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'[PIC]: Charge pump for LCD Vlc ?'
2001\04\17@140332 by Peter L. Peres

picon face
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.

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2001\04\17@151011 by Bob Blick

face
flavicon
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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

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2001\04\17@162811 by Barry King

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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
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2001\04\18@062215 by rubenj

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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

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2001\04\18@171426 by Peter L. Peres

picon face
>Why put in series resistors, except to reduce EMI?

The PIC electrical limits spec has a limit on the output current on any
pin. When the caps are discharged the limit is exceeded. This may collapse
my power supply (which is sometimes non-standard) and reset or glitch the
pic (it will recover and try again by watchdog). 220 ohms is a safe value,
I can actually use 110 ohms at 5V.

Peter

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2001\04\18@171430 by Peter L. Peres

picon face
> If the + supply is not regulated, you will see varying contrast as
> the batteries wear down.  Is this acceptable to your users?

Not really, but I plan to regulate it by changing the number or length of
the bursts (the pic monitors the supply voltage).

Peter

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2001\04\18@171435 by Peter L. Peres

picon face
>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

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2001\04\18@173055 by David VanHorn

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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.

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2001\04\18@194744 by Bob Blick
face
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>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

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2001\04\20@024720 by Peter L. Peres

picon face
> 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

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2001\04\20@034043 by Vasile Surducan

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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

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2001\04\20@040416 by Spehro Pefhany

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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,

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Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2001\04\20@043002 by Peter L. Peres

picon face
> 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

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2001\04\20@133315 by David VanHorn

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>
>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.

Say HI for us! :)

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2001\04\21@045415 by Peter L. Peres

picon face
>   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

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2001\04\21@045421 by Peter L. Peres

picon face
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

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