HC595 Outputs - How much current/Leds ?
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Hi stampers,
I'm designing a circuit using the HC595 as per page 357 BSM v1.09 and
am trying to find out how much current I can pull out of the 595.
Eg. If i design the circuit as per the stamp manual, how many leds
can I have on at any one time before the 595 overheats/overloads?
If I assume each led is using 20mA, then 4 leds on would be using
80mA. Can the 595 handle this? I can't seem to find it mentioned in
any of the pdfs that I have downloaded for this chip.
Thanks again,
Simon
I'm designing a circuit using the HC595 as per page 357 BSM v1.09 and
am trying to find out how much current I can pull out of the 595.
Eg. If i design the circuit as per the stamp manual, how many leds
can I have on at any one time before the 595 overheats/overloads?
If I assume each led is using 20mA, then 4 leds on would be using
80mA. Can the 595 handle this? I can't seem to find it mentioned in
any of the pdfs that I have downloaded for this chip.
Thanks again,
Simon
Comments
This indicates that the circuit in the Stamp manual is drawing more current
than the chip is designed for, about (4.9-1.2)/470~8 ma. (Actually, the IC
would probably current limit to less than this). To stay within 4 ma, you
need ~820 ohms, which would work, but the LEDS will be pretty dim. To get
more brightness, you should use transistors or a driver IC.
Good luck,
Ray McArthur
Original Message
From: <egroups@d...>
To: <basicstamps@egroups.com>
Sent: Thursday, November 30, 2000 9:38 PM
Subject: [noparse][[/noparse]basicstamps] HC595 Outputs - How much current/Leds ?
> Hi stampers,
>
> I'm designing a circuit using the HC595 as per page 357 BSM v1.09 and
> am trying to find out how much current I can pull out of the 595.
>
> Eg. If i design the circuit as per the stamp manual, how many leds
> can I have on at any one time before the 595 overheats/overloads?
>
> If I assume each led is using 20mA, then 4 leds on would be using
> 80mA. Can the 595 handle this? I can't seem to find it mentioned in
> any of the pdfs that I have downloaded for this chip.
>
> Thanks again,
>
> Simon
>The TI datasheet indicates the 595 can source or sink only 4 ma. per output.
>This indicates that the circuit in the Stamp manual is drawing more current
>than the chip is designed for, about (4.9-1.2)/470~8 ma. (Actually, the IC
>would probably current limit to less than this). To stay within 4 ma, you
>need ~820 ohms, which would work, but the LEDS will be pretty dim. To get
>more brightness, you should use transistors or a driver IC.
I really do hate to be contradictory, but the data sheet that I currently
work from shows only that the minimum output voltages on outputs QA thru QH
are guaranteed to be:
VDD=5V Vout HI >= 3.7V @ 6 mA, Vout LO <= 0.4V @ 4 mA
VDD=6V Vout HI >= 5.2V @ 7.8 mA, Vout LO <= 0.4V @ 7.8 mA
But these figures only tell you how much the output changes with load and
do NOT represent the maximum rating that the pin is good for. To be
honest, they really don't specify. But they do state, in the maximum
rating section, that maximum output current for any one output is 35 mA
maximum and that maximum VDD or ground current is 70 mA maximum.
The data sheet tells us a couple of things: the hc595 can sink current
better than it can source it, and that the total current from all outputs
must be less than 70 mA.
Now - you really don't want to run the chip at the absolute maximum current
rating. But 75% or 80% is not too bad, especially if you are sinking
current. Sinking current as opposed to sourcing current is important: the
amount of heat the chip dissipates is directly proportional to voltage drop
across the output FET. The data sheet states that at VDD=5V and Io=6 mA,
you lose 1.3V worst case if sourcing current versus 0.4V worst case if
sinking current. Bottom line: you can run your LEDs at 7 or 8 mA with no
problem at all.
All of the above info was taken from the MM74HC595 data downloaded from the
Fairchild site.
One other thing - I have (somewhere) another data sheet that shows maximum
ground current for a HC595 at something like 200 mA - all other parameters
being similar. That's the data sheet I used when I designed one of my
control panel display cards - this card has 40 LEDs all driven (sinking)
from 74HC595 shift registers. LED supply is 4.0 Vdc and the LED resistors
are 100R. That works out to about 23 mA per LED. The card routinely has
all 40 LEDs on at the same time. Even though I have violated Fairchild's
spec (but not that other data sheet), I have not yet seen one
failure. That would be several hundred cards (maybe a thousand) over 4 or
5 years, in industrial settings where down time is simply not tolerated. I
*would* find out about any failures!
I hope this helps.
dwayne
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 16 years of Engineering Innovation (1984 - 2000)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
The TI data sheet I looked at only specified Vout hi and Vout low at 4 ma,
with no max ratings. ... not nearly as complete as your data. Does this
mean that TI doesn't guarantee the drive levels Fairchild quoted?
Thanks,
Ray McArthur
Original Message
From: Dwayne Reid <dwayner@p...>
To: <basicstamps@egroups.com>
Sent: Thursday, November 30, 2000 11:32 PM
Subject: Re: [noparse][[/noparse]basicstamps] HC595 Outputs - How much current/Leds ?
> At 10:25 PM 11/30/00 -0500, Ray McArthur wrote:
> >The TI datasheet indicates the 595 can source or sink only 4 ma. per
output.
> >This indicates that the circuit in the Stamp manual is drawing more
current
> >than the chip is designed for, about (4.9-1.2)/470~8 ma. (Actually, the
IC
> >would probably current limit to less than this). To stay within 4 ma,
you
> >need ~820 ohms, which would work, but the LEDS will be pretty dim. To
get
> >more brightness, you should use transistors or a driver IC.
>
> I really do hate to be contradictory, but the data sheet that I currently
> work from shows only that the minimum output voltages on outputs QA thru
QH
> are guaranteed to be:
>
> VDD=5V Vout HI >= 3.7V @ 6 mA, Vout LO <= 0.4V @ 4 mA
> VDD=6V Vout HI >= 5.2V @ 7.8 mA, Vout LO <= 0.4V @ 7.8 mA
>
> But these figures only tell you how much the output changes with load and
> do NOT represent the maximum rating that the pin is good for. To be
> honest, they really don't specify. But they do state, in the maximum
> rating section, that maximum output current for any one output is 35 mA
> maximum and that maximum VDD or ground current is 70 mA maximum.
>
> The data sheet tells us a couple of things: the hc595 can sink current
> better than it can source it, and that the total current from all outputs
> must be less than 70 mA.
>
> Now - you really don't want to run the chip at the absolute maximum
current
> rating. But 75% or 80% is not too bad, especially if you are sinking
> current. Sinking current as opposed to sourcing current is important: the
> amount of heat the chip dissipates is directly proportional to voltage
drop
> across the output FET. The data sheet states that at VDD=5V and Io=6 mA,
> you lose 1.3V worst case if sourcing current versus 0.4V worst case if
> sinking current. Bottom line: you can run your LEDs at 7 or 8 mA with no
> problem at all.
>
> All of the above info was taken from the MM74HC595 data downloaded from
the
> Fairchild site.
>
> One other thing - I have (somewhere) another data sheet that shows maximum
> ground current for a HC595 at something like 200 mA - all other parameters
> being similar. That's the data sheet I used when I designed one of my
> control panel display cards - this card has 40 LEDs all driven (sinking)
> from 74HC595 shift registers. LED supply is 4.0 Vdc and the LED resistors
> are 100R. That works out to about 23 mA per LED. The card routinely has
> all 40 LEDs on at the same time. Even though I have violated Fairchild's
> spec (but not that other data sheet), I have not yet seen one
> failure. That would be several hundred cards (maybe a thousand) over 4 or
> 5 years, in industrial settings where down time is simply not tolerated.
I
> *would* find out about any failures!
>
> I hope this helps.
>
> dwayne
>
>
>
> Dwayne Reid <dwayner@p...>
> Trinity Electronics Systems Ltd Edmonton, AB, CANADA
> (780) 489-3199 voice (780) 487-6397 fax
>
> Celebrating 16 years of Engineering Innovation (1984 - 2000)
>
> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
> Do NOT send unsolicited commercial email to this email address.
> This message neither grants consent to receive unsolicited
> commercial email nor is intended to solicit commercial email.
>
>
>
>
>Good info, Wayne:
>
>The TI data sheet I looked at only specified Vout hi and Vout low at 4 ma,
>with no max ratings. ... not nearly as complete as your data. Does this
>mean that TI doesn't guarantee the drive levels Fairchild quoted?
>
>Thanks,
>Ray McArthur
Hi Ray, Duane, Wayne, et al -
Just a quick comment - many of the Fairchild datasheets DO NOT reflect the
original data sheets, regardless of the original producer. So far I have
run into this 6-8 times, and eventually I had to dig back thru my own
archives, or, in some cases, various producers have obso archives as well.
Seems strange, I know - but that's been my experience. Fair warning - so to
speak <shrug>.
Regards,
Bruce Bates
>Good info, Dwayne:
>
>The TI data sheet I looked at only specified Vout hi and Vout low at 4 ma,
>with no max ratings. ... not nearly as complete as your data. Does this
>mean that TI doesn't guarantee the drive levels Fairchild quoted?
I don't interpret it that way (yet). I'd have to take a closer look at
TI's data sheet, but I suspect that their maximum ratings section is either
in a different part of the data sheet or is at the front of the data book
in a section called "family characteristics" or some such thing.
What they are really telling you in that part of the data sheet is how the
device will drive other logic. In other words, you can use their numbers
to decide how many TTL loads you can drive with each output, or how many
LS-TTL loads, or whatever. They give you the guaranteed numbers, you as a
designer take those numbers and match them up with the input specs of
whatever logic family you want to drive and decide just how many inputs
that output can feed.
We are using those outputs to drive loads other than logic - LEDs in this
case. TI says in that part of the data sheet: we will guarantee that Vout
LO will be no higher than 0.4V when sinking 4 mA - and that's all they are
saying. That doesn't imply that 4 mA is the maximum sinking current.
Maybe look at it backwards. Most TTL or LS-TTL logic uses 0.4V as the
worst case logic 0 voltage when designing. TI is simply saying that to be
less than 0.4V, you must sink less than 4 mA.
Because we are driving LEDs, we don't care exactly what the output LO
voltage is. So long as we stay within the maximum current ratings of the
ground and VDD pins, the maximum current rating for each individual output,
and the internal temperature of the chip does not get too high, we are
fine. If I was using a uln2003 to drive LEDs, for example, the fact that I
lose about 1.2V across the output device isn't a problem at all.
I do recommend, though, to drive LEDs sinking rather than sourcing when
using the hc595. The LED driver chips on that display card I mentioned
earlier hardly get warmer than ambient, even when all the LEDs are
on. Plus - you have the luxury of having a separate LED supply if you have
to drive lots of LEDs. In the case of that display card, the 5V logic
supply comes from a simple 5V zener regulator (total logic current is only
5 mA or so) with a National Simple Switcher switch mode supply supplying
regulated 4.0 Vdc for the LED supply. This allowed me use a standard value
of SIP resistor network (100R 8 pin bussed) for the LED current limit
resistors. I simply adjusted the LED supply to give me whatever voltage I
needed to get the LED current I wanted. Incoming supply is 16 Vdc
unregulated - the card would have been too hot to touch if I had simply
used large value LED dropping resistors.
The card I have been talking about uses DIP parts. I re-did another
display card from many years ago early last year where I use 8- SO-16 parts
to drive 64 LEDs. This is the channel display for our pyro / explosives
control system. I don't have as much of a track record with it - there are
nowhere near as many cards out in use as the previous card but - no
problems there either. Its very similar technology - the same Simple
Switcher 4.0V supply, 100R SIP resistor networks. One card for each 32
channels of pyro, with red and green LEDs showing continuity and status for
each channel. The master console uses 4 display cards to show 128 channels
at a time. The 32 channel firing boxes use 1 card. The baby 8 channel
boxes use their own (smaller) version but otherwise identical technology.
I re-did the pyro system display cards because when I did the original
version back in '92, the 4094 CMOS shift registers I had available back
then would not drive LEDs directly - at least, not reliably. So I had a
matrix of 8- 4094 SRs with all the outputs bussed together and driving a
uln2803 NPN, a 4017 CMOS counter driving both the output enables on the
4094s and a uln2892 PNP darlington array. The LEDs were wired in an 8x8
matrix. It has worked OK since it was designed back in '92 but suffered
from 'ghosting' because I didn't have any dead time between each successive
output enable time slot. The new card fixes that problem quite nicely, as
well as being easier on the battery. But the little SMT hc595 chips do get
somewhat warmer than the DIP parts - I measured about a 10C rise above
ambient on a hot summer day. Again, though, no failures to date.
The pyro system is a bit interesting in that the technology used to build
it back in the early '90s has been disappearing. Its based entirely on
PLDs - I started off with Intel 5C series (5C032, 5C060, 5C090). Intel
dropped the whole PLD line several years back - that didn't worry me too
much because Altera was an identical second source. But Altera
discontinued the EP-320 (5C032) a couple of years ago - I ran out last year
and had to re-do the cards that used those parts. Now the other parts are
slated for obsolescence as well. <sigh> Life goes on.
Anyways, I've started rambling which is a bad sign. I'll stop now.
Do take care.
dwayne
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 16 years of Engineering Innovation (1984 - 2000)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
> >Good info, Dwayne:
> >
> >The TI data sheet I looked at only specified Vout hi and Vout low at 4
> >ma, with no max ratings. ... not nearly as complete as your data. Does
> >this mean that TI doesn't guarantee the drive levels Fairchild quoted?
>
TI makes a line of high current 595 compatible ICs. Look for TPIC6x595x