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LM5006: uncontrolled output voltage after a too short shutdown duration

Fabien Mestrallet
Prodigy 10 points
Part Number: LM5006
Other Parts Discussed in Thread: LM5161, LM5166

Hello,

 

When using a LM5006 to supply a 5V rail from a 15-70V source, I encountered an unexpected behavior concerning the shutdown/start-up sequence. If the LM5006 is started after a shutdown duration that is too low, the voltage output is not regulated at 5V as expected.

Here are a few pictures showing that, for a shutdown duration of 100ms, the LM5006 works as intended, but for a shutdown duration of 10ms, the output seems to be uncontrolled between 20V and 25V. Other durations have been tested: the LM5006 works properly for with any shutdown duration over 20ms.

The signal 1 (dark blue) is the shutdown signal controlling a MOS that pull the RT pin of the LM5006 down. The signal 2 (light blue) is the voltage output of the power supply.

The pictures are taken from a LM5006 operating with a 30V source and a load of 80mA. Here is the schematic of the system used:

As the shutdown pin is driven by a microcontroller, I could force the shutdown duration for at least 100ms. The problem is then: how can I be sure this behavior will not happen under other conditions? May I have your opinions about this unexpected behavior?

 

Best regards,

Fabien

  • 0
    TI__Mastermind 23291 points
    hi Fabien,

    I never used this part but I could guess 2 hypothesis:

    a) congratulation, you found a bug in the state machine of the part
    b) the design is marginal and this behaviour arises at the start-up, if the disable time is "too short" for the app (dependancy on VOUT, LOAD, VIN, etc...)

    the latter is the one I would bet on... there are 2 options to verify/confirm it:
    b.1) use the EVM or the EVM bom, since fits most likely to what you want to do and reproduce the test you made (I expect the EVM will not show the behaviour you are observing)
    b.2) use a newer part, like the LM5166 or the LM5161 (or any part of the family LM516x that fits) whose performance are impressive (LM5166 in particular)

    KR
    Vincenzo