12v DC supply for 5v and 7v loads on PCB.... direction needed

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
Thx guys for the new info. I will study this today and let it all sink in. I'll probably have a few more questions this evening.
Your input is very much appreciated!!
 

eetech00

Joined Jun 8, 2013
2,809
Lots of conflicting and some incorrect info in this thread.

Listen to Ian0 in post #3. or LesJones in post #4. They are essentially saying the same thing.

You want to drive the LEDs with a constant current of 450mA. Putting them in series does this with just one driver, whereas 3 would be required if you did it in parallel. They will need a supply that can provide constant current or 450mA at 21V.

Bob
The LEDs are CREE XLamp, XPE2, Red-Orange. There are three LEDs prewired in series.

The spec for each XPE2 LED is:

350ma@2.18v
1000ma@2.6v
 
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Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
To set the LED current to 450 mA Rset on the application circuit in the AL8843 data sheet will need to be 0.22 ohm.
Yo can control the brightness by connecting the PWM output from the ATtiny85 to the control input of the AL8843.
If your total load on the +5 volt rail is only 40 mA you will only waste 280 mW using a linear regulator. You could use an LM78L05 which is smaller than an LM7805. I think designing your own switching regulator based on an Arduino would waste more power than a linear regulator. If you want to use a switching regulator an MC4063 is one possible solution.

Les.
It sounds like the AL8843 is the IC version of the LD24AJTA module. Am I right?
 

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
@johnaustinkaty

The AL8843 will not produce a PWM output @ 450mA, it uses the input PWM signal to adjust brightness by changing the average output current.
450mA would be the maximum when pulse width is 255 (100%).
The PWM signal from the attiny is fading the LED up over 30 seconds, 100% for 3 min, then fades back down over 30 seconds. I may even go as high as 600mA, depending on testing results (brightness)
 

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
I don't think so. I can find no data on the LD24AJTA module. It does not look like it uses the AL8843 chip.

Les.
I agree. I was wondering if the circuit on the LD24AJTA module might be similar to the circuitry inside the ALL8843 IC, basically accomplishing the same thing?
 

LesJones

Joined Jan 8, 2017
3,655
It looks like it is functionally similar but I don't know if it achieves it in the same way. I would personally not use it as there seems to be only one seller of it on ebay. There seems to be a reliable supply chain for the AL8843 from reputable suppliers. (And a proper datasheet is available.)

Les.
 

ElectricSpidey

Joined Dec 2, 2017
2,027
@johnaustinkaty

I'm sorry my point was probably confusing, I was just trying to show that the output from the driver is not the same as PWM with a constant current 317.

Again, sorry for the confusion.

The AL8843 can produce the current you want just not the same way.
 

eetech00

Joined Jun 8, 2013
2,809
If you want to keep it simple, here's another way to do it.
Split the total power dissipated over two resistors (R1/R2) instead of one. Each resistor will dissipate 1/2 the total power.

1626730185936.png
 

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
I am using the following formula to find the required resistor for the AL8843 led circuit:

Iout = .1/RCS

So, I come up with a resistor size of .17 ohm (three .5 ohm resistors in parallel), which would give me 588mA.

I have decided to use a max current of around 600mA (LED @ 100%) instead of 450mA.
That is still less than 60% of the max current for my LED.

Does this sound right?
 

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
That chart is for analog dimming.
Aren't you using PWM dimming?

If so..

Rset=0.17 ohms
L=47uh, 0.2ohm
Fsw=376.7Khz
PWM_freq=500Hz (or less)
DC max=67% for 600ma

The values were generated using their calculator.

Yes, I am providing the PWM pulse from the attiny IC to the CTRL pin on the AL8843.
 

Thread Starter

johnaustinkaty

Joined Jul 16, 2021
37
you won’t need LED current limit resistor(s)
Awesome. I'm taking them out. I'll post my schematic and board shortly. Please be kind LOL. I am NOT an electrical designer, but it makes sense to me. This is six weeks of work for me. I welcome any and all input/recommendations of course.
 
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