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5v and 3.3v power supply for LCD and PIC

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BrownOut

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I suggest you connect both regulators to the 8V input, rather than cascading them.
 
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marcbarker

New Member
Just looked. Nominal supply is from 2.0 to 3.6 V.

The wide supply range could come in handy, you could tolerate a looser 3 V regulator.
 

marcbarker

New Member
You might be able to get away with just a 78L05 and if it's a low current, an LED as a crude voltage dropper. Choose whichever colour drops the best voltage.
 
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cyrusthevirus

New Member
The LCD requires 3 mA for logic and 120 mA for the backlight.

The max the PIC can handle is 250 mA.

If I get this correctly, the circuit would feed through the LM78L05 producing a regulated 5v. Then I would connect a LED with a forward voltage of 1.7 in series to the PIC, which would produce the 3.3 needed to power the PIC.

The PIC is being used to run a PWM circuit to a MOSFET, external crystal for time and an LCD. Does current have any affect on the I/O ports or on a MOSFET?

What size wall wart should I use?
 

marcbarker

New Member
The LCD requires 3 mA for logic and 120 mA for the backlight.

The max the PIC can handle is 250 mA.

If I get this correctly, the circuit would feed through the LM78L05 producing a regulated 5v. Then I would connect a LED with a forward voltage of 1.7 in series to the PIC, which would produce the 3.3 needed to power the PIC.

The PIC is being used to run a PWM circuit to a MOSFET, external crystal for time and an LCD. Does current have any affect on the I/O ports or on a MOSFET?

What size wall wart should I use?
If the PIC takes 250 mA you'd need a Luxeon Star high power LED. Then it might look a bit silly, and the light would hurt your eyes when you are testing the PCB. Maybe a 317 is a better idea.

Is the 3 V PIC driving a PWM and a MOSFET? Wow!, all off of 3V?, that's a clever design, how did you do it?

Choose a wall wart at least double the current it'll be supplying, they are not expensive. You could pick a 5 V regulated one and delete your 5V regulator.
 

marcbarker

New Member
I'd recommend psu that safely does the job and is cheap enough.

Where will max 250 mA of current be going (to do with PIC)? Is that the PICs supply current?

How did you drive all those fets with just 3 V?
 

cyrusthevirus

New Member
I do not know what current the PIC needs to be fed at. I was trying to determine that from the specs for the PIC.

As for the FETs, yes that was my hope. Four channels each feeding a logic FET with 3.3v.

Not possible? Any other suggestions?
 

kchriste

New Member
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I do not know what current the PIC needs to be fed at. I was trying to determine that from the specs for the PIC.
Feed the PIC the required voltage (3.3V) and it will only draw the current it needs. The amount of current it will draw depends on it's clock speed, how many peripherals are enabled, how much current it's IO pins is sourcing, etc. There is a chart in the datasheet which show the typical current draw at different clock speeds. See table 26-4 on page 225 of the datasheet.
There is nothing wrong with cascading the two regulators as you originally posted. In fact, the LM3940 has a max input voltage of 7.5V so that would be required if you were feeding them with an 8V supply. As mentioned, a 5V regulated wallwart is a good option too.
As for the FETs, yes that was my hope. Four channels each feeding a logic FET with 3.3v.
Yes, you can get FETs which work with 3.3V devices. For example, the Si4466DY has an RDS(on) of 0.013Ω at a Vgs of 2.5V.
 

cyrusthevirus

New Member
My main reason for going with the PIC24 was the need for multiple PWM outputs. I have no problem switching to a different PIC espically one that is 5V. Would this make feeding the FETs much easier? Are 5V logic driven FETs much more common than 3.3V?
 

cyrusthevirus

New Member
What if I rearranged my strings so they operated from 48V 2.2A power supply and powered the FETs using a pair of TC4425 (dual FET drivers)? Wouldn't this provide the same results?
 

cyrusthevirus

New Member
Sorry that was not my purpose. This is my first circuit and I saw it as several different components, LEDs, Power Supply and IC. It seems I have the wrong approach because as soon as I post a schematic, more questions are raised. Just trying to decipher everthing and lean from each of you.
 
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