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bq25504: High-impedance divider on VOC_SAMP required for correct function of MPPT ?

Martin Schmidt93
Intellectual 310 points
Other Parts Discussed in Thread: BQ25504

Referring to the TEG-application circuit of the bq25504 datasheet (Dec.2014, 9.2.2):

There are two resistors, Roc1 and Roc2 connecting to the VOC_SAMP input. They are 10M each. The source (=TEG) connects in parallel to this divider.

In my application, the "TEG" has a source impedance of 200R.
Will the device will still work nicely if I reduce the 10M / 10M - divider to a 22K / 22K-divider ?
The additional load on the 200R power source will be negligible, but will the MPPT still work correctly, when the divider impedance is so much lower ?

Thank you.

  • 0
    TI__Mastermind 20570 points
    We recommend 10M resistors because higher resistance provides low leakage path for the open circuit voltage sampling capacitor. You might be able to use 22k/22k divider, however, the performance might vary.
  • 0 in reply to Jing Zou
    Intellectual 310 points

    Hi J.Z.,

    In the reference design, the sampling capacitor charges in 256ms on a 10M/10M-divider. The sampling period is a long 16s. How can the capacitor hold the charge for so long, on a divider that charges it in ms ?

    In the block diagram, I see a switch between VOC_SAMP and VREF_SAMP. It is controlled by the "MPPT Controller". Isn't this switch separating the sampling capacitor from the divider output, so that a low-impedance divider should not matter ?

    Martin

  • 0 in reply to Martin Schmidt93
    TI__Mastermind 20570 points
    First of all, the input cap is not charged from 0 to VOC in 256ms, it is held at the MPP threshold druing the 16s and been charge up to VOC during 256 ms. If the input source does not have enough power to charge up the input cap, then the VOC sensed by the IC will be whatever the voltage on the input cap at the end of the 256ms. The input regulation voltage in the next 16s will be based on this sampled VOC instead of the actual VOC of the panel.
    Second, besides the acuracy of the VOC sampling, there is also the leakage at the VOC_SAMP pin, by using lower impedance divider, your overal efficiency might also be affected depending on the application power level.
  • 0 in reply to Jing Zou
    Intellectual 310 points

    Thank you J.Z.
    I now understand how it goes together.

    Martin