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LP3856-ADJ: Load Regulation > 0.08%

Wolfgang Zindler
Prodigy 100 points
Part Number: LP3856-ADJ

Hello
I use this component in a multi-power supply.

During the final test of the output voltage, I found that the deviations are greater than indicated in the data sheet.

Here are the results:

Here are the drawing of the 5V-output:

Why are my deviations greater than indicated in the data sheet?

Regards

Wolfgang

  • 0
    TI__Mastermind 32825 points

    Hello Wolfgang,

    Can you confirm that you are probing directly at the point of regulation?  The point of regulation is right on the top feedback resistor.  You will want the DMM at the top setpoint resistor and directly on the GND pin of the device.  If you are probing elsewhere, such as further away at the load of the LDO, then you will be including the IR drop across the board.  The values you collected suggest around 15 miliohms of DC drop on the forward and return paths if you are not measuring at the point of regulation, which is certainly possible in modern board designs.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 100 points

    Hello Stephen,
    Thank you for your feedback.
    I measure at the front jack against GND and a feedback cable goes directly from the front jack back to the resistor R10.

    This is intended to eliminate the cable resistance from the LDO output to the front jack.

    Regards

    Wolfgang

  • 0 in reply to Wolfgang Zindler
    TI__Mastermind 32825 points

    Okay, thanks for clarifying, I wanted to be sure IR drop was not it.  In this case the delta is due to the thermal change of the reference voltage.  At 2.9A load, for example, the device will dissipate nearly 2.5W in this setup.  The adjust pin voltage is typically 1.2165V +/- 3% across temperature.  With a 2.5W dissipation across the device the junction will raise significantly and slightly change Vout.  You are seeing a 1.5% change which is well within the datasheet EC table.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 100 points

    Hello Stephen,
    okay, that will be the reason.
    I measured the voltage on the ADJ pin and the value goes back to 1.195V at 2.9A,

    which corresponds to an output voltage of 3.252V.
    Thank you for the help and the explanation!

    Regands

    Wolfgang