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LM76003EVM-500K: Current Limiting Lower than Expected

Matthew Walker
Prodigy 10 points
Part Number: LM76003EVM-500K
Other Parts Discussed in Thread: LM76003

Hello;

I have a working design using the LM76003 and I'm seeing an issue in low volume production where some ICs will current limit much lower than expected, somewhere between 1 and 2 amps depending on the specimen. We've also noticed that the current limit will sometimes change on power cycle or toggling of the enable pin (in one instance going from ~1.2A to 2.3A.) The voltage waveform at the inductor looks fine even through the current limit (the duty cycle just reduces); and we've attempted to verify that all the passives around the IC are correct.

Boards that work will current limit somewhere upwards of 3.75A (the highest point my acceptance test procedure tests at.)

Does TI have any suggestions for things to look at when attempting to isolate why some of the boards exhibit the lower current limit?

Thanks

  • 0
    TI__Mastermind 27030 points

    HI

    do you float the SS, it may cause very fasts VO raising at on/off, it will cause extra current charging the output capacitor and the charging current related to the softstart time.

    Thanks

  • 0 in reply to Daniel Li14
    Intellectual 441 points

    I am a team-member on this project and can confirm SS is floating.

  • 0 in reply to Will Brown
    TI__Mastermind 27030 points

    HI Brown:

    OCP better to test in stable statues instead of toggling. As LM76003 current limit is peak and valley current protection, you can check the inductor's peak current and valley current in over current condition whether they are in sepc. 

    Thanks

  • 0 in reply to Daniel Li14
    Intellectual 441 points

    Well the point of the original question... is that regardless of when the load is applied, simply turning the power supply output on and off results in it giving a different maximum current limit.  Sometimes it's 700mA, sometimes it's 2.3A.  The power supply can be started with no load, and then the load increased.  After the current limit is reached (either 700mA or 2.3A), removing the load, disabling and re-enabling the power supply usually results in a different maximum current limit.

  • 0 in reply to Will Brown
    TI__Mastermind 27030 points

    HI Will:

    do you have oscilloscope to monitor the inductor current and output voltage? LM76003 will limited the inductor current when the inductor current reach 5.5A. 

    if output decrease when the peak current lower than min of spec, then we think something is wrong of the IC. by reading the load current may not reflect the real peak current value in the inductor

    Thanks