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LM25069

Will Haylock
Prodigy 190 points
Other Parts Discussed in Thread: LM25069, TPS53313

I am using the LM25069-2 as a current limiter - the dev board worked just fine, but my board 'trips' at 33mV, 3.3A with a 0.01R resistor - before I change the chip is there anyway that this can happen?

I am measuring the 33mV directly between pins 1 and 2 of the LM25069 and the current is being monitored.

Regards

Will Haylock

  • 0
    TI__Mastermind 42795 points

    Is your application circuit a direct copy of the EVAL board or are there differences. If your circuit is a copy, then you should be able to compare eval board v. your board circuit nodes. Make sure the FET is fully saturated when determining trip point to rule out power limiting (which can lower the expected circuit breaker trip point).

  • 0 in reply to Eric Wright
    Prodigy 190 points

    I have found something interesting - my LM25069-2 circuit  'feeds' into two Texas TPS53313 step-down regulators each set to step-down from 12V to 5V.

    If I disconnect the TPS53313s from the  LM25609-2 output and re-test my circuit then it now 'trips' at the expected 50mV (5A). There appears to be some interaction between the following switchers and the trip point of the  LM25609.

    I hadn't noticed this effect when cascading the LM25609-2 eval board with single TPS53313 eval board. Maybe I need a lot of capacitance on the output of the LM25069-2 circuit?

    Regards

    Will

  • 0 in reply to Will Haylock
    TI__Expert 7990 points

    Buck converters draw pulsed input currents, and having two in parallel also adds the "beat frequency" effect.  The hotswap is intended to respond to peak currents, so the DC measurement of the trip point is only applicable if the current is actually dc.  More than likely, the ac input currents of the converter circuits flowing through the LM25069 current sense resistor are causing the problem.  You can verify this using a current probe or similar technique.  You can either set the current trip point higher, or provide true filtering to the converter inputs. 

    Just adding input capacitance to the converter only changes the effect of the current divider between the capacitors and the source (through the hotswap).  A true filter requires another element such as an inductor or resistor.  A typical filter for this would be a PI filter where the converter input caps already provide the PI output function.  The PI input cap is required to control voltage undershoot on the hotswap and a stable load for the current limit function.

  • 0 in reply to Martin Patoka
    Prodigy 190 points

    Yes! this does seem to be the problem - it was further aggravated by the fact that I had insufficient output capacitance on the TPS53313 buck converters. It all became clear after I used a wideband current probe to monitor the currents.

    Hopefully I will be able adjust the current limit  now that I have removed the aggravating factor and avoid the need for a PI filter.

    Thanks for our reply it was very helpful indeed.

    Regards

    Will Haylock