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LM4051-N: Can not get IC to produce any kind of adjustable voltage

VALERIO KIOSE
Prodigy 30 points
Part Number: LM4051-N

I got two LM4051-ADJ's and when i wire them according to the datasheet they do not vary the output voltage at all. I first tried them with a source of 4.85 V and a Rs of 470ohm so it would not go over the 12mA limit and no voltage divider, my output is 0.8V and a Vref of 0.6V (measured from pin 3 Anode to pin 1 FB). After that i tried a source of 4.85V a Rs of 140ohm and R1 5kOhm, R2 6kOhm, the output voltage stayed at 0.8V again even though it should have been above 1V. I tried many more combinations, like Rs 130, R1 105kOhm, R2 105kOhm, Webench Power Designer suggested that one for an output of 2.48V, the output stays at 0.8V. I do not know what i am doing wrong, both my devices are out of the box and both do the same thing. I need a Vo of 2.48V with an input of 4.85V - 5V. Following are some pictures from the datasheet and a drawing of how i have my device connected on my breakout board.

  • 0
    TI__Expert 7930 points

    Hello,

    I think there was an error uploading the pictures. With a 5V input and a 2.48V output the smallest Rs value is 210 ohms. Less input resistance than this could cause damage. Is it possible that you have the anode and cathode connected in reverse? Can you please resend your schematic and setup pictures?

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 30 points

    So sorry for the delay Walter, i have uploaded the pictures again i hope they work, my breadboard is full so i drew exactly how i had the IC connected, ignore any random blobs of solder and extra pins, i had SMD resistors and decoupling capacitors before, but it made no difference, SMD and or breadboarded, i only got 0.8V output. I tried once to reverse the anode and cathode and i got as an output the VDD voltage, i got scared and i pulled everything, its still possible its wrong though. Let me remind that i have te Adjustable version of this IC.

  • 0 in reply to VALERIO KIOSE
    TI__Expert 7930 points

    Hello,

    Sorry but the pictures are still not showing. Can you try copy and pasting a screenshot? Do you have an unused part that you can test again with a 210 ohm or larger Rs value to limit the current below 12mA and a voltage divider on the FB pin? A voltage divider is required for this device.

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 30 points

    huh i did not know copy paste worked here, i hope you can see them now, i do not have an unused part to try, but i will try the 210ohm and a voltage divider on the one i have, lets hope i see something

  • +1 in reply to VALERIO KIOSE
    TI__Expert 7930 points

    Hi,

    According to your picture you need to swap the anode and cathode. Pin 2 needs to be connected to Rs and R1. Pin 3 needs to be connected to ground.

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 30 points

    Silly ol' me, thank you alot Walter sir.