Driving and dimming numerous LED strings together

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
I'll guess that NC = not critical
Total guess though..

personally I'd pick a different chip.. ISSI is to close to ISIS :D
Heh, yeah I thought the same thing when I first saw the name. Don't want to appear on any government watch lists, nevermind interpol...

I'll probably send them an email about it. Especially the zero-ohm resistor. They apparently make resistors with zero ohms (at least, DigiKey has a few -- I was curious), although I equate it to a left-handed screwdriver in that it's just marketing.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
That was fast, already heard back! Here's what they said:

Thank you for your interest in ISSI’s backlight driver, IS32BL3554. “NC” refers to “No Connect” meaning the component is not used.

The zero-ohm resistors you noted are really zero ohms or can be replaced with a solid PCB trace. The zero-ohm resistor and NC are included as “place holders” when the schematic is converted to a PCB layout. This way a component placement with solder pads will be available for future used. Future use could be to create an R/C filter if the power supply is too noisy.
In other words, they are not necessary. I'll exclude them. Now to finish the schematic!
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Schematic finished! To clarify, I'm connecting two boards via pin headers. This will allow me to surface-mount the LED board and (mostly) through-hole the power board. The connections are made at P1 and P2, which retain their numbers and labels for both schematics.

How's the below look? Any feedback, suggested changes, etc.?

Power_Supply.png

White_Test.png

(Note that both red and white share the same schematics)
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Err, I just noticed that the ISET math was misguided. If the pot is at 100% resistance, it will be at about 54 to 55 mA correctly. However! As the pot is lowered in resistance, the output increases to a maximum of just over 545 mA, which will easily blow the whole circuit up. Not good!

Unless I can source a 21.8k resistor, I'm likely to need two in series (or maybe a couple 44K in parallel). This is perfectly fine however as it shouldn't be that hard to find what I'm after. Pot would need to change to 30K however to get more range at first. Then the good news is that smaller range simply means a smaller pot value.

Side note: L1 is an inductor, while L2 is a ferrite bead. Not sure if that was clear or not, but I had to find the ferrite bead part off their demo board, where they spell that out in the parts list.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Sweet, thanks! I'm laying out the footprints now, and will report back on how it works. Probably will lay low for a week or two until it's done (unless someone spots a problem beforehand).

Thank you EVERYONE for all the help! This is really one of the better forums out there. Not only can I build it, but I can understand it too! No more copy-paste designs for me!
 
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