can somebody make sense of this datasheet for me?

snipped-for-privacy@green.rahul.net (Ken Smith) wrote in news:cr7nbg$ks2$ snipped-for-privacy@blue.rahul.net:

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

No. By definition, a logic input is what you can connect a logic signal to.

Lots of led are usefully bright at 1 ma.

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.

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.

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.

In article , John Popelish wrote: [...]

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.

In article , mc wrote: [...]

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.

In article , Rich Grise wrote: [...]

I'd do that too. I think the HC240 chip would be enough buffer.