6V -> 5V

[Dan East]

I'm currently powering a PIC12F629 with 4 AA batteries. The 629 is rated at 6.5 V max, so I'm within specs. However the internal oscillator is not accurate at 6V - I have to place a resistor on VDD or VSS or the frequency is off so much it causes communication errors.

So I thought I would do things right and properly regulate the voltage at 5V. However after looking at the 7805 datasheet I see the minimum input voltage is 8V.

Now, since the thing does work properly at 6V (batteries) with a resistor, do I even need a regulator? If so, what should I use to produce 5V from 6V?

Dan East

 

[2camjohn]

You need a 'low dropout' voltage regulator.
One which outputs 5V shouldn't be that hard to find in whatever package you want.

Always check the datasheet but generally a low dropout 5V voltage regulator will need less than 6V to work properly.

 

[Dan East]

Well, maybe I'm not reading the datasheet properly.

Take this one for instance:
https://www.st.com/stonline/books/pdf/docs/2144.pdf

In the L7805A table it specifies a dropout voltage of 2V. However on the second row ("Output voltage") it shows Vi of 7.5 to 20V. So is the dropout voltage the minimum input voltage? What if the input falls below 5V? Does it just output the input voltage, or does it maintain 5V?

Dan East

 

[Mike - K8LH]

The little LM2931A from Fairchild is a 5v/100ma regulator with a dropout voltage of 200-mv...

Regards, Mike

 

[justDIY]

try just using a diode or two in series with vdd ... each diode should drop between 0.5 and 0.7 volts

problem with using a regulator, is once your batteries aren't super fresh, you'll drop below the headroom voltage, and your regulator will either stop working all together or start oscillating or other weird anomolies

alternately you could switch to 4 rechargables and have 4.8 volts

or if you want a good hard challenge, look at the TPS61040 from Texas Inst... it's a boost regulator, takes 1.7 to 6 volts in and can be programmed to give a steady 3.3, 5, 12, up to 28 volts out.

 

[DirtyLude]

Why not just use an external crystal or ceramic oscillator?

 

[Mike - K8LH]

Why not just use an external crystal or ceramic oscillator?

It's an 8-pin PIC so I suspect he doesn't want to tie up two pins for an external crystal or resonator, not to mention the extra power that would be required...

Regards, Mike

 

[Dan East]

No, I definitely don't want to go with an external oscilator.

JustDIY, I was planning on using capacitors with the 7805, which is supposed to prevent the oscillating. Isn't the the minimum input voltage that will still yield 5V specified? Because I can't seem to find it.

Dan East

 

[Nigel Goodwin]

No, I definitely don't want to go with an external oscilator.

JustDIY, I was planning on using capacitors with the 7805, which is supposed to prevent the oscillating. Isn't the the minimum input voltage that will still yield 5V specified? Because I can't seem to find it.

Dan East

As already suggested, you CAN'T use a 7805, there's no where near enough headroom - a low drop-out regulator would be OK, but wouldn't give a great battery life as there's still not a lot of headroom.

 

[panic mode]

regulators are fine but MCUs draw so little, simple resistor and zener pair should do.