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Trouble shifting 5V clock down to 3.3V

Former Member

Hi Folks.

 

I have a pwm generated clock from my mcu which is at 5V. I want to step this down to 3.3V. I am using a 2N7000 N channel mosfet and the setup shown below.

 

image

 

This is the square wave generated by the mcu:

image

 

When I put it through the level shifter I get This output:

image

 

It is clearly offset and very distorted. When I apply 5V directly I get 3.3V out and when I connect ground to it I get 0V out as expected. However when I apply the 0 to 5V square wave it doesn't work.

 

Does anybody know why this is happening?

 

Thanks,

 

Mark.

Parents

  • Do it like this:

     

     

    image

     

    R1 should be chosen according to frequency - for 50kHz 1k will be acceptable. R2 will not be necessary with a 2n7000 (apply input directly to gate.)

     

    This type of level shifter is not very good at high speeds - where you should use a proper level shifting chip.

     

    MK

  • in reply to michaelkellett

    Thanks for the reply.

     

    I did need to use R2 at1K otherwise connecting directlyto the gate prevented the mcu from generating the clock. I got a decent looking wave out with perak voltage of 3.6V which should be ok for 3.3V device.

     

    How do I select R1 for different frequencies. I would like to use between 1MHz to 8MHz. If this is not good at high speeds like you said what chip would you recommend?

     

    Mark.

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  • in reply to michaelkellett

    Thanks for the reply.

     

    I did need to use R2 at1K otherwise connecting directlyto the gate prevented the mcu from generating the clock. I got a decent looking wave out with perak voltage of 3.6V which should be ok for 3.3V device.

     

    How do I select R1 for different frequencies. I would like to use between 1MHz to 8MHz. If this is not good at high speeds like you said what chip would you recommend?

     

    Mark.

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