High Side LCD Backlight Control

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High Side LCD Backlight Control

Josh Koffman
Hi all,

I'm working on a small project that uses a graphic LCD. I'd like to be
able to control the backlight (possibly PWM for brightness). On this
module the backlight ground is tied to the module ground, so I can't
switch the low side.

The backlight runs at 3.3V and maxes out at about 50mA. A bit of extra
capacity might be nice just in case I ever size up a bit on the
screen.

What would be a good way to handle this? Space will be a bit limited
so fewer parts would be better, and this is a low volume thing so I'm
happy to spend a bit more on a transistor if it means fewer external
parts. 3.3V is definitely not my area of expertise when it comes to
switching, so any help would be appreciated.

Thank you!

Josh
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Re: High Side LCD Backlight Control

John Lawton
What is your supply voltage, 5V or more?

You could use a high level linear constant current driver and switch it
via PWM, or use a buck regulator with current feedback from the
baclkight, then pwm control the regulator.

John

On 29/08/2020 21:27, Josh Koffman wrote:

> Hi all,
>
> I'm working on a small project that uses a graphic LCD. I'd like to be
> able to control the backlight (possibly PWM for brightness). On this
> module the backlight ground is tied to the module ground, so I can't
> switch the low side.
>
> The backlight runs at 3.3V and maxes out at about 50mA. A bit of extra
> capacity might be nice just in case I ever size up a bit on the
> screen.
>
> What would be a good way to handle this? Space will be a bit limited
> so fewer parts would be better, and this is a low volume thing so I'm
> happy to spend a bit more on a transistor if it means fewer external
> parts. 3.3V is definitely not my area of expertise when it comes to
> switching, so any help would be appreciated.
>
> Thank you!
>
> Josh

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Re: High Side LCD Backlight Control

Jason White-20
In reply to this post by Josh Koffman
For fun I ended up doing a lot of the work for you.

PNP BJT transistor, MMBT2907* SOT-23
Collector current 100mA, Base current 10mA. Estimated temperature rise
approx 5C at room temperature. (Vce-sat=0.1V @ Ic=100mA and Ib=10mA - >
10mW power dissipation - > 5C temperature rise )

A 220 ohm resistor to base driven by a micro would likely do the trick.

See in the datasheet ON Semiconductor "MMBT2907AL,SMMBT2907AL" the graph
"Figure 4. Collector Saturation Region" base current vs Vce.

*not necessarily the best transistor, just the first one I remembered.

Regards,
Jason White

On Saturday, August 29, 2020, Josh Koffman <[hidden email]> wrote:

> Hi all,
>
> I'm working on a small project that uses a graphic LCD. I'd like to be
> able to control the backlight (possibly PWM for brightness). On this
> module the backlight ground is tied to the module ground, so I can't
> switch the low side.
>
> The backlight runs at 3.3V and maxes out at about 50mA. A bit of extra
> capacity might be nice just in case I ever size up a bit on the
> screen.
>
> What would be a good way to handle this? Space will be a bit limited
> so fewer parts would be better, and this is a low volume thing so I'm
> happy to spend a bit more on a transistor if it means fewer external
> parts. 3.3V is definitely not my area of expertise when it comes to
> switching, so any help would be appreciated.
>
> Thank you!
>
> Josh
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
>         -Douglas Adams
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


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Re: High Side LCD Backlight Control

Jason White-20
Addendum to my email. I am assuming that the backlight has a current
limiting resistor installed and that the PNP transistor is tied to the 3.3
volt rail and that the micro can drive to required 10mA.

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Re: High Side LCD Backlight Control

Harold Hallikainen-3
In reply to this post by John Lawton

> What is your supply voltage, 5V or more?
>
> You could use a high level linear constant current driver and switch it
> via PWM, or use a buck regulator with current feedback from the
> baclkight, then pwm control the regulator.
>
> John

Most PIC pins are often good for 25 mA. You COULD maybe parallel two pins
and do a timer interrupt to switch the pins together for the PWM, then
have a resistor from those two pins to the backlight.

Another approach would be to use a PNP transistor with the emitter at Vcc,
a resistor from the base to the PWM pin, then a resistor from the
collector to the high side of the backlight. The PWM would be "upside
down" since a high output would turn off the transistor.

I'm assuming above that the PIC supply is high enough to light the LED. If
not, another approach would be to have the PIC drive an NPN transistor or
N channel FET. The drain would go to Vcc through an inductor. The
backlight high side would connect to the collector/drain. When the
transistor turns off, the collector voltage will increase as the inductor
magnetic field collapses. It will then force current through the
backlight. The current through the backlight at the start would be the
final inductor current, then linearly ramp down. You might do a constant
pulse duration so the inductor ramps up to a fixed current, then vary the
frequency go get varying duty cycle. There are likely to be some
oscillations, so an RC snubber might be required to clean up the
waveforms.

Good luck!

Harold


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Re: High Side LCD Backlight Control

Josh Koffman
In reply to this post by Jason White-20
On Sat, Aug 29, 2020 at 4:51 PM Jason White
<[hidden email]> wrote:

>
> For fun I ended up doing a lot of the work for you.
>
> PNP BJT transistor, MMBT2907* SOT-23
> Collector current 100mA, Base current 10mA. Estimated temperature rise
> approx 5C at room temperature. (Vce-sat=0.1V @ Ic=100mA and Ib=10mA - >
> 10mW power dissipation - > 5C temperature rise )
>
> A 220 ohm resistor to base driven by a micro would likely do the trick.
>
> See in the datasheet ON Semiconductor "MMBT2907AL,SMMBT2907AL" the graph
> "Figure 4. Collector Saturation Region" base current vs Vce.
>
> *not necessarily the best transistor, just the first one I remembered.

Hi Jason,

I appreciate the help! I feel like I recently saw some MMBT2907s in my
stash of parts....somewhere. I will have to figure out where...and why
I had them!

Thank you!

Josh
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Re: High Side LCD Backlight Control

Josh Koffman
In reply to this post by Harold Hallikainen-3
Hi Harold!

On Sat, Aug 29, 2020 at 8:00 PM Harold Hallikainen
<[hidden email]> wrote:
> Most PIC pins are often good for 25 mA. You COULD maybe parallel two pins
> and do a timer interrupt to switch the pins together for the PWM, then
> have a resistor from those two pins to the backlight.

This particular design isn't using a PIC. I realize now I've been
spoiled for years by high powered I/O. I can't even parallel anything
as I'm already about 3-4 pins short of where I want to be!

> Another approach would be to use a PNP transistor with the emitter at Vcc,
> a resistor from the base to the PWM pin, then a resistor from the
> collector to the high side of the backlight. The PWM would be "upside
> down" since a high output would turn off the transistor.

The resistor from the collector to the backlight is the regular
current limiting LED resistor, correct?

Thank you!

Josh
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Re: High Side LCD Backlight Control

Harold Hallikainen-3
Hi Josh!

Yes, the resistor from the collector to the backlight is the regular LED
current limit resistor. Ideally the transistor is driven into saturation,
so there's only a few hundred millivolts across the transistor.

Harold

> Hi Harold!
>
> On Sat, Aug 29, 2020 at 8:00 PM Harold Hallikainen
> <[hidden email]> wrote:
>> Most PIC pins are often good for 25 mA. You COULD maybe parallel two
>> pins
>> and do a timer interrupt to switch the pins together for the PWM, then
>> have a resistor from those two pins to the backlight.
>
> This particular design isn't using a PIC. I realize now I've been
> spoiled for years by high powered I/O. I can't even parallel anything
> as I'm already about 3-4 pins short of where I want to be!
>
>> Another approach would be to use a PNP transistor with the emitter at
>> Vcc,
>> a resistor from the base to the PWM pin, then a resistor from the
>> collector to the high side of the backlight. The PWM would be "upside
>> down" since a high output would turn off the transistor.
>
> The resistor from the collector to the backlight is the regular
> current limiting LED resistor, correct?
>
> Thank you!
>
> Josh
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
>         -Douglas Adams
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


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