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LM22677: Low current at the output of LM22677 buck converter

Rahul Gandhi
Prodigy 90 points
Part Number: LM22677
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hi 

I was trying to simulate the LM22677 IC in TINA-TI , converting 14V input to 5V. I used the same values as given by the calculator but still couldn't get exact output so I changed the values by trial and error a bit and got 5V output but when I measured the current I observed the following

  • The input current spikes high 4-5A and then falls to zero
  • The output current fluctuates between 600u A

I tried using the same values from Web-Bench as well but it too gave like 7V output and 60uA current.

Why does this error happen? What can be done?

Regards,

Rahul Gandhi

  • 0
    TI__Genius 9570 points

    Hi Rahul,

    The RT resistor of 1k is too low for adjusting frequency, it will pull the RT pin low and enter synchronization mode. You can choose a value based on the RT vs freq. graph in fig. 12 of the datasheet.

    Also, please check the feedback resistor values again, I think the top and bottom have to be swapped for 5V output.

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    TI__Prodigy 90 points

    Thanks for the changes the resistors were placed opposite. But still the input current peaks 2A and then falls to 60u A and the output current still is 50u A what might be the problem?

    Regrads,

    Rahul Gandhi

  • 0 in reply to Rahul Gandhi
    TI__Genius 9570 points

    Hi Rahul,

    The output current is set for 50 uA in your schematic because of the 100k resistor.

    Iout = Vout/Rout = 5/100k = 50 uA

    Input current will be high initially as the input and output capacitors are charged up then it will decrease to give only the output current.

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    TI__Prodigy 90 points

    Thanks, got it. I realized that just after I posted this doubt. 

    Also, I check the reference simulation for LM22677 in that they have used a diode myD40956 this is different from the ones that are available in TINA. If I remove this diode and replace it with a regular diode the output falls from 5V to 3V. What is different about this diode, where do I find the specifications of this diode ?

    Regards,

    Rahul Gandhi

  • 0 in reply to Rahul Gandhi
    TI__Genius 9570 points

    Hi Rahul,

    Apologies for the delay. I was on leave last week.

    I am not sure about the diode in TINA as we do not support it anymore. I would recommend using Pspice for TI as it is better supported.

    As for the diode in the reference design, myD40956, please see below image (from unencrypted spice model in product page) for specifications.

    Regards,

    Niranjan