Help with transistor selection

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?

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

Thanks in advance.

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.

Great, thanks for the 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.

You might also consider a different family of '138. 24mA source capability brings gain requirement down to 120 - lot more findable I think.

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

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

Hi Crossroads,

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

From the data sheet

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


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.

Thank you sir!