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TPS61090 - Large Inrush Current Causing Battery Voltage Drop

Nicholas Conn
Prodigy 140 points
Other Parts Discussed in Thread: TPS61090, TPS61020

I am using the TPS61090 in a portable application where the regulator is initially in shutdown mode (EN = 0). When the regulator is turned on (EN = 1), the inrush current to charge the regulator's output capacitor is causing the battery voltage to dip significantly, resulting in problems elsewhere in the circuit.

The TPS61090 is set to output 3V and the expected current draw on average is 20mA, with a maximum current of 400mA. During startup, the current draw is approximately 10mA. In order to reduce the output voltage ripple, the output capacitor was chosen to be 330uF with an ESR of 35mOhm (as per the datasheet).

The battery is a primary lithium battery with a nominal voltage of 3V with a bypass capacitor of 100uF. When the regulator turns on, the battery voltage dips to approximately 1.5V. This dip occurs well before the regulator starts switching during the soft-start phase. I believe that the inrush current is caused by the need to charge the 330uF capacitor.

Do you have any suggestions for reducing the inrush current? Or possibly another method for reducing the voltage drop in the battery? I will be trying out a 100uF capacitor in place of the 330uF capacitor, but I really do not want to increase the output ripple in my application.

Thank you for your help!

  • 0
    TI__Mastermind 46905 points
    hi Nicholas
    the start up current can't be controlled by external component. maybe you can also increase the input bypass capacitor. or change to a device with smaller current limit, like TPS61020 series.

    could you share the waveform of VIN, VOUT and SW to confirm the problem?

    if the output current can reach 400mA, the input voltage would drop when there is 400mA at the output.

    Thanks
    Jasper Li
  • 0 in reply to Jasper Li
    Prodigy 140 points

    Jasper, thank you for your response.

    I don't have easy access to the SW pin, but the attached image shows the battery voltage and the 3V output voltage.

    I believe that the high frequency oscillations on the battery voltage indicates when the TPS61090 starts switching. This matches what I would expect based on the datasheet. "During precharge, the rectifying switch is turned on until the output capacitor is charged to a value close to the input voltage." The battery voltage dips so much, that switching is initiated when the battery voltage (and output voltage) is approximately 1.5V.

    I have tried reducing the output capacitor to 100uF and placed 200uF (2x 100uF) on the battery (input voltage to TPS61090), but the problem remains. The dip in the battery voltage is actually the same, everything just happens on a much quicker time-scale.

    The two output capacitors that I have used:

    10volts 330uF ESR 35mohm - http://www.mouser.com/ProductDetail/Panasonic/10TPB330M/?qs=%2fha2pyFaduipppMK2qqevmr0Zf%2fKh6Xz9mUV0I7lNjRH46tS3SOQrQ%3d%3d

    and

    10volts 100uF ESR 45mohm - http://www.mouser.com/ProductDetail/Panasonic/10TPC100M/?qs=%2fha2pyFaduipppMK2qqevujaLhZioXjJV%2floQONHaf8qAklrOaOJCg%3d%3d 

    I am also using the following inductor:

    8.2uH 2.7A 0.053Ohm - http://www.mouser.com/ProductDetail/Wurth-Electronics/7447779008/?qs=XJfXErqHgA7ii2ZtDqeBew%3D%3D

    I'm not really sure the best way to proceed. While I can use a different regulator in the next version of the device, I still need to get this version with the TPS61090 working. Any suggestions would be very appreciated!

  • 0 in reply to Nicholas Conn
    TI__Mastermind 46905 points

    hi Nicholas

    from the waveform, the precharge current can be calculated. it is around 400mA. this current cause 1V voltage drop at the input. so the input impedance is about 2.5ohm. it look like the large input capacitor doesn't work. is there a long cable from input capacitor to the input pin of TPS61090.

    Could you share the schematic and the image of your real board? and the test point of the waveform.

    thanks

    Jasper Li

  • 0 in reply to Jasper Li
    Prodigy 140 points
    It has been a while, but I finally had time to revisit this problem and wanted to share with everyone what was going wrong.

    This issue only happens when I am measuring the current with a multimeter. When I remove the multimeter, this issue completely goes away. This leads me to believe it is caused by the multimeter switching current scales. Has anyone else had a similar issue? After the next board revision, I will test this on a benchtop meter without auto-scale on and see if the issue remains.

    Thanks for all of your help!
  • 0 in reply to Nicholas Conn
    TI__Mastermind 46905 points
    thanks for the update. it seem cause by the high resistor of the multimeter. i observed similar problem before, when i tested other device. you may need to set the current meter to fixed current scales, not auto-scale.

    Jasper Li