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BQ76930: Power supply for the REGSRC Pin

James Zhan
Expert 4365 points
Part Number: BQ76930

Hi guys,

This is the question from E2E China, could you please help to answer it? Thanks.

This is the original schematic design for the REGSRC.

Because our design only use 7S battery, whose voltage is not so high, and it will not a heavy load in the application. 

Plus, REGSRC current is about 6-15mA, which doesn't have a lot requirement for the heat dissipation. Therefore, we've changed the schematic as below:

R3 changed from 330 ohm to 1k ohm; use 0 ohm RD3 to short Q4. In this case, with RD3 soldered, we could save the circuit on the left of Q4, which lower our cost.

The question is:

Doing this change for the circuit, the voltage of REGSRC will change according to the change of the current on this circuit. 

Will this have some influence on the LDO output? and the EMC test?

By the way, could we increase the value of C18 to decrease the ripple of REGSRC?

Thanks.

  • 0
    TI__Guru 54715 points
    Hi James,
    See the VEXTLDO_LN line regulation specification in the bq76930 data sheet. 6 to 15 mA through a 1k would modulate the voltage 7V, with 7 cells the voltage would seem to drop out of regulation for the FET drivers which may be OK in your design.
    EMC results will vary with the design and implementation, the layout can have more effect than the components. More capacitance should reduce edge rates and seem to avoid EMC issues, but check the frequency response of the capacitors.