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BQ25570: BQ25570 and BQ25302 Circuit design inquiries

JP PARK
Prodigy 60 points
Part Number: BQ25570
Other Parts Discussed in Thread: BQ25302

Tool/software:

Hello.
I'm designing solar charging using BQ25570 and USB connection circuits using BQ25302.
The LOAD I wanted to use exceeded 100mA, so I connected the load directly to the battery without using VOUT of BQ25570.

So I attached the Schottky diode to the VBAT pin of BQ25570 and the BAT pin of BQ25302 respectively and connected it to the battery. (See also circuitry)

If I use BQ25570 and BQ25302 at the same time, do I need to design the circuit like this? Or I'm asking if you have any recommendation circuit design tips to use solar charging and USB charging at the same time.

Regards,

JP

  • 0
    TI__Mastermind 42205 points

    Hello,

    My schematic review is as follows:

    1. I see MPPT sampling at 80% of input open circuit voltage.

    2. If not using buck converter, VOUT_EN should be tied to GND, and you can leave off VOUT_SET resistors, the inductor, and the output capacitor on VOUT.

    3. If not using VBAT_OK this pin can be floated and OK_HYST and OK_PROG can be tied to GND.

    4. VBAT_OV is set for 4.18V.

    5. VBAT_OK_PROG is set to 2.79V.

    6. VBAT_OK_HYST is set to 2.99V.

    As for connecting the battery directly to the load, the main impact is that VBAT_UV will not function. In other words, if the battery is drained, the BQ25570 will not protect it from draining further. Please test according to your application.

    As for the diode or, it should be fine, whomever presents the higher voltage will determine which chip is powering the battery/load. Keep in mind the state of the battery and the load will determine the voltage each charger senses.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 60 points

    Thank you for your quick response.

    1. If BQ25570 and BQ25302 are added as shown in the circuit diagram and configured through the Schottky diode, I understand that charging to the high voltage side is done, so I decided there is no problem.
    2. I have simplified the circuit based on your advice as I only use the charging function via BQ25570 and only want to control overcharging at 4.2V. Finally, I would like to ask if there are any issues.
    3. VBAT_OV was connected to VBAT and adjusted the resistance value.
    4. LBUCK, VOUT, VBAT_OK, VRDIV, VOUT_SET were designed to be floating, and OK_HYST and OK_PROG were connected by GND.

    Regards,

    JP

  • 0 in reply to JP PARK
    TI__Mastermind 42205 points

    Hi JP,

    Now that I think about this, this scheme has a drawback. First, the diode drop will prevent the battery from reaching full charge as each part will regulate at the pin not the battery. This drop needs to be taken into account to ensure full charge. Please evaluate the lack of BAT_UVP on BQ25570.

    Here is my schematic review for the BQ25302:

    1. It is recommended to connect a 10uF capacitor to BAT.

    2. The /EN pin needs to be pulled low to enable the device.

    3. I would recommend a 10kOhm resistor on STAT to not overload REGN. 

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 60 points

    Hi Mike Emanuel,

    Thank you for your detailed response.

    The product I am designing has a scenario where it is normally charged by a solar cell via the BQ25570. If the charge is insufficient, I want to use USB power to charge the battery using the BQ25302.

    Based on our previous discussion, I believe that placing a Schottky diode on each VBAT output would prevent the battery from fully charging to 4.2V due to the forward voltage (Vf) of around 0.5V.

    Therefore, I have devised a new method and have some questions about it:

    1. I have installed a pull-down resistor on the BQ25570's EN pin. When a USB input is present, I will input a HIGH signal to the BQ25570's EN pin to stop its operation.

    2. I have removed the Schottky diodes from the previous design.

    My questions are: 3. In this state, with the BQ25570 disabled, if I charge the battery using the BQ25302, will there be any issues with the BQ25570's internal logic? 4. Conversely, when no USB power is connected, will the BQ25302's internal logic encounter any issues from a circuit perspective?

    In conclusion, I would like to ask if the attached revised circuit has any issues.

    Additionally, as you advised, I have connected the BQ25302's EN pin to GND instead of leaving it floating, and I have changed the STAT pin resistor to 10kΩ.

    Regards,

    JP

  • 0 in reply to JP PARK
    TI__Mastermind 42205 points

    Hello,

    There should not be any issue with charging BQ25302 while the BQ25570 is disabled. Likewise I do not foresee an issue with the BQ25302 USB disconnected affecting BQ25570. However, I have not setup up these devices in this manner. Please test according to your application.

    Regards,

    Mike Emanuel