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BQ77915: Desing Check Request

Tugba YILMAZ
Prodigy 70 points
Part Number: BQ77915

Hello, you will find a schematic design below. Before send this schematic to manufacturer I want to ask couple of quesitons. 

1. Is it okay if you can check and tell me if theres something wrong with it.

2.  In this pic, you can see R5, 20Ohm resistor. Well the spec of this resistor is 20 Ohm 0.5% 1W 1206. Is it suitable?

3.  Here you see me trying to make hibernate mode desirable enable. I found this part in a very different schematic. It says if you wire 1 and 2 ports of the switch, hibernate will be enabled. But there is already a 0ohm that connects them to each other. Little help here would be great.

4.  Here Im using two capacitors, 0.1uF, 50V, 0603 , Im thinking that if its okay to use 50V capacitors for 20s li-ion battery pack.

For more information, I would like to send you the full schematic in private.

  • +1
    TI__Expert 8526 points

    Hi Tugba,

    In response to your questions: 

    1. The external balancing circuit with NFETs looks okay.  The filter resistor and capacitor values are correct.  The value of the cell balancing resistor (R5, R12) will depend on your target cell balancing current and the voltage of your cells. 
    2. Here is the equation to determine what values to use: Rcb + Rdsonexternal = Vcell / ibal     |      After determining these values, you can calculate the specs on the cell balancing resistor, using P = ibal^2 * Rcb
      1. Rcb: cell balancing resistor
      2. Rdsonexternal: ON resistance of the external NFET
      3. Vcell: cell voltage
      4. ibal: desired balancing current
      5. P: power dissipated over the cell balancing resistor
    3. If PRES is floating, then the device will enter HIBERNATE mode after a certain amount of time.  If the PRES pin is pulled HIGH, then the device exits HIBERNATE mode and stays in NORMAL mode.  I believe the 0 Ohm resistor (R14) is populated for a disabled HIBERNATE mode and depopulated for an enabled HIBERNATE mode.  Please see Section 9.3.5 in the datasheet (BQ77915 3-Series to 5-Series Stackable Ultra-Low Power Primary Protector with Autonomous Cell Balancing and HIBERNATE Mode datasheet (Rev. L) (ti.com)) for more information.
    4. Yes, capacitors rated for 50V should be okay for your application.

    I will private message you for any other questions you may have.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Prodigy 70 points

    Hey, thank you so much for your help!