LM390 with MMC on/off switch

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Hi,

Is this going to work? The MMC card slot switch is to ground when the card lock is off. Not sure how to pull the logic pin low when the MMC is out of the card slot, or if current drain will drop the logic pin below 2V in a few seconds and shut the regulator down.

I want to powerup the M8 first, and then various periperals that have a large inductive load. I plan on booting from the MMC, so this powerup should be slow and steady.

http://www.national.com/ds/LM/LM...
..pg 4 for logic pin..

Thanks to any and all replies.

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Total votes: 0

Hi.

Because of the switch contacts instability, I think that the regulator ENABLE input should be driven by a latch. This could be a small transistor or mosfet, driven by the AVR state machine.

In the LM3490 datasheets there is no mention about an intrnal pull-up or a pull-down at the ON/OFF control input. I guess that this is a designer's responsibility.

A 10KΩ pull-down resistor should be sufficient to safely turn the regulator off. A PNP transistor (BC557) or a P-channel mosfet (BS250) could drive the ON/OFF pin high, to enable the regulator. There is also a need for a second open collector NPN transistor (or an open drain N-channel mosfet) to drive this high side driver input to GND, in order to isolate the AVR output when it is powered-off.

Inserting the MMC, the card slot switch enables the regulator temporarily: grounds the high side driver input to start the regulator. The AVR boots and secures the regulator enabled, untill the job is done. Then, it waits until the card is removed, monitoring the card slot switch and debouncing it on removal. Finally, the AVR releases the enable-pin drivers waiting in an infinite loop to be powered off.

Giorgos

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)