Current draw for VO and transistor drive : MOPAL204A

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Gryphon
LCD?
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Joined: Fri Mar 21, 2008 4:09 am
Location: Sydney, Australia

Current draw for VO and transistor drive : MOPAL204A

Post by Gryphon » Thu Oct 15, 2009 1:31 am

Hi All,

hope this isn't too silly a question but I was thinking of controlling the contrast and the backlight intensity of my MOPAL204A screens by using a couple of PWM outputs and transistors.

Basically the PWM drives a transistor with a cap on its output and this voltage gets fed into V0 for contrast.

For the backlight the transistor will be in series with a current limiting resistor for the max brightness and the PWM signal from my uc will allow me to adjust the brightness.

Anyone have any warnings against this?

Also what is the current flow into V0?

I can see that my white backlight MOPAL204A pulls 30mA for the backlight so I will be able to sze my circuit accordingly.

Thanks for your thoughts!

Cheers
Peter

Raquel
Matrix Orbital
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Post by Raquel » Fri Oct 16, 2009 12:01 pm

Hello Peter,

Thank you for posting on the forum.
Basically the PWM drives a transistor with a cap on its output and this voltage gets fed into V0 for contrast.
Yes, you should be able to get a PWM drive a transistor and I suggest to have a totempole at the emitter then connect V0 in the middle of the totempole such that it will be pulled to a voltage level all the time. Please see page 8 of the datasheet . The datasheet suggests 10k-20k range for the total of the totempole.
For the backlight the transistor will be in series with a current limiting resistor for the max brightness and the PWM signal
from my uc will allow me to adjust the brightness.
Yes, this is a good plan.
Also what is the current flow into V0?
As answered in your first question, the current flow in V0 is small, (imagine the current flow from the totempole), but it is not 0.

I hope this helps.
Raquel Malinis
Design and Development
Matrix Orbital

Gryphon
LCD?
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Joined: Fri Mar 21, 2008 4:09 am
Location: Sydney, Australia

Post by Gryphon » Fri Oct 16, 2009 10:42 pm

Thanks for your comments Racquel.

Had not thought of using a totem pole style output for the contrast but I can see that this would be a good idea.

cheers
Peter

AlphaByte
LCD?
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Joined: Tue Feb 03, 2009 3:33 pm

Re: Current draw for VO and transistor drive : MOPAL204A

Post by AlphaByte » Fri Jun 04, 2010 5:37 am

Gryphon wrote:Hi All,

hope this isn't too silly a question but I was thinking of controlling the contrast and the backlight intensity of my MOPAL204A screens by using a couple of PWM outputs and transistors.

Basically the PWM drives a transistor with a cap on its output and this voltage gets fed into V0 for contrast.

For the backlight the transistor will be in series with a current limiting resistor for the max brightness and the PWM signal from my uc will allow me to adjust the brightness.

Anyone have any warnings against this?

Also what is the current flow into V0?

I can see that my white backlight MOPAL204A pulls 30mA for the backlight so I will be able to sze my circuit accordingly.

Thanks for your thoughts!

Cheers
Peter
Regarding the backlight application:

Using a microcontroller the PWM output will work fine. The PWM output can
drive a low pass filter. The output of the filter should then be buffered with
a Darlington transistor (I'm using an SOT-223 SMT package without power
dissipation issues, white backlight only). The emitter drives the backlight.

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