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BQ77216: CHG Resistor

Buse Irem AZ
Prodigy 50 points
Part Number: BQ77216
Other Parts Discussed in Thread: BQ76952, TIDA-010208

We use BQ7721603PWR part numbers for protection purposes for your product. We designed our circuit by taking an example from your reference design. But when we energize the circuit, that is, when we insert the batteries, the R85 resistor suddenly heats up and then burns. What could be the reason for this? We suspect the direction of the D22 diode. You have specified this part in your reference circuit as we did, but we could not solve the source of the problem. We think that in your reference design the direction of the diode may be reversed, do you think so? Or what could be the problem? When we remove the Q36 and Q35 mosfets to which the CHG and DSG pins are connected, the R85 does not heat up and the problem goes away. Where exactly is the problem? Is it in the components we use or in the design? I am sharing screenshots of our own schematic files. If you need more information to solve the problem, please ask and I will share.

Q35-Q36 : FDT86113LZ // N-Channel 100 V 3.3A (Tc) 2.2W (Ta)

D22 : MMSZ5246ET1G // Zener Diode 16 V 500 mW ±5%

R85 : ERJ-8ENF4990V // 499 Ohms ±1% 0.25W, 1/4W Chip Resistor 1206 

  • 0
    TI__Mastermind 42060 points

    Hello Buse,

    In our reference design we do not have the BQ77216 included. Can you please share your whole schematic, it is difficult to understand what is going on. If this is to protect the gate-source of a FET, the polarity is correct.

    If you are also using the BQ76952, you should not have the drains of Q35/Q36 connected to the BQ76952 pins. The BQ77216 output should connect to a chemical FUSE, as the TIDA-010208 shows:

    If you are shorting the CHG to Vss through the BQ77216, then yes, the Zener would conduct.

    Best Regards,

    Luis Hernandez Salomon