• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 164 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS54040: Boot capacitor value

Tim Bossanyi
Prodigy 10 points
Part Number: TPS54040
Other Parts Discussed in Thread: LMR36510, LMR33610, LM3489, TPS64200, LM3475

We are using the TPS54040 in an application which requires operation at 100% duty-cycle with very light load for up to 50ms at a time.  Using the recommended boot capacitor value of 0.1uF, we cannot achieve this - the boot capacitor voltage falls below the 2.1V UVLO threshold and the internal FET turns off.  The low load current is insufficient to pull the switching node down and re-charge the capacitor.

We have achieved success by increasing the boot capacitor to 2.2uF - measured on the bench, this maintains sufficient voltage to keep the FET on for the required 50ms.  Are there any potential caveats to be aware of when using such a high-value boot capacitor?  Could this cause problems with the internal boot-charging circuit?

  • 0
    TI__Guru 57555 points

    Hello

    A value of 2.2uF is quite a bit larger than we could recommend.  I would not go much higher than about 0.47uF.

    As you surmised, higher values may cause issues with the re-charge circuit.

    One alternative is to use a small pre-load on the output to keep the boot capacitor charged.

    A better alternative is to use one of our synchronous devices that can achieve nearly 100% duty cycle

    be reducing the switching frequency in drop out mode.

    Devices such as the LMR33610 or LMR36510 may be of some interest.

    What is your input voltage, output voltage and load range ??

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 10 points

    Hi Frank,

    Thanks for your reply - unfortunately this is not what we were hoping to hear!  A simple pre-load on the output won't work since the output current is actually negative at times during the dropout operation - we have a diode connected from output to input to enable this.

    Our application is slightly unusual in that we're not aiming for a constant voltage output - we've actually re-arranged the feedback into something a bit different (can't give away too much detail on a public forum sorry!).  However this means that we wanted a part with external feedback compensation components, so that we could adjust the feedback loop response to our requirements.

    The input voltage range is 6-14V, output voltage can be anywhere from about 1.6V up to the full input voltage, and the max. load current is 150mA.  Do you have any parts that might fit this requirement?

    A 'simple' half-bridge and PWM controller would also work if you have any in this voltage/current range?

    Kind regards,

    Tim

  • 0 in reply to Tim Bossanyi
    TI__Guru 57555 points

    Hello

    You could retain the large boot capacitor.  However, there may be long term issues that could cause reliability issues.

    And, TI would not be able to provide much support should you encounter a problem down the road, since you are not using the

    device according to our recommendations.

    You could try one of our PFET controllers, such as the TPS64200 or the LM3475, or LM3489.

    Or maybe even an LDO would work??

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 10 points

    Hi Frank,

    Yes, this is what we were concerned about with the large value capacitor.  We will investigate other solutions.

    Thank you for your help.

    Tim

  • 0 in reply to Tim Bossanyi
    TI__Guru 57555 points

    Hello

    OK.  I will close this post.

    You can re-open any time you need more help.

    Thanks