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Original question:

LP87561-Q1: Doubt on over voltage protection

LP87561-Q1: Current Limit Functionality Doubt

Anoop Joshi
Expert 1125 points
Part Number: LP87561-Q1

Hi,

I had a doubt regarding the Current Limit Functionality for this part. Let's say it is used in a 4 phase application for a VOUT of 1V and normal load current of 2A. Assume the "ILIM_FWD" current setting is set to 4A for all phases. During normal operation, let's say for some reason the DUT developed some issue and the looking in impedance for the supply driven by the buck dropped and became 100mohms (ie not exactly a supply ground short, but almost like a weak short)

In this scenario, does the buck still continue to drive 10A (given that it is capable of driving 16A in 4 phase mode) in an attempt to maintain 1V at the load? I'm concerned that this high a current drive may damage the PCB path vias etc in the supply path. I suppose the "ILIM_FWD" setting is on a per phase basis, so even if we set it to 4A per phase, effective current limit is still 16A? And in any case, even if the "ILIM_FWD" limit does get hit, I understand the buck doesn't really limit the current based on it, it just raises an interrupt which the user has to read and take corrective measures (which may take time based on the implementation)?

Also, since the buck is capable of driving 10A across 100mohms to still get Vout=1V, I can't also rely on the Vout<350mV short circuit detection functionality that automatically disables the buck outputs when Vout dips below 350mV.

So my key doubt is if there is a true in built current limiting mechanism available that let's me limit the TOTAL load current safely to say 3.5A or so regardless of the load impedance value? Or is the usage of an external fuse (which adds to the power delivery network impedance) the only option here?

Thanks,

Anoop

  • 0
    Expert 1125 points

    Hi,

    I noticed the statement "If the load current is increased so that the current limit is triggered, the regulator continues to regulate to the limit current level (current peak regulation, peak on each switching cycle).", so it does seem like the current peak will indeed be regulated to the "ILIM_FWD" setting for each phase?

    If this is true, does the TOTAL effective load current limit for 4 phase operation end up being ILIM_FWD or will it actually be 4*ILIM_FWD because this limit setting is set on a per phase basis?

    Thanks,

    Anoop

  • 0 in reply to Anoop Joshi
    TI__Genius 10380 points

    Hi Anoop,

    I've connected your post with our expert for this device - he should get back to you by tomorrow. 

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    Expert 1125 points

    Thanks Gerard, will wait for the response

    Thanks,

    Anoop

  • +1
    TI__Genius 11020 points

    Hello Anoop,

    In case of overcurrent situation the bucks will limit the peak current per phase according to the ILIM setting. So if you have 4A limit per buck, that is 16A peak current total. Bucks will shut down only if the short circuit protection will trip (output voltage drops below 350mV as you said).

    You can set the ILIM through register write so that it can provide sufficient current in normal operation, but in case of overcurrent the output voltage would drop below 350mV. Note the ILIM setting is the peak inductor current per phase and when considering the load current requirement the current ripple and tolerances need to be taken into account. In the datasheet application section there is diagram on this.

    Thanks.

    Regards,

    Tomi Koskela

  • 0 in reply to Tomi Koskela
    Expert 1125 points

    Thanks Tomi for the details

    Thanks,

    Anoop