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BQ24650: MPPSET pin

Yoshinobu Kagawa
Expert 2030 points
Part Number: BQ24650

This time, two types of voltages are supplied to the VCC and MPPSET pins of the BQ24650RVAR on the same circuit.
Two types means that there are two different power supplies.
One is the main power supply, 17.4V, and the other is the secondary power supply, 12V.

Since 17.4V is the main power supply, R3 and R4 are selected with VMPPSET as 17.4V.
However, in this case, when the power supply is changed to the secondary power supply, the voltage to the MPPSET pin will be 12V, so the voltage to the MPPSET pin will be less than 1.2V due to resistive voltage divider by R3 and R4, and the charging current to the rechargeable battery will be reduced.

In order not to reduce the charging current even when the secondary power supply is used, is there a problem with supplying 3.3V to the MPPSET pin and keeping it active at all times when the secondary power supply is used?

Regards,
Kagawa

  • 0
    TI__Mastermind 40835 points

    Hello Kagawa,

    First, if you have the MPPSET directly programmed for 17.4 V with a 17.4 V power supply you may reduce the charge current straight away. You need to leave some headroom on this limit. Set your MPPSET to trigger at 17 V or 16.8 V for example. Pulling a couple of amperes may cause the MPPSET pin to dip below the divided 1.2 V or the full scale 17.4 V.

    I see no issue of keeping MPPSET at 3.3 V, but this will completely ignore the MPPT feature of the device. The charge current will not reduce no matter how low of a voltage is applied. You could consider using a FET to turn on another resistor in order to reprogram the MPPSET threshold to say 11.5 V when the 12 V power supply is active.

    Regards,

    Mike Emanuel