go_away

Author Topic: Advice needed, Going outside an IC's current rating.  (Read 1013 times)

0 Members and 1 Guest are viewing this topic.

Offline madsci1016Topic starter

  • Contest Winner
  • Supreme Robot
  • ****
  • Posts: 1,450
  • Helpful? 43
Advice needed, Going outside an IC's current rating.
« on: September 29, 2010, 12:34:25 PM »
As per my issues with my Arduquee project, I need some advice.

The first design, all loads were within (or close) the current limits of my shift registers. I was able to use the open-collector TPIC series shift registers which can sink 150+ mA, which is great for the worst case scenario of all 8 20mA LEDS on in a row.

Now that I have to change polarity, I need a high current shift register that can source the same amount of current. There doesn't seem to be such a beast.

I found this guy that is using a regular 595 (rated for 35mA per pin) to source all his rows. Worst case for him is 56 x 20mA with LEDS on in a row. It makes my head hurt on why that isn't exploding.

For me it would only be 16 x 20mA worst case because I will have a separate register per matrix, but two colors per pixel. Is this within the scope of sanity that this will work? Is the other guy getting away with it because scanning means it's only ever at 1/8 duty cycle? If that's the case, i'll have to break out the enable pins on the shift registers, so the LEDS aren't left 'on' during programming and power up.

Offline chelmi

  • Supreme Robot
  • *****
  • Posts: 496
  • Helpful? 15
Re: Advice needed, Going outside an IC's current rating.
« Reply #1 on: September 29, 2010, 02:48:07 PM »

Offline madsci1016Topic starter

  • Contest Winner
  • Supreme Robot
  • ****
  • Posts: 1,450
  • Helpful? 43
Re: Advice needed, Going outside an IC's current rating.
« Reply #2 on: September 29, 2010, 09:03:52 PM »
If only the TI chip was a serial input, it would be PERFECT.

Problem is I don't have the room to add another IC just for LED driving.


Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Advice needed, Going outside an IC's current rating.
« Reply #3 on: September 30, 2010, 01:44:29 PM »
Hi,

[...] using a regular 595 (rated for 35mA per pin) to source all his rows. Worst case for him is 56 x 20mA with LEDS on in a row. It makes my head hurt on why that isn't exploding.
Luck perhaps.


For me it would only be 16 x 20mA worst case because I will have a separate register per matrix, but two colors per pixel. Is this within the scope of sanity that this will work? Is the other guy getting away with it because scanning means it's only ever at 1/8 duty cycle? If that's the case, i'll have to break out the enable pins on the shift registers, so the LEDS aren't left 'on' during programming and power up.
If there's only 1 out of 8 on at any time, the off ones won't count of course.
If you, with the 1/8, go over the limits mentioned as normal working conditions, you are taking a chance.
If you go over (or even near) the max. ratings, all bets are off - you may get lucky and get better than the average component that the specs are for and you can monitor temperature rise in your own unit, but it gives bad Karma selling something that isn't designed to keep within specs (in your case, the Karma part may be quite tangible, as it's your "name" on the line) - ask yourself if you really wanna risk that and consider a redesign, whatever it takes to get your kit within limits.

There's enough poor design for sale already, better make it that bit more expensive and make sure you have a product that can handle itself (just my opinion of course, but there's a reason I don't buy Kitchen Sink Products, not because Kitchen Sink is necessarily a bad "company", but because there's so many poor hobby designs masked as real products and they're rarely that much cheaper, even though the established engineering businesses have years and years of experience and Quality Assurance that costs - and it's impossible to tell which will break and which will hold, if you've got nothing but a photo on a website to judge from.
That's why my advice is to make rock solid products from day one - it's hard to build a rep, but easy to shatter one by a single mistake.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016Topic starter

  • Contest Winner
  • Supreme Robot
  • ****
  • Posts: 1,450
  • Helpful? 43
Re: Advice needed, Going outside an IC's current rating.
« Reply #4 on: September 30, 2010, 02:05:17 PM »
I agree Soeren, it makes me nervous. Failing any miracle high current sourcing shift register ICs that someone knows about; I've already sent off a PCB design to be fabbed with the ICs I already have. I intend to build one and test the heck out of it, worst case for several weeks. Even that won't guarantee it 's a good thing.

Failing this, I might just drop the design for the 3mm matrix all together, and goto 5mm, with the room to add parallel source drives.

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Advice needed, Going outside an IC's current rating.
« Reply #5 on: September 30, 2010, 02:25:22 PM »
Hi,

Perhaps you could find room for 8 SOT-23 size MOSFET complimentary pairs to be driven by a low current shift reg?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016Topic starter

  • Contest Winner
  • Supreme Robot
  • ****
  • Posts: 1,450
  • Helpful? 43
Re: Advice needed, Going outside an IC's current rating.
« Reply #6 on: October 11, 2010, 04:30:18 PM »
Hi,

Perhaps you could find room for 8 SOT-23 size MOSFET complimentary pairs to be driven by a low current shift reg?


I like the idea, but would rather have less chips to solder. I've googled a bit, perhaps you know from experience of an array of pairs? At least in doubles, preferable quad.

EDIT

Found these but they are $2 a chip. So i'd be spending $2 to save $3 on LED matrices. Goodness, this stinks.

Can someone whip out a miracle and show me a TPIC6C595 equivalent high current sourcing shift register? plz!
« Last Edit: October 11, 2010, 05:10:14 PM by madsci1016 »

Offline madsci1016Topic starter

  • Contest Winner
  • Supreme Robot
  • ****
  • Posts: 1,450
  • Helpful? 43
Re: Advice needed, Going outside an IC's current rating.
« Reply #7 on: October 11, 2010, 05:23:42 PM »
Can someone whip out a miracle and show me a TPIC6C595 equivalent high current sourcing shift register? plz!


Nevermind, I can make my own miracles.

 


Get Your Ad Here