For a multiplex LED matrix, I am intending to use two ULN2803's as sink source for 16 times 20 mA per column (channel).
However, maybe I will try a test with 100 mA LEDs (10% duty cycle). For that I have IRLZ44Ns (see Datasheet IRLZ44N.
See the circuit below (simplified because of the circuit maker area restriction and somehow some connections are connected which should not, but the idea is clear I hope).
Below is the thermal resistance table on the first page:
Parameter Min. Typ. Max. Units RθJC Junction-to-Case –––– –––– 1.4 °C/W RθCS Case-to-Sink, Flat, Greased Surface –––– 0.50 –––– °C/W RθJA Junction-to-Ambient –––– –––– 62 °C/W
If I'm right the calculation is as follows:
16 (leds) * 0.1 (A) = 1.6A
For the voltage, I'm not sure. I intend to use a 9V adapter, however the UDN2891 source driver's reduce it with about 1 V, and I also have to remove the forward voltage of a LED. I'm using different colors, but let's say in worst case it's 1.8V, so I have a total voltage of:
9 - 1 - 1.8 = 6.2 V
This results in a power of
P = V * I = 6.2 V * 1.6 A = 9.92 W
With the RθJA value it would mean
Temp = P * RθJA = 9.92 * 62 °C/W = 615 °C
So I would definitely need heat sinks.
However, because of multiplexing, I only use a duty cycle of 10%, 100 Hz (per IRLZ44N). Does this mean, the current is (on average):
16 (leds) * 0.1 (A) * 0.1 (duty cycle) = 0.16 A
And the thermal calculation is:
Temp = P * RθJA = 0.16 A * 6.2 V * 62 = 61.5 °C
This means I do not need a heat sink; the absolute max rating is 175 °C.
Or does duty cycle have less impact on temperature this way?