Best schematic for driving 12V LED strips with TLC5940 with a possibility to PWM

Other Hardware LEDs and Multiplexing
1

I've searched a lot, but just cannot find any decent schematics for driving 12V LED strips using TLC5940.
I've seen some schematics that use N-channel MOSFETS, but then TLC5940 output is inverted, so LED are not fully turned off when they should. And that's not good for me, as most of the time LED will be turned off.
Please share schematics you are using to PWM LED strips with TLC5940.

2

Simply use a PNP in between :slight_smile: Drive the PNP with the TLC5940 and use that to drive the (power) NPN / mosfet

3

The outputs of a TLC5940 are not designed to drive n-channel mosfets.

You can indeed use a small-signal PNP transistor, to invert the logic, to drive the n-channel fet.
Emitter to VCC of the TLC (5volt).
Base to the TLC output (set to a low-ish current).
Collector to gate of the n-channel logic level mosfet.
~330ohm resistor from gate to ground.

Another option is to use PCA9685 breakout boards.
Leo..

4

Another option is to use PCA9685 breakout boards.

These are much better than the TLC5940 as they are a set and forget chip and do not need constant feeding like the TLC chip does.

5

Thanks! So it should look like this?

Wawa:
The outputs of a TLC5940 are not designed to drive n-channel mosfets.

You can indeed use a small-signal PNP transistor, to invert the logic, to drive the n-channel fet.
Emitter to VCC of the TLC (5volt).
Base to the TLC output (set to a low-ish current).
Collector to gate of the n-channel logic level mosfet.
~330ohm resistor from gate to ground.

Another option is to use PCA9685 breakout boards.
Leo..

6

So it should look like this?

No.

You need a base resistor in that PNP transistor.
The outputs of the TLC5940 are sink only. Therefore you need a pull up resistor to 5V.

I would also make R1 between 1K and 10K.

7

Grumpy_Mike:
You need a base resistor in that PNP transistor.

The outputs of the TLC5940 are sink only. Therefore you need a pull up resistor to 5V.

I would also make R1 between 1K and 10K.

  1. Isn't the TLC constant current out? You could set that to 1mA or so.

  2. Could add a 10k bleed resistor to VCC. Needed? See TLC datasheet for output off leak current.

  3. That would make PWM/switching slower. Wouldn't go more than 1k.

Leo..

8
  1. Isn't the TLC constant current out?

No it is a constant current sink, you get no voltage out of an output.

  1. Could add a 10k bleed resistor to VCC.

Yes that is what I suggested.

That would make PWM/switching slower.

No

9

I think we mean the same, but word it differently.
Current sink (with current limiting) is what is needed to drive the PNP transistor.
Voltage on the base is always (VCC - 0.65).

The resistor from gate to ground sets 'turn-off' time of the mosfet.
Leo..

10

What do you think now? :slight_smile:

11

Only thing missing is the value for the current set resistor. I would try to keep that low with this configuration. If it can do 1mA I would go for that.

12

A BUZ72 is an ancient mosfet with a high Rds(on), and it needs 10volt drive.
Not suitable for 5volt logic.

Use a logic level mosfet.
Leo..

13

Wawa:
A BUZ72 is an ancient mosfet with a high Rds(on), and it needs 10volt drive.
Not suitable for 5volt logic.

Use a logic level mosfet.
Leo..

Any suggestion on which one should I use? (I think I'll order a batch of them from ali or ebay).

14

I like the IRF7313.

But you don't necessarily need a logic level. You can connect the PNP to the 12V line and drive the n-mos with 12V :slight_smile:

15

Good catch. The outputs of a TLC5940 have an absolute max rating of 18volt.
What remains is the poor Rds(on) of that fet (0.2ohm). Gets too hot if the strip draws >1Amp.
Leo..