Post by thread:PIC select Vcc

I want to have a PIC switch it's power supply between 5.5V and 3.5V. Any ideas? It needs to default to 5.5V on powerup. And it can only use 1 pin. I was thinking: 5.5V | | -------------o | | \ | /470 | \ | | 10 | PIC--o---\/\/-- FET | | | Vcc to circuit This would work for the 5.5V supply. How do I do a 3.3V supply now? I'm using a FET because of the load I'm drawing. Thanks. Andy ================================================================== Montana Design Tech Support - http://www.montanadesign.com ==================================================================

On Wed, 12 May 1999 14:48:13 -0400 Andy Kunz <spam_OUTsupportTakeThisOuTMONTANADESIGN.COM> writes: >I want to have a PIC switch it's power supply between 5.5V and 3.5V. >Any >ideas? It needs to default to 5.5V on powerup. And it can only use 1 >pin. If you're regulating down from a higher voltage, use an adjustable regulator like a LM317. The PIC pin would be connected through a resistor to the adjust terminal. On power up, the pin is an input so there would be no current through the resistor and the regulator would run at 5.5V. If you make the pin an output and drive it low, then the current through the resistor would set the regulator up for a lower operating voltage. Making the pin an output and driving it high would raise the voltage to maybe 6.2 V, which you may not want. That could be avoided by using the open drain RA4 pin, adding a diode, or using a transistor to switch the resistor. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

5.5V----->|---------+---PIC | 3.5V----->|---------+ This will drop your voltage a little (use Schotkys). Just an idea, maybe not what you meant. Gabriel ----Original Message----- From: Andy Kunz <.....supportKILLspam@spam@MONTANADESIGN.COM> To: PICLISTKILLspamMITVMA.MIT.EDU <.....PICLISTKILLspam.....MITVMA.MIT.EDU> Date: Wednesday, May 12, 1999 4:20 PM Subject: PIC select Vcc {Quote hidden}>I want to have a PIC switch it's power supply between 5.5V and 3.5V. Any >ideas? It needs to default to 5.5V on powerup. And it can only use 1 pin. > >I was thinking: > > 5.5V > | > | > -------------o > | | > \ | > /470 | > \ | > | 10 | >PIC--o---\/\/-- FET > | > | > | > Vcc to circuit > >This would work for the 5.5V supply. How do I do a 3.3V supply now? > >I'm using a FET because of the load I'm drawing. > >Thanks. > >Andy > >================================================================== > Montana Design Tech Support - http://www.montanadesign.com >==================================================================

Improved: 5.5V | | -------------o | | \ | /470 | \ | | 10 | PIC--o---\/\/-- FET | o------|<|------- 3.3V | Vcc to circuit When the FET is on, supplying the 5.5V source, the diode is reverse biased and off. When the pic pin is activated, turning off the FET, the voltage at the diode's cathode is close to zero, thus forward biasing the diode and allowing supply of your 3.3V source. Chose a low dropout diode as suggested recently in the list. What kind of FET are you using that you need such a heavy pullup? Craig > {Original Message removed}

i'm not sure i understand why you want to do the switching: save power? then the 470R resitance is rather small: 11mA when you drive the pin low. you could use another circuit like this one for the 3.3V, but with a p-channel fet, i guess -- if you find one with a really low g-s threshold voltage. thinking of it, how's it with the gate-source threshold in your circuit? it seems that the "vcc to circuit" is at least 2V below the 5.5V. here you go... that's your 3.3V circuit... :) use a p-channel fet for the 5.5V (source to 5.5V) and pull the gate to ground -- this gives you -5.5V gate-source and lets the vcc output be lower than the 5.5V only by the on-state resistance of the fet. and use an n-channel fet for the 3.3V (drain to 3.3V) and pull the gate to 5.5V somehow (independently of the pic's supply voltage -- a transistor?) -- this gives you >2.2V gate-source, which should be enough for a low threshold fet. (if it have to be fets.) ge At 14:48 05/12/99 -0400, Andy Kunz wrote: {Quote hidden}>I want to have a PIC switch it's power supply between 5.5V and 3.5V. Any >ideas? It needs to default to 5.5V on powerup. And it can only use 1 pin. > >I was thinking: > > 5.5V > | > | > -------------o > | | > \ | > /470 | > \ | > | 10 | >PIC--o---\/\/-- FET > | > | > | > Vcc to circuit > >This would work for the 5.5V supply. How do I do a 3.3V supply now? > >I'm using a FET because of the load I'm drawing.

On Wed, 12 May 1999 14:48:13 -0400 Andy Kunz <EraseMEsupportspam_OUTTakeThisOuTMONTANADESIGN.COM> writes: > I want to have a PIC switch it's power supply between 5.5V and 3.5V. Any > ideas? It needs to default to 5.5V on powerup. And it can only use 1 pin. I'm not sure if it would help, but ISTR one of the Maxim power converter chips has a 3.3/5V select pin- it runs from a supply of down to 1.5V, so it might be an easy solution. Nigel -- Nigel Orr Research Associate O ______ Underwater Acoustics Group, o / o \_/( Dept of Electrical and Electronic Engineering (_ < _ ( University of Newcastle Upon Tyne \______/ \(

Andy Kunz wrote: >I want to have a PIC switch it's power supply between 5.5V and 3.5V. Any >ideas? It needs to default to 5.5V on powerup. And it can only use 1 pin. Some questions, Andy. Can you use pin RA4? The problem with your circuit as drawn is that the 470R resistor is gonna be above the PIC supply voltage when you want to operate from 3.3V. Is the 3.3V supply derived from the 5.5V supply? How much current do you actually need at both the 3.3V and 5.5V operation. How much drop can you tolerate? If the whole circuit is operating from a single supply that *changes* from 3.3V to 5.5V, you could just manipulate the Vadj pin on your voltage regulator. This works for both 317 type linear regs as well as LM2575 type switchers. dwayne Dwayne Reid <dwaynerspam_OUTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 15 years of Engineering Innovation (1984 - 1999) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. My posting messages to Usenet neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.

>Can you use pin RA4? The problem with your circuit as drawn is that >the 470R resistor is gonna be above the PIC supply voltage when you want to >operate from 3.3V. The 470 is a typo. It should have been 47K. This gives just enough trickle to bring the circuit up in 5V mode (slowly). >Is the 3.3V supply derived from the 5.5V supply? How much current do you >actually need at both the 3.3V and 5.5V operation. How much drop can you >tolerate? I need 250mA minimum available. >If the whole circuit is operating from a single supply that *changes* from >3.3V to 5.5V, you could just manipulate the Vadj pin on your voltage >regulator. This works for both 317 type linear regs as well as LM2575 type >switchers. Both are coming from a 13-16V supply. They can be two regulators, or one switched between two values, or a 5V supplying the 3V. Doesn't matter. Andy ================================================================== Montana Design Tech Support - http://www.montanadesign.com ==================================================================

At 16:57 05/13/99 -0400, Andy Kunz wrote: >Both are coming from a 13-16V supply. They can be two regulators, or one >switched between two values, or a 5V supplying the 3V. Doesn't matter. in this case i guess i would try a simple (switcher) regulator circuit and switch the feedback resistor divider. ge

Andy Kunz wrote: >>If the whole circuit is operating from a single supply that *changes* >> from 3.3V to 5.5V, you could just manipulate the Vadj pin on your >> voltage regulator. This works for both 317 type linear regs as well >> as LM2575 type switchers. > Both are coming from a 13-16V supply. They can be two regulators, or > one switched between two values, or a 5V supplying the 3V. Doesn't > matter. Fine. So use the idea you just quoted! The LM317 ( http://www.national.com/pf/LM/LM117.html ) uses a fixed resistor, nominally 240 ohms, between output and reference, while the resistor from reference to ground determines the drop from reference to ground. For 5V output, this would be 3.75V and the resistor 720 ohms. For 3.3V, the drop is 2.05V and the total resistance, 390 ohms. However, presuming your resistor for this setting is paralleled with the 720 ohms, you need 870 ohms. I'd try this: __________ V+ o-----|In Out|--+---------------o Vdd | LM317 | | | | < | Ref | < 240 |__________| < | | 820 +--------+--VVVVV--------o PIC control | Open for 5V < Ground for 3.3V < 750 (or 680) < | V- o----------+------------------------o Vss -- Cheers, Paul B.