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1. 16% PWM after applying power to the board. i.stack.imgur.com/cs93p.png

2. 97.3% PWM, not quite fully on yet.. i.stack.imgur.com/h5fqn.png

3. 97.7% PWM, the driver is on and full PWM range is available. i.stack.imgur.com/HhGkI.png

4. back to 16% PWM. i.stack.imgur.com/oV1Kc.png

1. 16% PWM after applying power to the board.

https://i.sstatic.net/cs93p.png

2. 97.3% PWM, not quite fully on yet..

https://i.sstatic.net/h5fqn.png

3. 97.7% PWM, the driver is on and full PWM range is available.

https://i.sstatic.net/HhGkI.png

4. back to 16% PWM.

https://i.sstatic.net/oV1Kc.png

i.stack.imgur.com/h9JQs.png

i.stack.imgur.com/l7kD8.png

i.stack.imgur.com/AxG5p.png

https://i.sstatic.net/h9JQs.png

https://i.sstatic.net/l7kD8.png

https://i.sstatic.net/AxG5p.png

I could slap a mcu on the board to do this but I thought that this would be a good oppertunity to learn how to accomplish this without using a micro. I have been searching google for ideas but I don't think I'm using the right key words.

Thanks John

I could slap a mcu on the board to do this but I thought that this would be a good opportunity to learn how to accomplish this without using a micro. I have been searching google for ideas but I don't think I'm using the right key words.

Thanks John

Edit: Here are some screenshots from my O'scope.

1. 16% PWM after applying power to the board. i.stack.imgur.com/cs93p.png

2. 97.3% PWM, not quite fully on yet.. i.stack.imgur.com/h5fqn.png

3. 97.7% PWM, the driver is on and full PWM range is available. i.stack.imgur.com/HhGkI.png

4. back to 16% PWM. i.stack.imgur.com/oV1Kc.png

Edit: since LTSpice didn't have a NOR gates built in, I found an example using some inexpensive BC817 BJTs. Here is what I came up with that seems to work ok based on the feedback provided in the comments.

I ended up with two NOR gates and one NOT gate which seems to do what I need it to do. When the device is powered on, it should hold the DIM pin high for about 1.5ms and then return back to 0V and stay there until the PWM signal is present, at which time it will control the DIM pin.

The output of the first NOR gate is also used to offset the feedback pin voltage at startup which should limit the output current and basically shutting off the output without adding a mosfet as a switch inline with the output.

Here are some screenshots from LTSpice.

i.stack.imgur.com/h9JQs.png

i.stack.imgur.com/l7kD8.png

i.stack.imgur.com/AxG5p.png