Why the voltage drop without any reason?

Hi guys
I’m using MSP430F2274 which needs a power supply of 1.8V - 3.6V.

I have a computer charger which gives 19V and maximum 1.5A. I regulate (correct word?) that with L7812CV down to 12V which I then use in a motor controller. I then regulate those 12V to 5V with L7805CV and use a voltage divider with 4.7kΩ and 9.4kΩ to get it down to 3.3V (I have some capacitors also connected just in case)

When measured without the MSP430 connected, it is just as predicted, 3.3V across. But when I connect the MSP430, it drops down to 1.04V which the MSP430 doesn’t run on.

Someone suggest me to use LM317, and I have check it online and found some datails about it. From the details(http://www.kynix.com/Detail/799868/LM317.html) ,do you think it is OK?

Any suggestions on what could be wrong? Thanks very much.

Yes regulate is the correct word. You are running in to ohms law. While under no load your resistor voltage divider works the MSP430 draws current, and thus effectively changes the resistance of your voltage divider and lowers the voltage. While you could adjust the resistance values a voltage regulator is a better bet. However rather than an lm317 (which will certainly work) I’d suggest an ldo 3.3V regulator (ldo meaning low dropout so that it will work from the 5V input supply). The advantage being that you don’t have to set the voltage as you would with an lm317. I couldn’t find (on a brief search) 3.3V ldo regulators on kynix.com but digikey.com has lots of them however.
(edit:) One more thing to watch out for: if you have 5V logic or sensors (running off the 7805) it looks like the MSP430 isn’t 5v tolerant so you would need level converters for any 5v level signals you want to put in to the micro.


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