LED matrix high side switching,which is better pmosfet or pnp darlington?

Using Arduino LEDs and Multiplexing
1

Hello , I've been beating my head against this matrix for some time and read lots of posts with conflicting answers/opinions.
I've got an rob led matrix and am using tlc5917s for the low side PWM stuff and was using UDN2982s for the high side supply. I'm trying to increase the current going to the leds and initially used TIP32s controlled by a ULN2803 which is controlled by a 74hc164 shift register but the 10k base resistor was getting red hot (there is also a 10k resistor acting as a pull-up on the TIP32. I'm now considering using a TIP125 darlington to get a higher hfe (1000 compared to 25) or a P channel mosfet IRF9630 which is a logic level unit. I have my pcb made up already and don't want to change it too much, the TIP32s are on a daughter board so i can make a new one or change it a bit. I'm driving the leds with 9v hence the use of the UDN/ULN "interface" chip. Does anyone have any preferences/opinions on the mosfet vs. darlington options . I'm running the "scan" rate at about 150hz using BAM so the switching is within the capabilities of both chips.

Thanks
Mike

2

How about a schematic of your design so far? Trying to picture what you're doing that could make the base resistor so hot.
For high side switch, I'd go with P-channel MOSFET.
Not a lot of choices - this one is good tradeoff of low Rds and price

3

OK, here is a drawing with my first usage of Fritzing. Please be gentle I'm not an expert. Please note that the base resistors are 15 ohms not 10K as I initially stated and their value was calculated using a web based calculator and seem right if I want about 1.5 amps output.

LED Matrix driver.pdf (987 KB)

4

Pretty good schematic - didn't know fritzing was capable of that.

What you want is to have the PNPs turn on fully (saturate) and let the downstream device (the PWM switching device) control the current flow. The PNP should just act like an electronic on/off switch. 15 is too low. Try 100, 150 there instead.

5

OK, I'll give some 150 ohm resistors a try and report back.
Cheers
Mike

6

Hello Crossroads, your suggestion of bigger resistors worked a treat. I used some 150 ohm 1 watt resistors and they are not getting even warm after 3 hours of 300 hz switching. Thanks for your time.

Cheers
Mike

7

"Cool" 8)
Probably didn't need 1W parts.