ76940

Hi,

1) I would like to ask that what is the purpose of R58 (1k) and R59(1k) on BQ76940EVM. 

I'm asking because, I have seen abnormal current on this line (Example: C10 to VC10X). Therefore, I see changes in cell voltages(Voltage unstability on C10, C11 C12 C13 C14 C15). The problem goes away when I change the resistor(Example R58) with 0 ohm resistor . So I want to know that how important is this resistance?

2) What are the potential negative effects of using 0ohm resistor instead of 1k resistor(R58) on my design?

3) Do you have any idea for voltage unstability? For more details(parts of schematic, etc.) we can use private messages on e2e forum.

Thanks a lot.

  • 0
    TI__Guru 54715 points

    Hi User,

    1. R58 and R59 are the power filter resistors for the intermediate power pins VC10X and VC5X.  These are important to provide a reference voltage for operating the cell groups.  The large Cf filter capacitors provide a reasonably stable operating voltage for the part.  The large value of Rf is required to keep the part alive during short circuit events.  See the application note https://www.ti.com/lit/pdf/slua749 section 3.  R61 and R62 on the EVM are current measurement shunt positions in case the R58, R59 would have been an odd value, they are not needed in a design.

    2. Setting Rf to 0 will almost certainly cause the part to shut down during a SCD event and may delay turn off the the FETs.

    3. The VC5X and VC10X voltages will vary somewhat during operation.  One of the most apparent is the thermistor temperature sample taken every 2 s.  The TS3 sample will pull up on VC10X, the TS2 sample will pull down on VC10X.  With room temperature (10k) the current should be about 165 uA or 165 mV, more when the thermistor is hot.  Similar effect will occur at VC5X, they are not synchronous.  It is unusual to have these affect cell voltage other than possibly the top cell of each group during that sample.  Check the recommended values in the data sheet or EVM as an example.