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LMR43606-Q1: Reverse current in FPWM

Leif Blake
Prodigy 20 points
Part Number: LMR43606-Q1

Tool/software:

Hi,

I'm looking at using the LMR43606-Q1, and evaluating whether it should be used in PFM or FPWM mode. When the inductor current ripple is more than twice the DC load current, the device allows CCM through reverse current. However, it is my understanding that this reverse current is present at the power supply input, and could damage other components further up the power supply chain. I have 2 questions:

1. If measures are taken to protect circuitry against reverse loads (such as diodes), would this not prevent the proper CCM operation of the power supply?

2. What types of power supply are suitable as input to an SMPS operating with reverse current?

Thanks,

Leif

  • 0
    TI__Guru 73175 points

    Hello

    In general the reverse currents are of short duration and the input capacitors should be able to absorb them.

    Perhaps with a small increase in ripple.  Also, this device is only 600mA so the current will be relatively small.

    You can also increase your input capacitors above those specified in the data sheet.

    In my opinion, allowing the device to operate in DCM/PFM (auto mode) is best, since that will

    provide good efficiency at all load currents.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 20 points

    Thanks for your response. How does PFM impact the sizing of the inductor? LMR43606-Q1 recommends that for calculating the inductor based on ripple current, the maximum current of the device should be used (0.6 mA). However, in auto mode regulation is achieved in part through frequency reduction, which in theory should require a larger inductance to keep a reasonable current ripple and reduce core loss.

    It's unclear to me how to properly size the inductor when the true operating frequency is unknown.

    Any advice on this would be appreciated.

  • 0 in reply to Leif Blake
    TI__Guru 73175 points

    Hello

    The PFM mode does not affect the inductor sizing.  This device has a type of peak-current PFM mode of operation.

    At the start of a cycle in PFM the inductor current is allowed to reach the level of "Ipeak-min" specified in the data sheet.

    Then the SW is turned off until the next cycle. In this way the converter regulates the output voltage by changing the switching

    frequency and the inductor current is controlled, regardless of how low the switching frequency shifts.

    Thanks