[EE] High Side LCD Backlight Control

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[EE] High Side LCD Backlight Control

Josh Koffman
Repost with topic tag - I am so ashamed...

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: [EE] High Side LCD Backlight Control

Josh Koffman
I'll reply to John's email here so it stays with the topic tag.

On Sat, Aug 29, 2020 at 4:38 PM John Lawton <[hidden email]> wrote:
>
> 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.

I have both 5V and 3.3V on the board already, so a solution that just
used the existing 3.3V supply would add the least number of parts, no?

Josh
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Re: [EE] High Side LCD Backlight Control

Isaac Marino Bavaresco-2
In reply to this post by Josh Koffman
Implement a buck regulator with a P-Channel MOSFET, inductor, Schottky diode and capacitor.
Use a PWM output of your MCU (without feedback) and limit de duty cycle in software.

Em 29 de agosto de 2020 17:42:55 BRT, Josh Koffman <[hidden email]> escreveu:

>Repost with topic tag - I am so ashamed...
>
>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: [EE] High Side LCD Backlight Control

Bob Blick-5
In reply to this post by Josh Koffman
Hi Josh,
Simplest is a P-channel Mosfet, recommend something from Alpha and Omega Semiconductor, part number AO3-something.
Cheapest solution is probably a PNP transistor with a base resistor.
Either will be fine. Hard to say which I'd choose. Both will invert, so 0 is ON.
Cheerful regards, Bob

________________________________________
From: [hidden email] <[hidden email]> on behalf of Josh Koffman
Sent: Saturday, August 29, 2020 1:42 PM
To: Microcontroller discussion list - Public.
Subject: [EE] High Side LCD Backlight Control

Repost with topic tag - I am so ashamed...

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


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Re: [EE] High Side LCD Backlight Control

Josh Koffman
On Sat, Aug 29, 2020 at 8:50 PM Bob Blick <[hidden email]> wrote:
> Simplest is a P-channel Mosfet, recommend something from Alpha and Omega Semiconductor, part number AO3-something.
> Cheapest solution is probably a PNP transistor with a base resistor.
> Either will be fine. Hard to say which I'd choose. Both will invert, so 0 is ON.


Hi Bob,

I'm leaning towards a P-channel MOSFET. I had forgotten about A&O, I
will check out their offerings!

Would I need a gate resistor? I have very little experience with
P-channel parts.

Thanks,

Josh
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Re: [EE] High Side LCD Backlight Control

Bob Blick-5
Hi Josh,
I have used the AO3401A without a gate resistor when doing low-speed switching. It has a fairly high amount of capacitance so if you were going to be switching inductive loads I would use 47 or 100 ohms just to soften the edges from ringing and isolate the microcontroller a bit. The output resistance of most 3.3 volt microcontrollers is more than a hundred ohms anyway so it's hardly worth it, switching speed is going to be at least a few microseconds already. But driving LED backlights isn't likely to present any weirdness, just use the raw MOSFET directly. If you want to do dimming, keep the PWM frequency under a few KHz and you'll be fine. I usually use 1KHz driven from the same timebase as the system tick.
Friendly regards, Bob
________________________________________
From: [hidden email] <[hidden email]> on behalf of Josh Koffman
Sent: Wednesday, September 2, 2020 12:49 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] High Side LCD Backlight Control

On Sat, Aug 29, 2020 at 8:50 PM Bob Blick  wrote:
> Simplest is a P-channel Mosfet, recommend something from Alpha and Omega Semiconductor, part number AO3-something.
> Cheapest solution is probably a PNP transistor with a base resistor.
> Either will be fine. Hard to say which I'd choose. Both will invert, so 0 is ON.


Hi Bob,

I'm leaning towards a P-channel MOSFET. I had forgotten about A&O, I
will check out their offerings!

Would I need a gate resistor? I have very little experience with
P-channel parts.

Thanks,

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
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Re: [EE] High Side LCD Backlight Control

Josh Koffman
On Wed, Sep 2, 2020 at 4:26 PM Bob Blick <[hidden email]> wrote:
> I have used the AO3401A without a gate resistor when doing low-speed switching. It has a fairly high amount of capacitance so if you were going to be switching inductive loads I would use 47 or 100 ohms just to soften the edges from ringing and isolate the microcontroller a bit. The output resistance of most 3.3 volt microcontrollers is more than a hundred ohms anyway so it's hardly worth it, switching speed is going to be at least a few microseconds already. But driving LED backlights isn't likely to present any weirdness, just use the raw MOSFET directly. If you want to do dimming, keep the PWM frequency under a few KHz and you'll be fine. I usually use 1KHz driven from the same timebase as the system tick.

Finally back on this! Because MOSFETs are not my area of expertise, I
just want to confirm that this will work with the controller I've got.
It's an NXP chip and the maximum output is 4mA at 3.3V. I _think_
based on my reading of the AO3401A datasheet that this will work, but
I'd love a double check.

Interestingly, the part is marked as not for new designs at Digikey.
They have over 247,000 of them in stock, but worth nothing if you're
relying on them in an existing design! For me 10 of them will last me
a while.

Also interestingly just noticed that one of the potential LCDs seems
to have pullups installed, so in order to PWM the backlight I need to
ground the pin. I guess I'll be putting footprints for both N channel
and P channel MOSFETs and only populating one or the other.

Thank you!

Josh
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