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TL594 Enable operation by RT pin

K.Hamamoto
Guru 12155 points

Hi

Our customer would like to use RT pin of  this device as Enable pin. Is it possible?

If it is possible, should we pull up the RT pin when it is disable? 

Regards,

Koji Hamamoto

  • 0
    TI__Mastermind 24975 points

    Koji-san,

    I apologize for the delay in response.

    Please refer to the schematic of the oscillator in the following application note. http://www.ti.com/lit/an/slva001e/slva001e.pdf

    When considering the RT pin, the current into the CT is controlled by a current mirror.

    You may be able to disable the device through the RT pin because if you pull the RT up to say 5V through a resistor then the current mirror will be off. When this is turned off, the current flowing into the CT will be 0 and the CT will not charge. If the CT voltage is below that of the dead time control (0.1V) and the voltage on the feedback pin, then the output transistors will be off. My concern is that if this happened in the middle of a CT charge, then the CT may not discharge and may be above 0.1V.

    I have also attached a TINA circuit displaying specifically how the current mirror works with a pullup through the RT resistor. Just run the transient analysis out to 10s and you should see what I am talking about.

    /cfs-file/__key/communityserver-discussions-components-files/196/8524.TL594-RT-Current-Mirror.TSC

    I hope this makes sense.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Guru 12155 points

    Hi

    Thank you for your reply.

    I understand.   

    Also I wonder that it may be possible to turn off the operation same as pulled-up RT pin if CT pin is pulled down.

    Is that correct?

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Mastermind 24975 points
    Koji-san,

    Yes, that is correct. If you pull the Ct pin down, it should disable the device.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 12155 points

    Hi

    Thank you for your support.

    I think CT pin is easy to use for Enable pin more than RT.

    By the way, as you mentioned the CT may not discharge and may be above 0.1V if this happened in the middle of a CT charge.

     >My concern is that if this happened in the middle of a CT charge, then the CT may not discharge and may be above 0.1V.

    So, what happen to the operation when the RT pin is pulled up in the middle of CT charge?   Does it mean it does not stop immediately?

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Mastermind 24975 points
    Koji-san,

    Yes, I agree, pulling CT pin down is more direct than using the RT pin.
    As for the using the RT pin, yes, that is a concern of mine. I am not 100% sure if the capacitor will discharge or not, in which case it may not stop immediately.

    Best,
    Michael