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LM5166: Strange power off behavior

Trevor van Ginkel
Prodigy 10 points
Part Number: LM5166

Hello,

We are using the LM5166x in a design which shows odd power off behavior.

This behavior makes LEDs briefly flash and flicker (but always the same pattern) when removing the power, which to us is undesirable.

The LM5166x is configured as follows:

When removing the power yields the following voltage on VCC:

The voltage over an LED (very similar):

VIN of the LM5166 (yellow) vs VCC (blue)

Note how the input voltage (+24V, yellow) falls and after a certain threshold rises again, to then slowly fall again.

VCC (blue) shows the spikes which correspond to the short flashes we see on the LEDs but then both VIN and VCC very slowly fall towards 0V, but never quite reach it.

Using the LM5166x in another project with different hardware yields the same behavior.

Thusfar the only fix we found was to increase the input capacitance but before going any of these band-aid routes, we are wondering what might cause these spikes and very slow decrease in voltage and if it pertains to the LM5166.

  • 0
    TI__Guru* 76205 points

    Hi Trevor,

    Thank you for using our products.  It seems to be caused by the lack of hysteresis at the EN pin.  When the input voltage decreases, it can trigger the EN pin to stop switching. Because there in input line impedance, the EN pin in your circuit sees a voltage given by

    VEN= VIN - I_IN x Rs

    where Rs is input line impedance, and VIN the input source voltage, and I_IN the input current.  

    Once the switching is suddenly stopped, I_IN becomes zero. Then VEN jumps up and it may trigger the circuit to restart. Then the above process can occur again, producing the non-smooth VCC drop.

    To resolve the issue, you need to add EN hysteresis as mentioned in the datasheet, which I copy and paste here for your convenience.

    Hope this clarifies.

    Best Regards,

    Youhao