Driving LEDs from 74HC using PWM.

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AndyGadget

Senior Member
I've just decided that a couple of 74HC595 latched shifters is the best way to drive a matrix of 15 LEDs for a project I'm working on, but I'm planning to do it the dirty way by driving them with no resistors and using the HPWM of a 20x2 into OE of the 595s to control the average current (brightness). Looking at the HC595 absolute max ratings I think I'll get away with it as long as I don't get too ambitious.
Just wondering if anyone's tried anything similar.
 
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BeanieBots

Moderator
Not advised!
Unlike an LED which is fairly happy with an average current via PWM, the 595 output current should not be exceeded even for a brief moment.
You might get away with it but it's not advised.
Totem pole output transistors can blow very quickly when abused.
 
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Andrew Cowan

Senior Member
Max current per pin = 35mA
Max current for package = 75mA

You could drop the voltage to avoid needing a diode - look at the Vf/If graph in the LED's datasheet. Don't forget the internal drop in the 595 (0.7v) when calculating.

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AndyGadget

Senior Member
Ta for the replies. In the light of Beaniebots' I'll probably do it the proper way and take the component count hit. Those were the current figures I was looking at and assuming thermal momentum would allow averaging, but I've never tried abusing MOSFET gates in that way. (However, I may just sacrifice a chip and see how far I can take it #;¬)
 
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AndyGadget

Senior Member
Thanks - The 5916/17 look more interesting as they come in a 16DIL package rather than small outline. Farnell do them too.

Twenty years ago when I was designing kit and writing in-circuit test programs for a living I could tell you what just about any chip around was, and give you the pinouts for a fair proportion. Now I'm doing this as a hobby and there's so many weird and wonderful devices around I've lost track of what's out there.
 
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BCJKiwi

Senior Member
Two 5916/7s would be required as they are 8 channel.

Had good results from the Philips SAA1064 (an oldie but goodie). It is a 24 pin 16 channel wide DIP, bank switched unit with brightness control. Designed for 4 x 7 segment displays and can be used with or without the bank switching.
Just watch the full chip current capacity if running a lot of Leds at once at higher brightness settings.

http://www.nxp.com/#/pip/pip=[pip=SAA1064_CNV_2]|pp=[t=pip,i=SAA1064_CNV_2 ]
(The link doesn't work as the forum doesn't like the sq brackets - just copy and paste to the browser)

and available from the usual suppliers - price at Futurlec $3.20
 
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