ADD5211 power on

Dear Sir/Madam,

I`m using the ADD5211 to drive a LCD backlight, but sometimes (very rare) it doesn´t power on.

After connecting the EN-pin or the UVLO-pin shortly to ground it starts again.

Unfortunately I didn´t connect the Fault-pin, so I can`t monitor any fault state.

I tried to connect EN-pin or UVLO-pin via a 100kOhm resistor to VIN, but without any success.

I can also reproduce it by permanent powering the ADD5211 and switching off/on the EN-pin.

I already ordered the evaluation board and use quite the same values for all components,

but could not reproduce the faulty behavior.

Production date ADD5211: 1319 @ADD5211-Evalboard and 1634 in my design

Any idea?

Many thanks,


  • 0
    •  Analog Employees 
    on Jul 5, 2017 8:27 PM

    Hi Sven,

    The only latched fault conditions causing the ADD5211 to turn off are LED short or LED open, these can also be indirectly triggered by another condition such as in-rush current. Since you could replicate the issues by repeatedly enabling and disabling the part I would exclude connection issues with the backlight.

    Could you please let me know the following:

    - Is LSD pin biased as suggested in the datasheet or tied to VDD? If biased, could you try to force it to VDD and see if the problem goes away? I am not suggesting this as a solution but to isolate the origin of the issue.

    - How many LEDs, color and strings are used in your application?

    - Could you confirm the values for the passive components used and the input supply voltage?

        -> I would need the exact p/n for COUT, L1, D1, Q1, Rcs and Css 

    - If needed, would you be able to probe the FAULT pin together with VOUT with a two channels oscilloscope?

    Best regards,


  • Hi Maurizio,

    thank you for your quick answer and your guidelines.

    First I figured out that the FAULT pin stays high under fault condition.

    After I found the main different between my circuit and the eval board.

    *** 100nF for emc reasons between U_out and FB ***

    Without the capacitor the ADD5211 works well!

    Thank you again and best regards,

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:52 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin