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Original question:

BQ25504: Schematic with MPPT to micro controller.

BQ25504: Schematic with MPPT to micro controller.

Kawsar Ali
Intellectual 320 points
Part Number: BQ25504

I tried with a series PFET (CSD23381F4T) with its source connected to VREF_SAMP, but as soon as the FET turns off, VREF_SAMP drops quickly to zero, probably because of the leakage through the FET. So then I tried with the ADG701 switch which advertises only 10 pA leakage current, but still could not hold the VREF_SAMP voltage constant (leaking slowly). Do you have any suggestion for a sample-and-hold with large hold time (5 seconds) for this situation? The key is probably extremely low leakage components.

Also, the bq25504 datasheet specifies a maximum limit of 11 nF for C_REF. Can you please tell me the reason?

  • 0
    TI__Guru*** 143880 points
    Kawsar,

    How were you measuring the voltage on VREF_SAMP capacitor? Did the device function as expected without measurement of VREF_SAMP capacitor. I have only been able to measure without the meter or probe leakage collapsing the voltage with a JFET probe having several Giga-ohm impedance.

    The source that charges VREF_SAMP is very low current and must recharge it during the short sample period, hence the small C_REF value.
  • 0 in reply to Jeff F
    Intellectual 320 points
    Thank you for the response.

    I observed the VREF_SAMP on a Keithley 2000 multimeter. Also, even when I do not measure VREF_SAMP at all, I can observe it falling just by probing my Vin. Please suggest some solution I can try.

    Thank you for the second answer as well. I presume you are refering to the RC time constant from the resistive divider on the OC_SAMP pin (4.42 Mohm) and the C_REF. So, when I am giving MPPT refernce voltage from outside, there should not be this limit on C_REF value as long as I can supply the necessary current, is that right?
  • 0 in reply to Kawsar Ali
    TI__Guru*** 143880 points
    The datasheet shows an over simplification. There is a 1x buffer in between the MPPT resistor divider at VOC_SAMP and the VEF_SAMP with fixed output current. Lowering the MPPT resistors will not significantly speed up the capacitor charge time.

    If you set your multi-meter to highest output impedance (10Gohm?), you might be able to get a got ready without draining the capacitor.
  • 0 in reply to Jeff F
    Intellectual 320 points
    Thank you Jeff!
    As I mentioned earlier, even without measuring VREF_SAMP I can observe it falling by looking at my Vin (because Vin is following VREF_SAMP).

    I am trying to design a sample-and-hold with large hold time (5 sec or more) without any leakage for this external MPPT reference. I shall be grateful if you recommend any part number or device (is there any off the shelf S&H like this?).
  • 0 in reply to Kawsar Ali
    TI__Guru*** 143880 points
    All capacitors have a leakage current. The capacitor on the EVM should be low leakage.

    I am aware of an off the shelf S&H circuit.
  • 0 in reply to Jeff F
    Intellectual 320 points
    That is nice! Would it be possible for you to give me the part number of the said S&H please?
  • 0 in reply to Kawsar Ali
    TI__Guru*** 143880 points
    Sorry, meant to say I am NOT aware.