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BQ29200: BQ29200 use with an under voltage cutoff IC

schwac
Prodigy 130 points
Part Number: BQ29200

Hello, we are prototyping some battery management circuits that use a combination of an over/under voltage protection IC with the BQ29200 for cell balancing on a 2S 18650 LIPO pack. We are effectively using the circuit in figure 11 of the datasheet. One issue we are having though is that when the under voltage cutoff IC trips, it uses a low side FET cutoff, so that Pack+ and Pack- become 0V, (due to the load on the pack pulling pack+ to pack-) but the mid cell input into VC1 stays more negative than both of them (about -2.5V). This is a direct violation of the BQ29200 datasheet. Is this likely to cause a problem? Are there clamp diodes on this input that would cause current to flow from the -2.5V mid cell voltage to the Pack- voltage?

We have also added a shotkey diode to the low side balance FET to block that reverse path.

Our other option would be to move the BQ29200 to the pre-undervoltage cutoff side of the pack, which would keep the cell voltages at proper levels in relation to each other, with the negative effect of the BQ29200 IC consuming it's Icc supply current and VC2 input consumption current would in theory damage the cells over time. This should be approx 3uA + 2.5uA = 5.5uA, which would then take over a year to drain 100mAH from a battery, which seems acceptable. Is connection of this IC to bare un-protected cells the typical recommended design? 

Thank You!

  • 0
    Prodigy 130 points

    Hello, would someone from TI please take a look at this soon? Thanks in advance!

  • 0 in reply to schwac
    TI__Guru 54715 points
    Hi Cory,
    What is probably happening is that with PACK- disconnected the supply current of the bq29200 or other load pulls PACK- up to PACK+. Current then flows out VC1 and/or VC1_CB through internal paths and loads the upper cell. With the various external resistances the current may not be damaging but likely does create voltages which violate abs max of the inputs and the currrent is undesirable for the pack. There are no special diodes in the part to prevent damage from this situation, but even if you add schottky diodes externally to limit the voltage the current would still be undesirable on the cell.
    Yes, the design of the device expects that it is always connected to the cells, if protection is used that it be on the right side of the data sheet figure 11. Additionally you may want to disable cell balancing during low voltage.