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BQ24074: Input Voltage Regulated Down to 4.3V (potentially Vdppm?)

Catherine Denisowski
Prodigy 10 points
Part Number: BQ24074

I am using the BQ24074 within Adafruit's USB/DC/Solar LiPo battery charging board (https://www.adafruit.com/product/4755). I have connected a 1W 6V solar panel (https://voltaicsystems.com/1-watt-panel/) into the DC adapter and have verified that it is functional. While I do measure variable solar input (measured through the VBUS pin, connected the input pin of this chip) based on the panel's position, the input voltage never goes above ~4.3V. The solar panel does react to different lighting conditions, but it seems as though all of the voltage values over 4.3V are regulated down to this value.

I believe this may be related to the Vdppm functionality in the system, as Vdppm is listed as 4.3V in the datasheet. While the DPPM functionality for the system should only affect the output voltage (as Vin-dppm is only for BQ24074s configured for USB100 or USB500 modes and my board is set to EN1 = 0, EN2 = 1), I still wanted to rule out a larger current draw than expected.

The board is configured for a programmable input current limit based on EN1 = 0 and EN2 = 1, and this limit is based on the default EN setting for this board (set by 1k resistor, = 1.55 A). The maximum current of the solar panel (short circuit) is 0.20 A, the charge rate is currently configured for 1 A, and while there is currently no load attached the maximum current draw of the future load is 0.11 A. While that should stay within the acceptable current range (1.55 > 1+0.2+0.11), I decided to reduce current draw where I could. However, reducing the charge current to 0.5 A had no effect on the input voltage, as it stayed at 4.3 V .

  • 0
    TI__Mastermind 23200 points

    Hi Catherine,

    Welcome to E2E!

    You're correct that this is VDPPM. What's happening is your solar panel output (the charger's input) beings to provide current that the charger is asking for. As the charger is trying to pull more current that the panel can handle, the voltage of the panel will drop based on the IV curve of the panel. Because the input is dropping on the charger, the OUT pin voltage will also drop. Once the OUT pin falls to 4.3 V, the device enters DPPM. Because you have no load, the IN pin voltage on the charger will be relatively close to the OUT pin voltage. 

    DPPM wants to prevent the OUT pin voltage from dropping further. To do this, it reduces the charge current until the OUT pin voltage can stay constant. 

    Catherine Denisowski said:
    I decided to reduce current draw where I could. However, reducing the charge current to 0.5 A had no effect on the input voltag

    As your solar panel is only capable of a maximum 200 mA, you'll need to reduce it below this. 

    Depending on your panel's IV curve and what you want the voltage to be, you'll need to adjust your current below that. Do note that the IV curve will vary based on the irradiance and temperature of the panel. 

    Let me know if that helps and if you have any other questions.

    Best Regards,

    Anthony Pham