[sdiy] current sink idea

[brianw]

Thanks for the Maxim application note. So, Analog Devices bought Maxim...

The last display backlight that I looked at would accept either a control voltage or a PWM signal. I made the assumption that this particular device has a conversion circuit that operates PWM internally, and either converts the input CV to PWM or uses the input PWM directly. If that assumption is correct, it seems more efficient to drive it with PWM if that's how it's working internally. The point of this line of questioning is that a poorly shielded display that's running PWM internally probably won't be any worse feeding it PWM than CV.

I suppose I could be wrong in my assumption. A simple LED plus resistor might react well to either CV (within the appropriate range) or PWM. The caveat here is that most LED PWM runs the LED at a much higher current for shorter bursts, and that current level is way too high for continuous current. This is generally how PWM achieves brighter LED output without burning out the LED.

At any rate, I'm sure that the reason such display backlights accept both CV and PWM is for maximum compatibility with various use cases or designs that might have only one option.

Brian


On Nov 30, 2024, at 12:11 PM, Roman Sowa wrote:
> Thank you.
> 
> Aparently this is old news anyway. Someone found Analog Devices article from 2006 where such current sources are discussed:
> https://www.analog.com/en/resources/technical-articles/variations-among-currentsinksource-circuits-with-a-single-op-amp.html
> 
> Roman
> 
> W dniu 2024-11-30 o 17:33, Jean-Pierre Desrochers pisze:
>> To me it's a very good idea and a very different way to solve this design problem..
>> I've always liked and promoted the 'weird' ways to solve problems
>> that are too often analyzed and cured with 'mostly' available solutions.
>> Easy problems can be solved by easy solutions though..
>> But sometimes you need to use your 'grey matter'.
>> 
>> Good job Roman !
>> JP
>> 
>>> -----Message d'origine-----
>>> W dniu 2024-11-30 o 09:17, Roman Sowa pisze:
>>>> Since criticizing circuit ideas is a new trend, here's something I
>>>> came up with 2 days ago.
>>>> Special needs require special solutions - I needed to control
>>>> display's backlit intensity from internal DAC of the microcontroller,
>>>> which unfortunately is unbuffered and rather high impedance. Obvious
>>>> simplest solution with emitter follower works, but even high beta
>>>> transistor loads the DAC too much. I could replace it with Darlington,
>>>> but then there's even more of DAC range lost and possibly even worse
>>>> repeatability between units.
>>>> Next step was well known current source/sink using opamp driving
>>>> transistor with current sensing resistor. But that's 2 active parts
>>>> and space is limited. So I made this (hoping ithis shows up in the
>>>> email body or attachment):
>>>> Current sink schematics
>>>> I moved the sensing resistor to opamp's power input thus eliminating
>>>> the need of external transistor. Quiescent current is 60uA so it
>>>> doesn't affect performance. It works up to 23mA of LED current, which
>>>> is all I needed. It can also be controlled by PWM with 1st order
>>>> very-low pass RC filter, like 1M+100nF adding free fade in/out effects.
>>>> 
>>>> And here's how it looks:
>>>> assembled prototype
>>>> 
>>>> Roman