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LM5165: Abnormal current increase in input current

Chris van Niekerk
Prodigy 130 points
Part Number: LM5165
Other Parts Discussed in Thread: LM5161,

Hi

I have a power supply that requires a wide input voltage (14VDC to 50VDC) and a 12V output with a max current requirement of aoubout 50mA. The main requirement was extremely low Iq. I selected the LM5161 for this and achieved most of the requirements. I selected PFM mode of operation to achieve the lowest possible Iq. The 12Volt output drives another regulator for powering the MCU at 3.3Volt.

Upon testing the supply, I noticed that there is a steep current change when going from 24Volt  to 30Volt. The change is about 5-fold as you can see from the attached table. The circuit diagram is also attached for reference.

I have been through the datasheet several times to see if I have missed something, but I cannot explain the increase in current. The current measurements were taken with the MCU in idle mode, consuming a constant current in all measurements.

The application requires extremely low supply currents as hundreds of nodes needs to be powered over a distance of 30+ kilometers and 1mA extra current means a few kilometers less that can be attained.

Your help will be appreciated.

Best regards

Chris

https://ticsc.service-now.com/sys_attachment.do?view=true&sys_id=7b3292761b66acd0602cf715464bcb12

  • 0
    TI__Expert 4391 points

    Hi Chris，

      We can't see your pictures. Could you please kindly upload your image document again？ Thanks

    BR，

    Victor

  • 0 in reply to Victor Yu
    Prodigy 130 points

    Hi Victor

    Here it is.

    Regards

  • 0 in reply to Chris van Niekerk
    TI__Expert 4391 points

    Hi Chris，

       I don't have a LM5165evm on hand. Could you please kindly offer waveform Vsw and Iinductance when input voltage is 24/30V？ I suspect switching behaviors are different between two conditions. 

       It will be better if you can also test the output voltage/current to confirm the load didn't change while the input voltage changed. Thanks.

    BR,

    Victor

  • 0 in reply to Victor Yu
    Prodigy 130 points

    Hi Victor

    As this is a very small module, it would be extremely difficult to measure the current between the two regulators as the components are very close together and the tracks are as wide as is possible for obvious reasons.

    I will get the waveforms when I get a chance and submit it.

    Regards

    Chris

  • 0 in reply to Chris van Niekerk
    Prodigy 130 points

    Hi Victor

    Attached are the waveforms as requested.

    The red traces are the switch waveforms, the blue are the 12V outputwaveforms and the grey are the 3.3V outputs of the secondary regulator.

    Your insight will be appreciated. Please let me know if you need anything else. The inductances and other values are on the schematic.

    Reagrds

    Chris

  • 0 in reply to Chris van Niekerk
    TI__Expert 4391 points

    Hi Chris,

      Sorry for the delay. I didn't find anything wrong from the  waveform. Please remove the secondary regulator to see if the current is still abnormal.

    BR,

    Victor

  • 0 in reply to Chris van Niekerk
    TI__Expert 4391 points

    Hi Chris，

      Your waveform showed that LM5161 work normally. The switching loss between 24V/30V input won't lead to a 5-fold current change. So I suspect the load have some change.

       I have apply for an EVM board yesterday  and will test on LM5165EVM as soon as it arrive. Once I get the result. I'll let you know.

    BR,

    Victor

  • 0 in reply to Victor Yu
    Prodigy 130 points

    Hi Victor

    Apologies. I did the split, but your site was down when I wanted to load the info and it slipped my mind till I checked for an update now.

    I removed the secondary regulator and placed a 2.7mA resistive load on the output of the LM5165.

    At 24V it drew 2.1mA and 1.2ma at 30V. This gives a 36mW consumption at 30V versus a 50mW at 24V. As the efficiency should improve at lower voltages, this still does not make sense to me.

    Please let me know your view.

    Regards

    Chris

  • 0 in reply to Chris van Niekerk
    TI__Expert 4391 points

    Hi Chris,

      Sorry for the delay. Our site is updated last weekend and it didn't remind me that you have new update on this thread.

      According to datasheet, the efficiency should improve at lower voltages. You can check if the SW pulse frequency is the same. Because LM5165 is working in pulse frequency mode at 2.7mA load, the power consumption is also determined by the switching frequency. Lower input voltage will lead to more switching pulse in same time period, which may lead to higher power consumption. 

    BR,

    Victor