can somebody make sense of this datasheet for me?

M

Michael Noone

Guest
Hi - I'm trying to figure out if a Holtek HT12D
(http://holtek.com/pdf/consumer/2_12dv110.pdf) can source enough current on
the data output pins (D8-D11) to drive an LED (I think most leds run off of
about 10-30ma). Anyways - in the datasheet it lists 1ma as the minimum d8-
d11 current, 1.6ma as the typical current, and "-" as the maximum current.
What does this mean? I'd really like to avoid causing the chip to release
it's black smoke. Thanks for your help!

-Michael Noone
 
kensmith@green.rahul.net (Ken Smith) wrote in
news:cr7nbg$ks2$2@blue.rahul.net:

In article <Xns95D1BA1ED3C51mnooneuiucedu127001@216.196.97.136>,
Michael Noone <mnoone.uiuc.edu@127.0.0.1> wrote:
Hi - I'm trying to figure out if a Holtek HT12D
(http://holtek.com/pdf/consumer/2_12dv110.pdf) can source enough
current on the data output pins (D8-D11) to drive an LED (I think most
leds run off of about 10-30ma). Anyways - in the datasheet it lists
1ma as the minimum d8- d11 current, 1.6ma as the typical current, and
"-" as the maximum current. What does this mean? I'd really like to
avoid causing the chip to release it's black smoke. Thanks for your
help!

It looks like they are specifying the current the chip will deliver
and not a destruction point.

Your design should expect less than the 1mA number at the 0.5V point.
Many of the chips will drive more than that giving the 1.6mA spec.
The maximum could be just about anything. Chances are, the output
will source and sink at least 4mA into a short circuit.

This is not enough to run LEDs, if you want them very bright. You can
see ultra-bright LEDs at 1mA if you are in doors. If you aren't using
ultra-brights or need bright LEDs, you should use a buffer.

If you are running at 5V, a HC240 or HC244 may work as the buffer.
Click to expand...
As luck would have it - I have a number of HD244s lying around. Stupid
question, but I don't need to put anything between the d8-d11 outputs of
the HT12E chip and the inputs of the HD244, do I? Like I don't need any
resistors to limit current or anything, right? I'm pretty sure I don't -
but I'm just sort of used to using resistors for that purpose with
microcontrollers, so I thought it best to ask :) Thanks,

Michael
 
In article <41D78302.A1BA182D@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
[...]
The data sheet specifies that the output transistors drop no more than
.5 volt (4.5 volt high output with 5 volt supply or .5 volt low
output) with at least 1 ma. passing through the output transistors
typically 1.6 ma.). That means that under those conditions the output
transistors have an on resistance of less than 500 ohms. typically 313
ohms. If that resistance does not provide the LED current you need,
you will have to add some sort of driver.
Click to expand...
Beware that the resistance is non-linear. The output MOSFET typically
looks like a constant current source when the voltage across it is more
than 2V.

--
--
kensmith@rahul.net forging knowledge
 
In article <41d7953d$1@mustang.speedfactory.net>,
mc <mc_no_spam@uga.edu> wrote:
[...]
Good point. Summing it up: The output current can probably be considerably
more than 1 mA, but then the output voltage will not be a valid logic level.
As long as the LED is the only thing connected to that output pin, that's
not a problem.
Click to expand...
You should not expect to get more than about 4mA when the chip is running
on 5V. Remember these are just MOSFETs not real resistors.

--
--
kensmith@rahul.net forging knowledge
 
On Sun, 02 Jan 2005 14:27:45 +0000, Ken Smith wrote:

In article <41d7953d$1@mustang.speedfactory.net>,
mc <mc_no_spam@uga.edu> wrote:
[...]
Good point. Summing it up: The output current can probably be considerably
more than 1 mA, but then the output voltage will not be a valid logic level.
As long as the LED is the only thing connected to that output pin, that's
not a problem.

You should not expect to get more than about 4mA when the chip is running
on 5V. Remember these are just MOSFETs not real resistors.

And I wonder what it does to the dissipation of the chip, driving LEDs
Click to expand...
directly with them and depending on their Rdson for the LED dropping
resistor.

I'd drive an LED with a buffer; I wouldn't put them right at the
output of that chip, and a resistor would make the LED even dimmer. )-;

Thanks,
Rich
 
In article <Xns95D1BA1ED3C51mnooneuiucedu127001@216.196.97.136>,
Michael Noone <mnoone.uiuc.edu@127.0.0.1> wrote:
Hi - I'm trying to figure out if a Holtek HT12D
(http://holtek.com/pdf/consumer/2_12dv110.pdf) can source enough current on
the data output pins (D8-D11) to drive an LED (I think most leds run off of
about 10-30ma). Anyways - in the datasheet it lists 1ma as the minimum d8-
d11 current, 1.6ma as the typical current, and "-" as the maximum current.
What does this mean? I'd really like to avoid causing the chip to release
it's black smoke. Thanks for your help!
Click to expand...
It looks like they are specifying the current the chip will deliver and
not a destruction point.

Your design should expect less than the 1mA number at the 0.5V point.
Many of the chips will drive more than that giving the 1.6mA spec. The
maximum could be just about anything. Chances are, the output will source
and sink at least 4mA into a short circuit.

This is not enough to run LEDs, if you want them very bright. You can see
ultra-bright LEDs at 1mA if you are in doors. If you aren't using
ultra-brights or need bright LEDs, you should use a buffer.

If you are running at 5V, a HC240 or HC244 may work as the buffer.

--
--
kensmith@rahul.net forging knowledge
 
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