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Original question:

BQ40Z80: Power supply (VCC, BAT, and PACK) questions

BQ40Z80: VCC and PACK series res

Fabrizio Raggioli
Intellectual 360 points
Part Number: BQ40Z80

Related to this thread:

https://e2e.ti.com/support/power-management/f/196/t/794338?tisearch=e2e-sitesearch&keymatch=BQ40Z80%25252520vcc

It is not clear the address for the design.

The two series resistors for VCC and PACK in the datasheet are VCC->100 ohm and PACK -> 10K

In the EVM (reference design) : 

 VCC-> has 510 ohm (R25) and PACK -> 1.1 K (R13).

It is not clear which indication I should follow! 

1. What is the rationale to choice the series resistance?

2. Is the EVM design wrong?

3. Are the resistances, configuration dependent (I want to use for 2s and 6s configuration, maximum to 25.2V)?

Thanks,

Fabrizio

  • 0
    Intellectual 360 points

    I'm reviewing the schematics and I found another inchoerence beetween EVM and datasheet:

    4. Why in the EVM is there a series resistence  (R37) between BAT and Diode (D6)? Nothing of similar is suggested on the datasheet...

  • 0
    TI__Mastermind 34775 points

    Hello Fabrizio,

    Fabrizio Raggioli said:
    1. What is the rational to chioche the series resistance?

    No series resistor is required once you can guarantee you application never exceeds the ABS Max  in the datasheet and during normal operation never exceeds the Recommended Operating Conditions in the datasheet. If you can't you can add a series resistor to make your design more robust to ESD and voltage spikes. 1k or 10k is your choice based on your BOM cost and application design size requirements trade off with expect voltage spikes and max current you want to limit the voltage spikes and or shorts to.

    Fabrizio Raggioli said:
    2. Is the EVM design wrong?

    Neither is wrong as mentioned above. You get to choose which works best for your application design

    Fabrizio Raggioli said:
    3. Are the resistences, configuration dependent (I want to use for 2s and 6s configuration, maximum to 25.2V)?

    .

    Not entirely as mentioned before but 2S application is less likely to exceed ABS Max or Recommended Operating Conditions than 6S application.

  • 0 in reply to Fabrizio Raggioli
    TI__Mastermind 34775 points

    Fabrizio Raggioli said:
    4. Why in the EVM is there a series resistence  (R37) between BAT and Diode (D6)? Nothing of similar is suggested on the datasheet...

    That resistor help with filtering (RC filter), instead of diode and a cap.

  • 0 in reply to Damian Lewis
    Intellectual 360 points

    I didn't find any equation on the datasheet that can help myself to calculate the two resistances in relation with the maximum voltage.

    Can you write here?

  • 0 in reply to Fabrizio Raggioli
    TI__Mastermind 34775 points

    Hello Fabrizio,

    Fabrizio Raggioli said:

    I didn't find any equation on the datasheet that can help myself to calculate the two resistances in relation with the maximum voltage.

    Can you write here?

    It's really just Ohm's Law. If you want to limit the current from a transient voltage spike to 100mA then the resistor value is (Max Spike Voltage - ABS Max Voltage)/0.1A