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LM5160: why VIN = 5V is NOT ok at SCP result compared to VIN = 6V

Steve Hsiao1
Prodigy 85 points
Part Number: LM5160

Dear TIer,

We are evaluating LM5160 for new projects. However, we find a over current (4.87A) issue at EVM while testing SCP when VIN=5V.

=====

Configuration and Preparation before SCP test

  1. Take  LM5160NTFBKEVM
  2. Remove R10 to let EVM support VIN(min) = 4.5V
  3. Modify Rfb to config Vo1 (primary side) = 2.5V.

=====

Issue : Abnormal current occurs, out of current limit spec shown in LM5160 datasheet, Ipeak = 4.87A 

Conditions: Vin = 5V, short Vo1 to GND in purpose

 =====

Benchmark case :  Ipeak = 2.4A within current limit spec, ok to me

Conditions: Vin = 6V, short Vo1 in purpose

=====

  • 0
    TI__Mastermind 29690 points

    Can you please share the oscilloscope shot plots and figure demonstrating how primary side current is being measured, please?

  • 0
    TI__Mastermind 22299 points

    Hi Steve,

    At VCC < 5.5V (VIN <~5.6V)  the high-side peak current detection becomes noisy.

    In this case the current limit detection is sensed on the low-side FET.

    When the lowside MOSFET measures over-current it will extend the off-time until the inductor current falls below ILIM.

    This is valley current limiting.

    The valley current is sensed during the OFF-time, which is AFTER the ON-time, so the peak current should be  (ΔIL + ILIM).

    You use the LM5160DNTFBKEVM, which has a transformer that has a saturation current of 0.84A.

    This transformer also has bad saturation performance when the current is greater than 1.5A,the current is uncontrollable. 

    In saturation, peak current limiting can work somewhat, although the current might exceed ILIM by a little.

    In saturation, valley current limiting will NOT work, because the during the constant ON-time the inductor current will increase uncontrollably.

    You need a transformer with a higher saturation current for the valley current limiting to work.

    You need ISAT >  ILIM + ΔIL

    I hope this answers your question,

    -Orlando

  • 0 in reply to Marshall_Beck
    Prodigy 85 points

    Hi Marshall,

    Here is osc. scope shot @ abnormal current (Ipeak = 4.87A )

    Pls check if this is what you want?

    Pls also let me know your comment.

    Steve

    LM5160 SCP over current at VIN=5V.docx

  • 0 in reply to Orlando Murray
    Prodigy 85 points

    Hi Orlando,

    Noted and clear. ok, we agree transformer saturation is the key reason of this issue. 

    However,

    We need to know how much current (max current capacity) of LM5160's high-side and low-side MOSFET are able to handle without damage?

    Could you provide us this information?

    Steve

  • 0 in reply to Steve Hsiao1
    TI__Mastermind 29690 points

    Hello Steve,

    Sorry for the delay.

    I will get in touch with the product expert to get this resolved for you ASAP.

  • 0 in reply to Marshall_Beck
    Prodigy 85 points

    Hi Marshall,

    More investigation from our side below, for your information and pls comment why?

    1. Config LM5160 EVM is NOT ok at SCP when VIN=5V

    2. Keep VIN = 5V and abnormal behavior of SCP remains

    3. Apply Pin10 (VCC) =6V externally and still keep VIN=5V, we observe SCP becomes ok (normal).

    Steve

  • 0 in reply to Steve Hsiao1
    TI__Mastermind 22299 points

    Steve,

    At VCC < 5.5V,  the high-side peak current detection circuit becomes noisy.

    In this case the current limit detection is sensed on the low-side FET. This is valley current limiting.

    With external VCC > 5.5V , the high-side peak current detection circuit is better and peak-current limiting can work even at low VIN.

    Hope this helps,

    -Orlando