Hi all,
I'm powering an MSP430 project from batteries through a TI BQ24030RHLR..BQ24030RHLR.., having an input voltage to my system (out of the BQ24030RHLR..BQ24030RHLR..) between 3.7v and 5v, thus I'm inserting the National (TI) LP3878-ADJ as a regulator down to 3.3v for the whole system.
On the functional side, I have to be able to send a shutdown command to the display before the system is turned off:
- my first approach was to monitor the SVS interrupts for sending the command as soon as the voltage drops, but I don't think it is feasible. Maybe using a capacitor? If any of you can help me with this approach I would highly appreciated.
- as that approach didn't seem to work I'm trying to implement a soft power switch, which allows me to shutdown the system from the uC and turn it on from a push button. For doing this I'm relaying in the shutdown (~SHDN) pin of the LP3878MR-ADJ/NOPBLP3878MR-ADJ/NOPB. When using the push button I feed Vin = 5V into SHDN, and I want the uC to take over that pulling up once it has booted, but it seems that the 3.3v I've got in the system are not enough to keep SHDN high. I've been reading the datasheet and it seems that the ON threshold is 1.4v when Vin=3v and Vout=1v, but it says nothing about what is the relation of this voltages with the SHDN on and off thresholds. I've found, empirically that a voltage of 3.7v holds the SHDN high and thus powers the system. Can you help me finding that threshold?
- Finally, in my desperation, I've tried to make it work driving a transistor, but didn't managed either. I only have a couple of npn transistors at hand, and I think I should be using pnp for this. Am I right?
As a side note, my project is a portable equipment, so battery conservation is highly relevant. Do you think the 1st approach with a mechanical OFF slider button would be much better (less consumption) than the shutdown of the regulator? (I think I've read the regulator only draws 10uA when in shutdown.)
Thanks a lot for any help or light you can through, best regards,
Asier.
