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LM5085: Help with Buck converter. Increasing Vin increases Vout

Lasse Birk
Prodigy 10 points
Part Number: LM5085

Tool/software:

Hello I have designed a buck converter around the ic lm5085.

Vin is 22-57V and Vout is set to 21.5V with Rfb1 and Rfb2.

Increasing voltage on Vin increases voltage on Vout.

Vin = 22V Vout = 21.26V
Vin = 24V Vout = 22.17V
Vin = 26V Vout = 23.05V
Vin = 28V Vout = 23.83V
Vin = 30V Vout = 24.55V



Layout:

I have tried manually increasing Rt with other resistor to lower the frequency in steps from 380khz to 250khz, 200 khz and 180 khz without removing the issue.

Adjusting Rfb1 and Rfb2 relationship to regulate to 10V instead, seems to improve the performance.

To Vout is attached 22 Ohm power resistor to have around 1A draw during testing.

Output has clean output voltage at all measurements

Output from Source on pmos (sorry for screenshots):
24Vin vs 30V vin:

It seems the frequency is changing with Vin?

Worth noting: I messed up footprint of pmos and corrected this manually on development board.
pgate is manually soldered to lm5085 with short copper wire. <1cm

  • 0
    TI__Guru 59001 points

    Hello Lasse

    You have to check the FB pin voltage together with VOUT. Looks like the feedback divider is not working well matching with your expectation. Please keep in mind that VFB=VOUT × (RFB_BOT/(RFB_BOT+RFB_TOP))

     -EL

  • 0
    Prodigy 10 points

    Hi Eric,

    Thank you for looking at my problem.

    I have to add a note to my description that after 30V and beyond the output voltage stays constant at 24.55V.

    output from drain of pmos vs fb.
    FB is yellow (2v/d) blue is output on drain (20V/d)



    Decreasing input voltage of below 21.5V increases duty cycle to 100%, so I naively assume the FB divider is correct? Have also measures correct resistances.

    Increasing the input voltage from 21.5V and up seems to decrease the duty cycle, but then at around 30V the frequency starts increasing along decreasing duty cycle, as seen from first images. 

  • 0 in reply to Lasse Birk
    TI__Guru 59001 points

    Hello Lasse

    You said 'Increasing the input voltage from 21.5V and up seems to decrease the duty cycle' but that doesn't match with the waveforms 

    <21.5V>

    <30V> 

    Basically, the LM5085 device is not a fixed frequency device, the switching frequency varies widely by changing VIN and/or VOUT. 

    -EL 

  • 0 in reply to EricLee
    Prodigy 10 points

    Hi Eric,

    Okay thank you for clearing that up.

    Do you have any other ideas for improving the behavior with input voltage in range 22-30V where regulation is gradually increasing output voltage?

    Vin 22-30V = Vout 21.3-24.55V
    Vin 30V-57V = Vout 24.55V

  • 0 in reply to Lasse Birk
    TI__Guru 59001 points

    Hello Lasse 

    Usually, this happens when the VOUT regulation target is higher than VIN.  Please capture the FB-GND  voltage (Zoom-in waveform) at these two conditions at no load. 

    1. Vin 22-30V = Vout 21.3-24.55V
    2. Vin 30V-57V = Vout 24.55V

    -EL

  • 0 in reply to EricLee
    TI__Guru 59001 points

    I am closing this thread since I have not heard from you for sometime.
    - EL