Help with transistor selection

Other Hardware General Electronics
1

I'm making a 24x16 LED matrix. If I drive a whole row of 24 LEDs @ 120mA I'll need a row transistor that can handle 2.88A, so 3A.

If the supply voltage is 5V then peak power is 15W; average dissipation (each transistor on 1/16th of the time) under one watt.

The driver for the transistors is the output of a pair of 74hc138 3:8 decoders, active low, so I need PNP transistors.

Am I missing anything, and if not is this part acceptable?

http://www.diodes.com/datasheets/ds31329.pdf

Finally, do I need a resistor between the decoder pin and the transistor base?

Thanks in advance.

2

What LEDs do you have that need 120mA? Sounds high.
74hc138 only sources 4mA
If you want 2.88A, you need a part with gain of 720.
This part only has a max gain of 390, so I would say it wasn't acceptable.

3

Great, thanks for the info...now I know to look for gain as well.

The LEDs are just standard ones normally driven @ 20mA but the spec sheet says I can drive to 150mA as long as I keep it under 0.1ms, which I'll be doing so I can PWM. They're being driven by 3 TLC5916s which support 120mA per channel.

4

You might also consider a different family of '138.

24mA source capability
brings gain requirement down to 120 - lot more findable I think.

5

I think this one will work, gain up to 1500?

http://www.diodes.com/datasheets/ds31325.pdf

6

Thanks for the chip info as well...so much to learn...

7

Hi Crossroads,

Earlier you said the 74hc138 only sources 4ma, where did you get that number?

From the data sheet

http://www.jameco.com/Jameco/Products/ProdDS/45330.pdf

wouldn't it be either DC output current per pin (25mA) or DC ground current per pin (50mA)?

Thanks

8

Look in the table - Iout 4 & 5mA to be able to get the rated output voltage.
You are looking at Absolute Maximum - you can't design to that and hope to have any longevity with the part. It will run hot and fail prematurely.

9

Thank you sir!