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Topic: Power-on delay circuit (Read 1 time) previous topic - next topic

Andy Brown

I've got a device that has 3.3V and 1.8V power inputs. I'm providing 3.3V directly and 1.8V via an LDO regulator. The device requires that there is at least a 10ms delay between the 3.3V (first) and 1.8V (second) supplies on power up.

It's tempting to do the lazy way and buy a 1.8V regulator with an EN pin and drive that from the MCU but I've got some MCP1700 devices in stock that only have Vin/Vout/GND pins and I'd like to use these.

So how would you design a (minimum) 10ms delay circuit around the MCP1700 using the smallest number of components? I was thinking an RC delay circuit to the base of a transistor. Yes?
Home of the Nokia QVGA TFT LCD hacks: http://andybrown.me.uk

BillO

What are the current requirements on the 1.8V supply?
Facts just don't care if you ignore them.

Andy Brown


What are the current requirements on the 1.8V supply?


The maximum current requirement on the 1.8V supply is 82mA.
Home of the Nokia QVGA TFT LCD hacks: http://andybrown.me.uk

BillO

Hmmm.  If it were about 30ma or less I would have suggested running the regulator directly off an i/O pin on the MCU.

Okay, just use a P-channel MOSFET to turn on power to the 1.8V regulator using the MCU.
Facts just don't care if you ignore them.

Riva

Just use a simple Resistor/Capacitor combination to create a delay
http://ladyada.net/library/rccalc.html
http://forum.arduino.cc/index.php?action=unread;boards=5,67,10,11,66,12,15,17,21,22,23,24,25,29;ALL

pwillard

Quote
If it were about 30ma or less I would have suggested running the regulator directly off an i/O pin on the MCU.


To contradict that statement, I would never consider using an I/O pin as a power source for a "circuit", an LED or other single low current component "maybe", but to be used for generic "power source", I call that pin abuse.

My philosophy:  Output pins are TTL High/Low "signals", not power sources, that "tell" other components to do some work.

Considering removing the "magic smoke" from an $0.11 transistor versus a $5.00 MCU... the math is easy.

BillO

I did say 30ma or less.  Or did you miss  that?  30ma or less is not pin abuse and it matters not the tiniest squat what that 30ma is used for.  Your post, however, might just be a little nasty.
Facts just don't care if you ignore them.

dhenry

Use a mcu's pin to float the regulator's GND pin to shut it down. Ground that pin to turn on the regulator.

Andy Brown

Good suggestions guys, thank you very much.
Home of the Nokia QVGA TFT LCD hacks: http://andybrown.me.uk

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