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LM3644: Is it safe to add bulk capacitance on OUT in mF range for pass-mode operation?

Magnus H
Prodigy 10 points
Part Number: LM3644

Tool/software:

Hi,
We are using the LM3644YFFR to drive an IR projector and are seeking advice on any unwanted effects of placing a large bulk capacitance on the OUT pin. In our design the LM3644 will be operating in IR mode, but we are not expecting to ever enter boost mode, since we have a VIN of 3.9V, much higher than the IR projector will go (even considering the VHR & PMOS drop margin). We wish to add bulk capacitance in the order of 1-4mF on the OUT pin to reduce peak current into the device.
  • From the data sheet, the PMOS will act as a current limiter of 200mA when the output is below 2.3V, such as during startup of the device. With 1-4mF on OUT, this would result in several milliseconds of PMOS regulation every time we enable the output. Do you expect this to shorten the lifespan of the device?
  • As far as we understand, the device should never enter boost mode, but would the bulk capacitance still negatively affect the stability of the device in some way?
  • Given our use case, can we remove the SW inductor without consequences?
Thanks!
  • 0
    TI__Intellectual 1840 points

    Hi Magnus, 

    Got your idea of application. About the 3 questions you mentioned, 

    • This will bring no harm to lifespan of the device. 
    • Since the device always works in pass mode, there will be no risk of stability. 
    • In this case, you could save the inductor and connect IN with SW. But please make sure device in pass mode. 

    Thank you! 

    Best, 

    John.