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LM2678: allowed peak current pulse of LM2678

Uchida Yasuhide
Prodigy 180 points
Part Number: LM2678


Hi,

The peak current pulse of Iout is over rated current 5A in my schematic.

The peak current pulse is 7A at 264us.

The continuous current is 1.7Arms.

In this case, The current limit don’t work.

I want to know that what value is allowed peak current pulse.

Could you tell me it?

  • 0
    TI__Guru 74055 points

    Hello

    The output current rating on the front page of the data sheet, in this case 5A, is the maximum

    average current that the converter can supply under most operating conditions.  The maximum output

    current that can be supplied to the load before current limit is reached is always somewhat higher.  Once current

    limit is reached the output voltage will fall out of regulation.  The following equation is used to calculate the 

    maximum output current before the current limit is reached for a typical buck DC/DC converter:

    Ioutmax = ICL-0.5*(Vin-Vout)*Vout/(Vin*L*FSW)

    Where ICL = the current limit given in the converter data sheet (on page 6 in this case)

    L = your inductor value and FSW is the 260kHz switching frequency.

    As an example, with 12Vin, 5Vout, 22uH, and ICL=8.3A (form data sheet)

    The maximum output current before current limit is reached is about 8A, under this example.

    By using an adjustable resistive load you can find the maximum output current just before the

    output falls out of regulation on the bench.  A current probe can be used to monitor the inductor

    to check the peak current limit value.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 180 points

    Hi,

    Thank you for your reply.

    I calculated Ioutmax on minimum ICL=6.1A

    Ioutmax becomes 5.8A(16.8Vin  5Vout  27uH)

    Measured Ioutmax by current probe is 7A in my schematic.

    So Iout of my schematic is over allowed Ioutmax5.8A.

    7A is short time(264us) so the current limit does not work.

    Can I use LM2678 in present schematic ?

    I concern that LM2678 is damaged or not.

  • 0 in reply to Uchida Yasuhide
    TI__Guru 74055 points

    Hello

    Notice that the current limit specification is between 6.1A and 8.3A, with 7A at typical in the data sheet.

    If you are measuring 7A, then your device is at the typical value and seems to be OK.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 180 points

    Hi,

    Thank you for your reply.

    I would like to understand the current limit deeply.

    Is the cause of the variation of current limit value(between 6.1A and 8.3A) the variation of the devices? or Ioutmax waveform?

  • 0 in reply to Uchida Yasuhide
    TI__Guru 74055 points

    Hello

    The variation of the current limit specification in the data sheet is due to the internal circuit variation of the regulator.

    Also, the current limit feature on this regulator is not trimmed.  So, you can expect a rather wide variation for this parameter.

    Most of the newer regulators from TI have a tighter specification for the current limit protection feature.

    Thanks