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bq25505 efficiency curve on datasheet

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: BQ25505

There are efficiency curves on bq25505 datasheet. Input voltage is set by Keithley Source Meter. At this time, how is the MPPT setting?
Is the input voltage of datasheet efficiency curve the value after MPPT?
Or is it possible to set MPPT 100% by connecting VIN_DC to VOC_SAMP?

Best Regards,
Kohei Sasai

  • 0
    TI__Mastermind 20570 points

    The source meter voltage is considered as the open circuit voltage (VOC). Typical characteristic of high impedance sources is at VOC, the output current is 0.

    For bq255xx, every 16s, the charging is disabled for 256ms for the IC to sample VOC and the MPP setting. Then the input VIN_DC is regulated at VOC* MPP%. Thus the efficiency is calculated using the MPPT value not the open circuit value.

    The MPP percentage can be adjusted between 50% to 80%. Tying VOC_SAMP to VIN_DC does not set the MPPT threshold to 100%

  • 0 in reply to Jing Zou
    Guru 10200 points
    Jing-san,

    Is the X-axis voltage value of datasheet Figure 1 efficiency curve VOC*MPP%?
    If it is so, I don't know why the X-axis value is rise up to about 2.9V. If X-axis value is VOC*MPP%, I think it becomes less than 2.4V because source meter setting is 0 to 3.0V.

    And you said MPP percentage can be adjusted between 50% to 80%. However 40% setting is done on datasheet P.26. Is the datasheet setting violation?

    Best Regards,
    Kohei Sasaki
  • 0 in reply to K. Sasaki14
    TI__Mastermind 20570 points

    Sasaki-san,

    Sorry for the confusion. Regarding on the MPPT setting, you are correct. Please refer to datasheet page 11 to 12. When tying VOC_SAMP to VSTOR, the MPPT is set to 80%. When tying VOC_SAMP to ground, MPPT is set to 50%. If input source does not have either 80% or 50% of VOC as its MPP point, the exact ratio for MPPT can be optimized to meet the needs of the input source being used by connecting external resistors ROC1 and ROC2 between VIN_DC and GND with mid-point at VOC_SAMP. With this said, the MPPT threshold can be set to 40% as in the example on page 26. However, tying VOC_SAMP to VIN_DC cannot set the higher limit to 100% since VIN_DC voltage is normally lower than VSTOR level. 

    Regarding on figure 1, the efficiency data in figure 1 was taken with MPPT disabled.

    Regards,

    Jing