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Original question:

BQ24630: Size restrained: How to disable Input Adapter Current Regulation

BQ24630: Select between 5A and 10A charging

Frits Jan van Kempen
Prodigy 215 points
Part Number: BQ24630

I would like to have the option to 'select' between 5A charging and 10A charging by changing the ISET1.

I will be charging 1 cell (3,6V) and have an input voltage of 12V

Questions:

1. CAN I use the components selected for 10A also for charging with only 5A ? (Lo = 4.7uH and Co = 30-40uF )

2. And if so, what are the consequences? Will efficiency be less (or more) when charging 5A with a 4,7uH inductor? 

3. with 4,7uH, I can use Co=30uF resulting in 13,4Khz resonant freq OR Co = 40uF resulting in 11.6Khz which is better and why?

Thanks in advance for the swift reply.

  • 0
    TI__Mastermind 24075 points
    Hello Frits,
    I am evaluating the use of the same capacitor and inductor for the 2 charging currents and will get back to you as soon as possible.
  • 0 in reply to Kedar Manishankar
    Prodigy 215 points

    Thanks for your effort! I will wait for your findings.
    As far as I understand now, is that lower uH inductors can handle bigger currents, so that is why for 10A a lower uH inductor is chosen.
    But then the C needs to be bigger to store the energy.

    But I do not know for sure the impact on efficiency in both use cases.

  • 0 in reply to Frits Jan van Kempen
    TI__Mastermind 24075 points
    Hello Frits,
    Smaller values of inductance are used for higher currents mainly because a higher inductance involves a higher DCR (DC resistance). Higher current means more voltage drop across the DCR and decreases the efficiency. Inductance is also related to the current ripple, and higher inductance will reduce current ripple which is used to charge the output cap, thereby reducing output voltage ripple.
    To answer your original post:
    1) Yes you may use components selected for 10A also for 5A.
    2) Efficiency of 5A relative to 10A with the same inductor will definitely be better as with the same DCR of inductor, increase in charge current will decrease the efficiency(larger voltage drop). For capacitor, keep in mind the ESR as well, as this also influences output ripple. Higher capacitance means less voltage ripple, but can also reduce output voltage transient response.
    3) As you are within the stable region you are fine with regards to loop stability. Within this range the higher resonant frequency will result in faster loop response.
  • 0 in reply to Kedar Manishankar
    Prodigy 215 points
    That is good news, thanks for the explanation.

    As for voltage ripple, it is better to have almost no ripple right? Just a clean output.
    Does this need to be as low as possible (even 0) or is a certain range better?

    As for ESR, selecting X7R or X5R should be fine right? (although ceramic caps are getting rare these days...)
    Is a lower graded cap also an option when the higher graded caps are not available? In my calculations I have a voltage ripple of 'only' 24mV which does not seem a lot to me (1 cell, 12v input)

    As for the loop response, a faster loop reponse is better? So better get near 15kHz instead of the 10kHz?

    Thanks for your answers
  • 0 in reply to Frits Jan van Kempen
    TI__Mastermind 24075 points
    Hello Frits,
    Ideally yes, no ripple is desirable which would imply higher capacitance but the trade off you have is your loop stability response. To keep it stable your inductor value would also have to change to stabilize the feedback loop. It is your choice as a designer as to what features you prioritize as there are tradeoffs in size, cost and performance. Yes, ceramic capacitors are an ideal choice due to their very low ESR.
    As for loop response, datasheet recommends a tight stable range for you optimal performance. Faster response is ideal, however within this range you should not see too much variation.