Post by thread:[PIC]: Charge pump for LCD Vlc ?

Hello list, I have several battery run applications using PICs and standard ASCII LCD modules. The devices run on 4.8V nominal (NiCd) and the LCD contrast is not good enough. I am going to try to make a small charge pump to make a Vlc of -0.8 to -5V to fix this. I have made charge punps before, and I would not like to use a chip for this, therefore please do not suggest one (I don't want a 7660 in it). The following ideas come to my mind: 1. Voltage doubler charge pump driven from pic pin using 0.01 uF + 220 R to drive a SOT32 double diode as rectifier with 0.1 uF filter cap to contrast potentiometer. This should make -3.2V or so. I cannot afford an interrupt so the pic will output pulses on that pin whenever not busy doing something else (bursts of 10's of fast pulses several tens of times per second). 2. A simple booster converter circuit with one PNP bipolar transistor and a transformer, controlled by a PIC pin (on/off). This can make as many volts as I want but it contains a coil that needs to be built. The output needs to be regulated (or clamped) somehow (or not ?). 3. A simple booster using a PNP bipolar and a standard inductor, driven by the PIC with short pulses from time to time (as in 1). 4. Give in and use a proper micropower switching supply for the whole system(s). I like #1 and this is what I am going to try out soon. What would you pic(k) ? #1 adds only four standard parts to my design. All the other options are expensive. tia, Peter PS: The Vlc potentiometer is 10K and the LCD is standard (Vlc 0..5V). I suspect that -0.6V of Vlc is all I am going to get with these LCDs but I have to try. Those 0.6V are the 0.6V that are missing to make a proper contrast display imho. The other option is to add another cell to the battery and use some sort of low loss voltage dropping scheme (not a regulator) when the battery is full. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Peter, I have used the PIC's crystal out pin before for this, no problem. I was able to use an "extended temperature" type LCD this way. The amount of voltage you get depends on the oscillator type you select, and I suppose also the frequency and load capacitors to some extent. With 2 conventional small signal diodes and two small capacitors I got -1.2 volts with XT oscillator selected, and almost -3 volts with HS oscillator selected(running a 4 MHz xtal). I forget what value capacitors I used, I think 1000pF. Did not have any problems with oscillator startup either way. Just make sure it's the out pin not the in pin :-) Cheerful regards, Bob Blick -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Peter, you suggest: > 1. Voltage doubler charge pump driven from pic pin using 0.01 uF + 220 R > to drive a SOT32 double diode as rectifier with 0.1 uF filter cap to > contrast potentiometer. This should make -3.2V or so. Yes, this is what I do. My displays are wide-temp-range fluid, so they must have the negative supply. Why put in series resistors, except to reduce EMI? >I cannot afford an > interrupt so the pic will output pulses on that pin whenever not busy > doing something else (bursts of 10's of fast pulses several tens of times > per second). I use the CCP in PWM mode, on a low end PIC without CCP, software bursts sould work fine, but you may see varying contrast if its uneven. If the + supply is not regulated, you will see varying contrast as the batteries wear down. Is this acceptable to your users? -Barry. ------------ Barry King NRG Systems "Measuring the Wind's Energy" http://www.nrgsystems.com Check out the accumulated (PIC) wisdom of the ages at: PIC/PICList FAQ: http://www.piclist.com/faq -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

What is the lowest voltage the LCD needs to operate? Put a diode in series with the LCD 0V to raise it about 0.6V above 0V. Connect LCD Vlc to real 0V which will give about -0.6V for contrast. However the digital signals to the LCD will be at -0.6V to the LCD instead of 0V when low. I don't know if this will work, just a thought. Best regards, Ruben {Quote hidden}> Hello list, > > I have several battery run applications using PICs and standard ASCII > LCD modules. The devices run on 4.8V nominal (NiCd) and the LCD > contrast is not good enough. I am going to try to make a small charge > pump to make a Vlc of -0.8 to -5V to fix this. I have made charge > punps before, and I would not like to use a chip for this, therefore > please do not suggest one (I don't want a 7660 in it). The following > ideas come to my mind: > > 1. Voltage doubler charge pump driven from pic pin using 0.01 uF + 220 > R to drive a SOT32 double diode as rectifier with 0.1 uF filter cap to > contrast potentiometer. This should make -3.2V or so. I cannot afford > an interrupt so the pic will output pulses on that pin whenever not > busy doing something else (bursts of 10's of fast pulses several tens > of times per second). > > 2. A simple booster converter circuit with one PNP bipolar transistor > and a transformer, controlled by a PIC pin (on/off). This can make as > many volts as I want but it contains a coil that needs to be built. > The output needs to be regulated (or clamped) somehow (or not ?). > > 3. A simple booster using a PNP bipolar and a standard inductor, > driven by the PIC with short pulses from time to time (as in 1). > > 4. Give in and use a proper micropower switching supply for the whole > system(s). > > I like #1 and this is what I am going to try out soon. What would you > pic(k) ? #1 adds only four standard parts to my design. All the other > options are expensive. > > tia, > > Peter > > PS: The Vlc potentiometer is 10K and the LCD is standard (Vlc 0..5V). > I suspect that -0.6V of Vlc is all I am going to get with these LCDs > but I have to try. Those 0.6V are the 0.6V that are missing to make a > proper contrast display imho. The other option is to add another cell > to the battery and use some sort of low loss voltage dropping scheme > (not a regulator) when the battery is full. > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > ============================== Ruben Jvnsson AB Liros Elektronik Box 9124, 200 39 Malmv, Sweden TEL INT +46 40142078 FAX INT +46 40947388 spam_OUTrubenTakeThisOuTpp.sbbs.se ============================== -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>I have used the PIC's crystal out pin before for this, no problem. I was >able to use an "extended temperature" type LCD this way. Uhh. Okay, you win the prize for simplicity ;-). However, do you need a broadcast license for this ? Some of my projects require 'quiet' operation from time to time or go to sleep and I don't know how to achieve this with your scheme. What I did do in the past is drive the gate of a logic level FET from Oscout (through series RC and resistor to GND). This drove a standard inductor to obtain higher voltage. This was using 455kHz ceramic resonators on the PIC. It was noisy enough to make me think twice before using it again. thanks, Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 10:38 PM 4/18/01 +0300, Peter L. Peres wrote: > >I have used the PIC's crystal out pin before for this, no problem. I was > >able to use an "extended temperature" type LCD this way. > >Uhh. Okay, you win the prize for simplicity ;-). >However, do you need a broadcast license for this ? Highly likely! You're better off, if you must, to do a charge pump off one of the I/O pins. >This drove a standard inductor to obtain higher voltage. >This was using 455kHz ceramic resonators on the PIC. It was noisy enough >to make me think twice before using it again. 455k is getting a little adventurous, but I would only expect layout-driven issues. Gonna be a lot of RF current into that gate cap, and the source needs a solid link back to the uP gnd and VCC pins. I'm wondering if you were turning the fet fully on and off, since the output drive on the pic is relatively wimpy for such things. -- Dave's Engineering Page: http://www.dvanhorn.org Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>What I did do in the past is >drive the gate of a logic level FET from Oscout (through series RC and >resistor to GND). This drove a standard inductor to obtain higher voltage. >This was using 455kHz ceramic resonators on the PIC. It was noisy enough >to make me think twice before using it again. I'd expect that would be noisier than what I described. Hey, Whatever fits all your specs, do it. For a display going inside of a computer case I didn't see any problem using the crystal out. Cheers, Bob -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> Put a diode in series with the LCD 0V to raise it about 0.6V above > 0V. Connect LCD Vlc to real 0V which will give about -0.6V for > contrast. However the digital signals to the LCD will be at -0.6V to > the LCD instead of 0V when low. I don't know if this will work, just > a thought. > > Best regards, Ruben This is an idea to try. I have my doubts because the magnitude of Vdd - Vlc determines contrast but it is worth a try. I will try to put a Shottky diode between LCD Vss and GND and see what happens. thanks, Peter -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

On Fri, 20 Apr 2001, Peter L. Peres wrote: > > Put a diode in series with the LCD 0V to raise it about 0.6V above > > 0V. Connect LCD Vlc to real 0V which will give about -0.6V for > > contrast. However the digital signals to the LCD will be at -0.6V to > > the LCD instead of 0V when low. I don't know if this will work, just > > a thought. > > > > Best regards, Ruben > > This is an idea to try. I have my doubts because the magnitude of Vdd - > Vlc determines contrast but it is worth a try. I will try to put a Shottky > diode between LCD Vss and GND and see what happens. > I don't think -0.6V is enough for a good contrast, that's depend of LCD type ( viewing angle and temperature), I found lcd's with good contrast at vlc=+2.5v but also lcd's with optimum vlc=-2V. ( all potential reffer to ground ) That diode solution will alter also the logic input levels, so I will not use it. Obtaining -Vlc by rectifying and inverting a square wave generated by one pic pin, sound to be the better choice. Up to -6...-8V can be easily obtained using a voltage doubler. Vasile -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

At 08:33 AM 4/20/01 +0300, you wrote: >> Put a diode in series with the LCD 0V to raise it about 0.6V above >> 0V. Connect LCD Vlc to real 0V which will give about -0.6V for >> contrast. However the digital signals to the LCD will be at -0.6V to >> the LCD instead of 0V when low. I don't know if this will work, just >> a thought. Since I'm going to be seeing Bob Pease tomorrow it reminds me of the following trick - not applicable to this case because the available current is very low, microamps. Also you need +12V or so. +12------- 1K ------ -------> -0.4V E \ / C --------- Q1 | B | 0V Q1 C8050 etc. NPN BJT Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam.....interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> 455k is getting a little adventurous, but I would only expect > layout-driven issues. Gonna be a lot of RF current into that gate cap, > and the source needs a solid link back to the uP gnd and VCC pins. I'm > wondering if you were turning the fet fully on and off, since the > output drive on the pic is relatively wimpy for such things. 455k is about standard in consumer equipment smpsus now (<10W) so you can get coils for it. The manufacturers are moving up across the AM band now ... The gate cap of a 0.8W logic level fet is about 5 to 20 pF. I wanted to series tune it by making it a part of the PI network but then it turned out this was not necessary. The fet turned on fully and made a nice square wave on the drain which made me add the series RC and slow it down. However, that time I had a full 5V regulated power available. The PIC was burned with XT oscillator option and the signal on Oscout was nearly rail to rail square wave. The logical level fet usually switches at about 1.8V Vgs with lots of S. Peter -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> That diode solution will alter also the logic input levels, so I > will not use it. Obtaining -Vlc by rectifying and inverting a square > wave generated by one pic pin, sound to be the better choice. Up to > -6...-8V can be easily obtained using a voltage doubler. > > Vasile The Shottky diode will put the input voltage (Vil) for the LCD to -0.3-0.4V which is within spec. Normal (not extended temperature) LCDs accept 0..5V on Vlc, not -2.5V. -6..-8V can NOT be obtained by running a voltage doubler in inverting mode from +5V. -4V is about the limit with Shottky diodes and no load. Maybe Ge diodes will increase it a little bit more but it will be less than -5V in any case. Peter -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

Spehro wrote: > Since I'm going to be seeing Bob Pease tomorrow it reminds me of the > following trick - not applicable to this case because the available > current is very low, microamps. Also you need +12V or so. That one I thought about too but discarded it immediately because of the 10K contrast pot which would kill the voltage on the spot (the available current is about 2 x 10^-8A (20 nA) at -0.4V. And you don't need 12V. You can do it with 5V with transistors with Vber < 5V like UHF antenna amplifier transistors. An interesting exercise would be to plot the available current vs. injected current. Who knows maybe a 2N3055 with 200mA into the inverted BE can supply more than I thought ;-). Peter -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu