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Noob question here.

Currently I'm using a darlington transistor array to drive about 14 IR leds. I want to be use a CD74HC4067 multiplexer to simplify and shrink things up a bit. The IR led's I'm using require 100ma of current, so from what I understand, the problem is that this multiplexer cannot handle this amount of current.

From the multiplexer's datasheet, it says at 4.5V, the "on resistance" is 70 Ohms. Correct me if I'm wrong, I'm assuming this means that the maximum current that it can handle is around 64ma given by Ohms law.


Put simply, this is the what I'm trying to achieve:

From this: enter image description here

To this: enter image description here


My questions are:

  1. Assuming the aforementioned is correct, is there a way to change the "on resistance" to handle 100ma of current, or temporarily reduce the amount of current passing through until exiting the multiplexer?

  2. If so, is this as simple as adding a resistor somewhere or how to do so?


If I'm completely off, I'd appreciate suggestions or advice.

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  • \$\begingroup\$ How would a multiplexer simplify and shrink things? It is not clear whether you need to control the LEDs individually and what you are trying to achieve. \$\endgroup\$ – Wesley Lee Apr 19 at 13:50
  • \$\begingroup\$ @WesleyLee I am trying to control them individually, and using a multiplexer would reduce the amount of space needed to do so, but that is least important \$\endgroup\$ – Dante Biase Apr 19 at 13:54
  • \$\begingroup\$ One of the ways of doing it would be to plug the output of the multiplexer on the input of the darlington array. Another option is to use darlington arrays that are controlled via SPI. \$\endgroup\$ – Wesley Lee Apr 19 at 14:22
  • \$\begingroup\$ @WesleyLee After viewing the illustrations I added, wouldn't that defeat the purpose of using the multiplexer in the first place? \$\endgroup\$ – Dante Biase Apr 19 at 14:25
  • \$\begingroup\$ If you want to control 14 LEDs with a darlington array, you need 14 outputs from the MCU. If you use a multiplexer you need less outputs (2~3 for say SPI or I2C). So that does simplify routing and I/O requirements. \$\endgroup\$ – Wesley Lee Apr 19 at 14:37
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Posting a schematic may help us to understand the problem better. If you are using a Darlington array the output resistance of your digital IC is irrelevant, as it would be in series with a much higher resistance.

enter image description here

To explicitly answer your question(s): no, you can't change the current capabilities (internal resistance) of the digital IC. If it can't handle the load, you need some kind of driver, as you implemented.

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  • \$\begingroup\$ I updated the question with illustrations to show what I am trying to achieve, with that in mind, is there still nothing I can do? \$\endgroup\$ – Dante Biase Apr 19 at 14:22
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You'd be seriously exceeding spec-sheet maximum ratings of this CD74HC4067 chip:
limits
Its "ON" resistance of about 60 ohms is inherent to its design, and can't be changed. Running at a high DC supply might help reduce \$ R_{on} \$ a little, bit. Don't forget, this is a "typical" value and could be greater.
This resistance would likely dominate the total resistance in the LED current path. Don't forget that the LED will drop voltage of about 2V.

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