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BQ24074: Question about output capacitance.

Kazuya Nakai59
Guru 21665 points
Part Number: BQ24074
Other Parts Discussed in Thread: BQ24075

Tool/software:

Hello guys,

One of my customers is evaluating BQ24074 for their new products.

At this moment, they have the following question.
Could you please give me your reply?

Q1.
They are going to use 300uF for the capacitor placed between OUT pin and GND
to reduce voltage drop by load transient.
Is it no problem to use 300uF as the output capacitor?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    Guru 21665 points

    Hello guys,

    I'm very sorry that I mistook to input part number.

    The correct part number is BQ24075 not BQ24074.

    Could you please give me your reply for BQ24075?

    Thank you and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Expert 8515 points

    Hello Nakai-san,

    In Table 7-1 of the BQ2407x datasheet, we recommend using a 4.7uF to 47uF ceramic capacitor between OUT and GND.

    The reason for this recommendation is regulator stability. The ESR of the capacitor, together with its capacitance, forms a zero in the frequency response which can impact the stability of the charger's feedback loops.

    Board layout also affects how much the output capacitors affect stability. Therefore, I recommend that the customer checks the stability of the BQ24075 on their board by performing a load step test from no current to their max load current. They can replace their actual loads with an e-load and monitor the output for any ringing or overshoot which would indicate instability.

    Using output capacitors outside of the recommended range may impact stability, so it's important to verify their effect in the application.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Guru 21665 points

    Hello Alec,

    Thank you very much for your reply.

    Could I ask you an additional question as the follow?

    Q.
    If a test using an electronic load reveals that ringing or overshoot are not occurred and the output voltage is stable,
    is it okay to add a 300uF capacitance?

    Is there any other concerning point?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Expert 8515 points

    Hello Nakai-san,

    Yes, if your testing shows no significant ringing or overshoot on the output, then your system is likely stable. Another consideration with adding 300uF to OUT is inrush current, as more capacitance can lead to higher inrush when BAT is connected to the charger IC. I'd just make sure there are no startup issues when powering from BAT.

    300uF is outside the datasheet recommendation, but if your testing confirms stability, it should be fine for your design.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Guru 21665 points

    Hello Alec,

    Thank you very much for your supports.

    I understood.

    Could I ask you one more question?

    Q.
    The customer wants to change EN1/2 setting like MODE1 -> MODE2 or MODE2 -> MODE1
    when power is supplied to VIN and battery is not connected to VBAT pin.

    MODE1: EN1="H", EN2="L" (USB 500mA setting)
    MODE2: EN1="L", EN2="H" (1.5A setting by a external resistor)

    Is there any problem at this operation?

    Your reply would be much appreciated.

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Expert 8515 points

    Hello Nakai-san,

    I tested this on a BQ24075 EVM and found that there could be issues depending on whether there is a load on SYS while changing modes with only an input source and no battery present. If there is no load on SYS, there shouldn't be any issues. However, if there is a load, the input FET may turn off when changing modes.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Guru 21665 points

    Hello Alec,

    Thank you very much for your strong supports and your testing on your side.

    Do you mean that the OUT pin voltage may be dropped a moment during EN1/2 level changing because input FET (Q1 in the block diagram of datasheet)?

    If the customer think the voltage drop is not problem,
    is this EN1/2 setting change no problem for BQ24075?
    Is there any damage to BQ24075 in the situation?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Expert 8515 points

    Hello Kazuya,

    I don't expect any damage to occur, but there could be problematic behavior. If the input FET turns off and SYS drops below the output short-circuit threshold, the device will behave as if there's a short on OUT. Without a battery and with a SYS load present, OUT could get stuck low and have trouble recovering. I'd recommend the customer verifies this in their particular application.

    Best regards,

    Alec