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BQ25713: How to measure Bode plot

K. Sasaki14
Guru 10200 points
Part Number: BQ25713

We would like to measure Bode plot of bq25713.

Please tell me the injection resistor point we should put.

- During charging operation
   Is it between VBAT and SRN pin?

- When BATFET is OFF
   Is it between VSYS line and VSYS pin?

- During OTG mode
   Is it between Vin line and VBUS pin?

Best Regards,
Kohei Sasaki

  • 0
    TI__Expert 5330 points
    Hello,

    I have not done this measurement specifically myself, and today was a holiday in TI US. I will give you an answer when I return to the office on Monday.

    Thanks,
    Steve
  • 0 in reply to Steve Preissig
    TI__Expert 5330 points

    As you have stated in your question, the measurement point is dependent on the mode of operation of the device.  One important distinction is whether the device is regulating a voltage (CV charging, regulating VSYS, regulating VBUS) or regulating a current (CC charging, IIN_DPM). I spoke to our design team, and they cautioned that it would probably be difficult to measure loop stability when the device is in a current regulation mode, such as constant-current charging or IIN_DPM, because the current sense amplifier is integrated into the device and there would be no simple way to break the differential input lines (SRP/SRN or ACP/ACN) in order to inject a signal. 

    To test loop stability / take bode plot while the device is in voltage mode regulation:

    - During charging operation  (CV charging)

      During CV charging, the BQ25713 is regulating battery voltage which is sensed through the SRN pin. So place injection resistor at SRN pin.

    - When BATFET is OFF

      In this case the device is regulating sys voltage through the VSYS pin.  So place injection resistor at VSYS pin.

    - During OTG mode

      The device is regulating VBUS voltage sensed through the VBUS pin.  So place injection resistor at VBUS pin.

    So the three points that you mentioned in your question are the correct locations.  

    Basically, you want to put your injection resistor just above the three resistor dividers shown on this diagram from the datasheet (depending on the mode to be tested as per the above):

    Note that different effects can cause the device to transition between modes.  For instance the device could initially be in CV charging and the injected signal could move it into a CC charging mode.  If this happens it will produce very confusing results, so please be careful when taking the measurements to assure that the device remains in the same control mode throughout the test.

    Here is an interesting TI app note on measuring loop stability that you may also be interested in :

    http://www.ti.com/lit/an/snva364a/snva364a.pdf

    Regards,

    Steve