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BQ25504: BQ25504 MPPT not work

Part Number: BQ25504


The customer want to use BQ25504 for thermoelectric energy harvesting. The output of the thermoelectric chip is expected to be 250mV, which is equivalent to an internal resistance of 2Ω. They use a 250mV voltage source through a 2 ohm resistor to equal the thermoelectric chip to the VIN_DC pin of BQ25504. Connection, the BQ25504 circuit schematic is as follows.

The input capacitor Cin is 4.7uF ceramic capacitor. Vstor is connected to a 4.7uF, and Vbat is connected to a 1.5F supercapacitor, which has been precharged to 2.9V.

The MPPT ratio is set to 50%. What I want to see is that the average voltage of the VIN_DC pin is 125mV, and the average output current of the voltage source is 125mV/2Ω=62.5mA.

But the actual situation is that the average voltage of VIN_DC is 175mV, and the average current is waveforms of 38mA, VIN_DC (CH1) and LBST (CH2) pins are as shown in the figure:

The initial guess was that the input capacitance was small, causing the input voltage ripple to be large, and the input voltage could not be well regulated. So the input capacitance was changed to 47uF, and the situation improved somewhat. The waveforms of the VIN_DC (CH1) and LBST (CH2) pins are as shown in the figure. The average input voltage becomes 159mV, and the average current becomes 46mA, but the input continues to increase. Although the capacitor can reduce ripple, the average value of the input voltage and current will not change.

Reducing the voltage dividing ratio of the MPPT resistor will not cause any change in the waveform. After changing the input resistance to 3Ω, the average input voltage is 140mV and the minimum value is 125mV. Because the datasheet describes the input voltage regulation rate is ±10%, I think the MPPT is working normally at this time.

To sum up, I would like to know why the MPPT of BQ25504 does not work properly when the input voltage is 250mV and the equivalent internal resistance is 2Ω. Can the BQ25504 get the maximum power input (VIN_DC) when the input conditions remain unchanged?(VIN_DC=0.5*250mV=125mV)

Please help check the schematic.