LNK306 (w/ transformer) - Expecting 5.0V output... getting 10.4V output instead

Discussion in 'General Electronics Chat' started by Mahonroy, Mar 24, 2016.

  1. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    Hey guys,

    I used the LNK306 to handle the 5.0V rail on my PCB. (120VAC input). I used the datasheet and the Expert Suit to come up with the component values.

    The problem is that I am getting a 10.4V output instead of the desired 5.0V. I double checked the feedback resistors, and they are 2K and 3.84K which should generate the desired 5.0V output.

    I attached my circuit schematic, and the PCB layout of the circuit. Any help would be greatly appreciated as I am at a loss as to what might be wrong, Thanks!

    Here are the part #'s I am using for the Inductors, Diodes, and Capacitors:

    D1, D3: DFLU1400-7
    D2: US1JHE3/61T
    D4: FM4007W-W
    L1, L2: 46105C
    C4: GRM219R61E106KA12D
    C6, C7: 475CKE450MLN
    C8: 35ZLH100MEFC6.3X11
    R3: WP2S-7R5JA25

    lnk306_problem3.jpg
    lnk306_problem2.jpg
     
  2. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    I also wanted to add that as an experiment, I swapped out that 3.84K resistor with another 2K resistor, and nothing changed... still getting 10.4 volts. I'm pretty confused.
     
  3. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    I've been looking at this for a while now and I don't see any obvious issues.

    It seems to be regulating (you've got 110V input and only 10.x V output) but it's not regulating to your desired setpoint.

    Have you tried going to a higher setpoint? Try using the FB resistors they had in the spec sheet reference design for 12V output of R1 = 13.0K and R2 = 2.05K. If it then regulates to 12V, it could be symptomatic of a minimum regulated voltage at your 10.4V observed level.

    Try those resistors and post back here.
     
  4. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    Wait a minute!

    Your pinout in your layout is radically different from the datasheet. Do you have any explanation for that?

    I was looking at their recommended layout. When laying out a switching power supply, ALWAYS follow the recommended layout closely.

    upload_2016-3-24_17-18-6.png
     
  5. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    When laying out a switching power supply, ALWAYS follow the recommended layout closely.

    upload_2016-3-24_17-24-52.png
     
  6. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    Hey InspectorGadget, appreciate you taking the time to check this out.

    I am using the SO-8C package (referenced as "D" package), it has slightly different pinout than the larger version you mentioned. Here is the part number for the LNK306 IC:
    LNK306DN-TL
    lnk306_problem4.jpg
     
    Last edited: Mar 25, 2016
  7. tsan

    Member

    Sep 6, 2014
    41
    4
    How much output load you have? Minimum output load is 3 mA according to datasheet. Minimum load is required to maintain output regulation.
     
  8. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    I have a 3.3V linear voltage regulator connected up to it, a microcontroller, and a radio transceiver. I don't have the exact output load value but it should be plenty (between 25 and 50 mA while idling)
     
  9. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    hh
    OK, thanks for the update. Good job with the layout. And it sounds like you have the necessary load.

    If you can try that 12V FB resistor configuration, it would be really interesting...
     
  10. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    So before I try another resistor configuration... it mentions in the datasheet on page 7 under "Feedback Resistors R1 and R3" that the voltage is supposed to stabilize at 1.65V, and that is how the gain is adjusted, am I reading that correctly? I took a voltage reading of that pin and I am getting 12 volts here (14 volts directly behind the voltage divider)... shouldn't these voltages be much lower?

    Datasheet reference:
    http://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf
     
  11. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    189
    5
    I finally figured this out... turns out the problem was my circuit was apparently not drawing enough current for the LNK to operate properly. I soldered a 3.3K resistor directly ontop of the 0.1uf capacitor (C10) and I am now getting a solid 6 volts. However, it is still not the desired 5 volts, but it is getting closer.

    According to what I have been experiencing (e.g. 2k & 3.84K = 6 volts, 2k & 2k = 4.48 volts), am I to assume that I need a 2k & 2.63k to get the desired 5 volts?

    And by the way, thanks again for the feedback & advice!
     
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