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BQ77905: BQ77905 sames not work on my board

jiangbin zhu
Prodigy 100 points
Part Number: BQ77905

Dear,

we met some problems while using BQ77905, schematic is put on attachment.

We use three 18650 each with voltage 4.15V serial connected. It's strange that AVDD is 2.4V but VTB output is 0, as I think it should be same as AVDD.

DSG output is somewhat 11.5V, but CHG is 0. Total load current is about 10mA.

I try to find some document on IT web, but only datasheet. 

Please help me on this.

BRs

zhujiangbin

BQ77905 question.pdf

  • 0
    TI__Guru 54715 points
    Hi Jiangbin,
    The thermistor is sampled for temperature measurement as indicated in the data sheet section 8.3.1 and shown in figure 9. VTB is only turned on for the measurement to save power from the battery. It will match AVDD only during the sample. The datasheet does not show detail on the timing, but if you inspect the signal with an oscilloscope you should see it pulse high approximately 80 us every 120 ms. With a meter it will measure about 0 V.
    Since DSG is high, the parts are operating and discharge is enabled. The bq77905 does not have communication to tell its mode, the fault must be determined from the observed conditions. See table 5 of the datasheet for a summary. Since the cell voltage is 4.15V, and for several of the parts such as bq7790500 the OV threshold is 4.2V with a hysteresis of 100 mV. If one of the 3 parts has experienced a cell voltage over 4.2V, it has likely tripped the OV fault and CHG will not turn on until OV has recovered with the cell voltage below approximately 4.1V or CHG will turn on with a discharge current sufficient to trip the state comparator(-2 mV typical). The other possibilities from table 5 are OTC or UTC without OTD or UTD.
  • 0 in reply to WM5295
    Prodigy 100 points
    Thank you very much, I do think it might be in hysteresis zone, so I check battery several times and replaced BQ77905 with other boards. I am not sure if problem is produced when I solder battery on board. It might be complicated while soldering. If it is possible chip make some wrong measurement during soldering or connect battery to board? Such as OV etc... Then enter hysteresis zone
  • 0 in reply to jiangbin zhu
    TI__Guru 54715 points
    Hi Jiangbin,
    It may depend on connection order of the cells. Datasheet section 8.4.1.1 says "If device reset occurs when any cell voltage is above the OV hysteresis range, the CHG driver will remain off until an OV recovery condition is met."
    Since signals are referenced to VSS connecting it first (BATTERY-) is typically recommended. If cells are connected bottom up the part will have normal voltages when power is connected. If connection mates the top cell before a lower cell, the single ended cap on the unconnected input will hold the pin low. The part is likely to detect some combination of OV, UV and OW faults. At 4.15V the UV and OW faults will clear on connection, but the OV would remain until the recovery condition is met.
    I have generally heard packs are assembled with cells at midrange voltage, except during prototyping. Certainly you will need to determine what is possible for your situation. Some suggestions may be to adjust the connection order, load the battery after assembly to drop the voltage for recovery, or discharge the pack to the recovery level after connection.