How do I connect the ON/OFF pin of LP2981-50DBVR to a microcontroller ??

ajmal khan

Other Parts Discussed in Thread: LP2981

Hi all, I would like to be able to turn on and off the LP2981 using a microcontroller. The output voltage of this device is 5V and my input voltage is anywhere from 6V to 12V. My microcontroller runs on 5V. A link to the datasheet is below http://focus.ti.com/lit/ds/symlink/lp2981-28.pdf On page 7, it talks about the function of the ON OFF. All the talk about totem pole and open collector is getting me confused. My question is:- Can I connect pin 3 (ON/OFF) to Vcc (5V, supplied by a separate regulator ) using a pull up resistor and connect it to the microcontroller?? Your quick response is appreciated. I want to get this schematic out for a quote ASAP Ajmal

over 14 years ago

0 Chris Glaser over 14 years ago

TI__Guru**** 150650 points

Yes, if your uC needs a pull-up resistor, then it has an open-drain output. Connect the uC output to the ON/OFF pin and connect that node to a pull-up resistor to the LDO's Vin source (another voltage is ok too).

0 ajmal khan over 14 years ago in reply to Chris Glaser

Prodigy 30 points

Thanx a bunch

0 Chris Glaser over 14 years ago in reply to ajmal khan

TI__Guru**** 150650 points

Also, make sure that the voltage that you use for the pull-up resistor does not exceed your uC's pin ratings. When the uC output floats high to turn the LDO on, the pull-up voltage will appear on the uC's pin. Most likely your Vin of 12V is not the right voltage to use.

0 Ron Michallick over 14 years ago in reply to Chris Glaser

TI__Guru**** 164456 points

Hello Ajmal,

The simplest solution is to use a microcontroller output that drives both 0V (low) and 5V (high).
If the microcontroller has open collector or open drain ports then a pull up to 5V (not 12V) will be required.

The LP2981 will be on when the OFF/ON pin voltage is greater than 1.6V
The LP2981 will be off when the OFF/ON pin voltage is less than 0.15V

Regards,
Ron M

0 ajmal khan over 14 years ago in reply to Ron Michallick

Prodigy 30 points

I looked up the data sheet of my microcontroller (Atmega16) and found that only a few pins are "open drain". I want to use a regular input output pin which outputs a 5V for a high and a 0V for a low to control the regulator.

Do I even need a pull up resistor in this case?

0 Chris Glaser over 14 years ago in reply to ajmal khan

TI__Guru**** 150650 points

No. Since you have a 'push-pull' type output, no pull-up resistor is needed.

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How do I connect the ON/OFF pin of LP2981-50DBVR to a microcontroller ??