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LM5069 setup for light loads?

Jeff Sauer
Prodigy 10 points
Other Parts Discussed in Thread: LM5069

I've been using LM5069 with great success as a branch circuit on/off switch with soft-start and current limiting. Vin=24VDC and current limit is set to 5.5 Amps (10 mOhm Rsense).

Now I have an application where the current limit should be 200mA, but the data sheet has warnings about exceeding 0.1 Ohm on the sense resistor (data sheet pg12 paragraph "Current Limit"), and 150K Ohms on the power limit resistor (data sheet pg 15 paragraph "Power Limit Threshold").

Does anyone know why the LM5069 cannot be used at low pass currents? I can compensate (or ignore) errors caused by input bias current on the Vin, sense, and output pins if I know what those currents are. And I can add protection to prevent excessive drops across the higher value sense resistor in back-flow situations.

But is there any fundamental reason why I cannot use this part under light load?

  • 0
    TI__Genius 13245 points

    Hi Jeff,

    The power regulation loop amplifier will saturate with power resistor values larger than 150 kOhms, and the device will no longer be able to regulate the FET SOA power accurately at that point.

    The warning about the large sense resistors is exactly what you mentioned - if the Vsns voltage is too large (either from a very fast forward current surge or any reverse current surge) the anti-parallel diodes can conduct potentially destructive amounts of current. 

    I hope this helps.

    Mike

  • 0 in reply to Michael Hartshorne
    Prodigy 10 points

    Hi Michael,

    I have a similar problem as Jeff. Power-limiting is not used, the current limit is wanted to 100mA,so  the Rs is set to 0,47R.

    Is it possible to insert a Resistor from Rs to Pin "sense" to limit possible surge-currents through the internal anti-parallel diodes?

    What is the maximum current these diodes can hold? Or is the better way to place antiparallel schottky-diodes at Rs?

    Best regards, Joerg

  • 0 in reply to Joerg Bammert
    TI__Genius 13245 points

    Hi Joerg,

    Yes.  A 10 Ohm resistor between the SENSE pin and the Rs resistor can protect the diodes.  However it comes at the expense of current limit accuracy.  The sense pin will have around 30 uA current draw that will change depending on VIN, and there is also a 1MOhm resistor from SENSE to OUT that will contribute to the error when OUT is low.  If the design can tolerate the amount of error that comes from this, then that should be OK. 

    Anti-parallel Shottky diodes will also help protect the diodes on the LM5069.  They may not be necessary with the series resistor, but probably a good idea to recommend in the case that the series resistor is not enough.

    Regards,

    Mike