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BQ76PL536A VC0-6 bypass caps

David Wahl
Prodigy 145 points
Other Parts Discussed in Thread: BQ76PL536A

I'm comparing the reference schematic in the datasheet for the BQ76PL536A to the EVM schematic. I notice that in the EVM schematic the 0.1uF caps connected to the VC0-6 inputs bypass to the adjacent cell input, whereas in the datasheet schematic they bypass to the ground plane. What's the reason for this?

  • 0
    Prodigy 10 points
    I have the same question - is there a 'right' way to do this? Are there advantages/disadvantages to do it like in the EVM or the datasheet?
    (I am talking about C30, C32, C33, C38, C40, C43 in Figure 18 in the datasheet vs. C25, C27, C28, C32, C36, C37 in the EVM-schematic)
  • 0 in reply to Michael Schneider88
    TI__Intellectual 2795 points

    The recommendation is to have single-ended filter caps to GND on all VCx inputs. With differential caps, the effective capacitance at each VCx input is different, with the least filtering at the highest input and VBAT, thus allowing more noise into the device. For example, using 0.1uF differential caps, the cap value seen by VC1 will be 0.1uF, but at VC6 will be 16nF. There will be different filter response curves at each input. We recommend a 0.1uF cap close to the pin for best performance with the switch-cap ADC in the PL536A.

    If a very low cutoff frequency is desired, a 2-pole filter can be employed, with 0.1uF to GND close to the input pin, and an additional RC filter with the cap deferentially connected just in front of it. 

  • 0 in reply to Stephen Holland
    Prodigy 145 points

    I understand why bypassing to ground is preferable, but why would the EVM not do this as well? Perhaps it was just a mistake?

  • 0 in reply to David Wahl
    TI__Intellectual 2795 points

    The EVM is old, and is in the process of getting updated with the latest recommendations.