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bq77PL900 - Reference Design vs. Absolute Maximum Ratings

Jörg Selig
Prodigy 231 points
Other Parts Discussed in Thread: BQ77PL900

Hello,

I just stumbled upon a detail that I previously missed. On the bq77PL900EVM reference design, VC11 is connected to the cell via a resistor, as with the other VCx (SLUU324, p.21). However, in the datasheet, it's absolute maximum rating is stated as -0.3 V .. +0.3 V with respect to GND. Wouldn't this likely be exceeded when the balancing fet is turned on (4,2V * 400 / (400+2*510) = 1,2 V) ?

We used the bq77PL900 in low volume designs with schematics very similar to the EVM (including the forementioned resistor at VC11, that is). There haven't been failures so far. Plus, the EVM will have been rather widely spread as well, and appears to work fine in our case. What am I missing in the calculation above? What is the motivation for the narrow range of VC11 maximum ratings?

Thanks in advance,

Joerg

  • 0
    TI__Guru 54715 points

    Yes, when the part balances the inputs will pull together and the input will rise above the abs max. But the part knows it is balancing and this self induced voltage is OK since it has turned off the safety measuring system.  When the part is measuring it is referencing to VC11 and ground and the input voltage has a more limited range. A voltage above the abs max must not be applied during measurement or a high input current could flow and could damage the part.  The abs max limit is the more restrictive voltage.

    The cell monitor output measuring system is independt of the safety and balancing system but will measure similarly.  It does not have an interlock to prevent operation during balancing, but should not be used during balancing to prevent disrupting balancing and the balance stressing the input.