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TPS61288: Charge time of high output capacitance

Part Number: TPS61288
Other Parts Discussed in Thread: , TPS61046

Hello,

I currently use a TPS61288EVM-064 in standard configuration and an output voltage of 15V. I connected 4700uF directly to the output of the Booster. As soon the voltage in the capacitor has reached 15V, I turn off the booster and use the energy in the capacitor to control a solenoid over a h-bridge.

It works perfectly fine BUT to charge the 4700uF capacitor it takes about 3.5 seconds which is too much for my application. In theory it should take much less time. I guess this is due to the PFM feature? 

Is there a way I can speed that up?

If not I will go for a simple switcher like the LMR62014 but it would be much more convenient to use the TPS61288 which is already on our device.

Thanks for your help and kind regards,

Manuel

  • Hi Manuel,

    It may be restricted by the input supply you use.

    Please have a measurement of your input current and ensure the current limit is larger than 15A.

    On my bench, I add 6800uF*3 capacitors and the start time is within 100ms.

    You can add some waveform if you get some detail finding.

    Best Regards

    Fergus

  • Hi Fergus,

    You are right, I had a problem with the current limit in combination with the resistance of my cables. Further I forgot to mention my input voltage of 4.2V down to 2.75V from a Li-ion battery with 8700mAh and max. 2C of discharge current.

    The problem is that the voltage of the battery drops while drawing that much current so it can't start up properly. This is what I can see while connect the battery without long cables directly to the evalboard. By using cables it works, as there is some series resistance but it takes much more time then.

    Is there any way to reduce the inrush current for this booster? I guess no, as the switch turns off as soon as it reaches this high current. I tried by pulsing the enable pin and it worked arround 100Hz but this can't be the solution as it depends on the input voltage as well.

    Well, I didn't consider this problem while evaluating a booster which has much power and can go down to 2.4V for the startup voltage...

    Thanks for your help,
    Manuel

  • Hi M B,

    Sorry for reply late.

    I don't quite understand what you said about the reasons.

    This is what I can see while connect the battery without long cables directly to the evalboard.

    You mean that there is quite large internal resistance inside the battery so that when drawing large current, the input voltage of device will drop below 2V, causing the shutdown. Is this right?

    So, the device should be in modes of starting up and shutting down, right?

    By using cables it works, as there is some series resistance but it takes much more time then.

    It is weird when using cables, the device can start without Undervoltage protection.

    If the problem is caused by UVLO, you can connect Vin pin to a signal input(JP1 can be an input),after battery is connected, then pull the signal to high level.

    In this condition, the chip can still work even power input is very low. 

    Best Regards

    Fergus

  • Hi Fergus,

    Yes you are correct with your assuptions. I have two screenshots here:

    This is with the battery (LP70103122-PCM-LD) directly connected to Vin only with the cables mounted on the battery. As you can see, Vout cannot reach 15V.

    I also used an external power source to drive VIN high after battery is connected as you suggested. But then the battery instantly drops to 0V because of the high current.

    This is with the battery connected to Vin by using two additional jumper cables. As you can see the booster can startup within a few 100ms.

    We also use this booster in another project with even smaller batteries (HCB CR26500 3V, Maximum pulse discharge current: 3000mA). But I have not tested it in my own design, only with a testsetup by using evalboards. But I guess it won't work neither without some external cables. How can I even use this booster as the inrush current is so extremely high at startup even if there is no load. Internal softstart seems not to help and I cannot limit inrush current.

    Thanks and kind regards,
    Manuel

  • Hi Manuel,

    It seems that TPS61288 is not fit for this battery charge application.

    You can choose the device with small current limit and true disconnect.

    With true disconnect function, the battery will not discharge when EN is low.

    TPS61378's current limit is 4.8A

    TPS61046's current limit is 0.8A with small size.

    you can apply the EVMs to have a try.

    Please feel free to ask me if you have any further question.

    Best Regards

    Fergus