Level shift 3V3 1kHz PWM to 12V

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I am considering what would be the "best" way of level shifting two signals from 3.3V micro to 12V logic levels.
The other signal is a 1kHz PWM signal, other is just on/off control.

According to target datasheet the 12V logic inputs have a 30k pull-down, and logic levels are <0.4V for low and >9.0V for high. Unfortunately there is no spec about rise/fall times.

This is just for a quick prototype so I am tempted to find a 2-channel level converter chip that does this (FET gate drivers?) so can anyone suggest these or something similar?

Or should I just whip up some discrete transistor circuit and fine-tune and simulate different resistor values ad infinitum until it works? Something like in the picture, but obviously there are several permutations how to connect the base and pull resistors.

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The dual transistor solution is nice and simple, but you could also use an ICL7667 f.i. That's a fet gate driver.

cheers

Nard

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Well, maybe I'm missing something, but:

If one inverted the PWM output in software, (i.e. 20 % On becomes 80 % On), and flips the On/Off signal, then wouldn't a simple NFet with a pull up to +12 work?

(And add a Gate pull down, for start-up state, if needed.)

JC

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Are there 3.3V logic level mosfets?

Imagecraft compiler user

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dependeing on how/where this is used it could be that a optocoupler could be the best solution.

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Clever ideas everyone. This is actually for a backlight module enable and brightness control.

Sparrow2:

Could be. I'll be sure to look up optocouplers, either as high-side switch or if there are some with 12V logic-level outputs.

Bob:

There should be 3V3 FETs. If I do the transistor thingy I don't care if it's a FET or BJT as long as it does the switching. Thanks for reminding, FETs could make this easier.

DocJC:

Pull-up with only NFET pulling down could work if two things are considered. As there is a 30k pull-down on module, the passive pull-up must be high enough like 3k to 6k. Thats about 5mA or 60mW to keep one line low when module is turned off - I could live with that. The nasty thing is the 3.3V is made from the 12V, so these lines go high before MCU supply outputs 3.3V, which may turn on the backlight at boot and can disturb the module inrush current limiting and soft-start, if sequenced improperly. Plus I don't know the rise time requirement if any, or the capacitive load that need to be charged. That's too many unknowns already.

Plons:

I have been looking few datasheets of FET drivers, thanks.

As it seems the FET driver will do the job but it might be a bit overkill. I am still leaning towards this. But has anyone use CD40109 or other nice 3V to 12V logic level translators?

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Yes. There are lots of small, and even big, fets that will turn on with 3.3v logic.

Don't forget to put a pulldown resistor on the gate, or the thing may turn on when you go to program the chip and the output pin goes to high impedance. Or would that be high impudence?

 

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