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LMR36503EVM: Low power, high efficiency buck converter not as efficient as expected with low load

Ole-Martin Hanstveit
Prodigy 10 points
Part Number: LMR36503EVM
Other Parts Discussed in Thread: LMR36503

Hi, I received the evaluation board (LMR36503RFEVM) today and did some testing with it. As per the datasheet, it seems to be efficient at 24V input / 3.3V output with low load. However, I am measuring a much higher current consumption than expected from reading the datasheet, even without any load attached. The inefficiency was also verified by IR-camera which measured about 6 degC rise on the LMR36503 IC when powered up (similar temperature rise both with and without any load attached).

The evaluation board was tested without any modifications after receiving it. The input was supplied by a 24V power supply and current consumption was measured through 24V PSU to VIN on the evaluation board using a multimeter. The output was original 3.3V with the default configuration (but also verified the same behaviour when setting output to 5V).

No load attached: measuring 8.98mA consumption on 24V side.
500Ohm load attached: measuring 9.87mA (equals approx 9.2% efficiency)
240Ohm load attached: measuring 10.86mA (equals approx 17.4% efficiency)

Can you please review these findings and provide insight as to why it might have such poor performance?
The intended application is severely limited on current consumption, which is why the buck converter must perform efficiently at these low loads. I am not able to find a connection between my measurements and the listed performance in its datasheet.

If it is of any use, the intended purpose for the buck converter is to be used in a circuit which will have a total current limit of as low as 6.5mA at 24V. As such, it is clearly not working when the buck converter itself consumes 9mA without load. The original buck converter I used was  LM5165DRCR, but I am looking at a substitute as it is out of stock with long lead-time.

Any insight into this issue is appreciated.

  • 0
    TI__Mastermind 34775 points

    Hi Martin,

    LMR3603RF is FPWM version converter which keeps switching at freq set by RT no load condition, that's why input current is high at light load. You need to use PFM version device for your application.

    B R

    Andy 

  • 0 in reply to Andy Chen
    Prodigy 10 points

    Hi Andy,

    Thank you for explaining this to me! Looking at the datasheet, it seems the adjustable output voltage version of this buck converter (LMR36503MSCQRPERQ1) has selectable PFM/FPWM. However, it does seem like this must factory-set.

    I can only find one version of the device, which states that PFM/FPWM is selectable, how can I choose the PFM version?
    Also, is there any way for me to make it run on PFM mode myself?

    Note that the fixed output version is not suitable for me as I require access to the FB pin to apply external current limiting.

  • +1 in reply to Ole-Martin Hanstveit
    TI__Mastermind 34775 points

    Hi Martin,

    For commercial grade, only LMR36503R5RPER 5-V Fixed has PFM at light load, user can't change the setting.

    For Automotive grade, only LMR36503MSCQRPERQ1 has adjustable output and PFM feature. you can try this one.

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 10 points

    Thank you Andy!

    I ordered both of these to test with my application. I appreciate your insight, it was most helpful!