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Voltage of CT pin about TL7700

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: TL7700

We have a question about TL7700.

Please tell us the voltage applied to the CT pin.

Voltage of CT pin doesn't depend on capacitance of CT.

 we can calculate the voltage with the following formula.

Is this idea correct?

Voltage of capacitor at both ends : V=Q/C、Q=I*t

V=I*t/C
  =Ict*(Ct*10^5)/Ct
  =15uA*10^5
  =1.5V

Best regards,

Takahiro Nishizawa

  • 0
    TI__Genius 10385 points
    Takahiro-san,

    I am going to verify this with some bench testing and I will report back what I find.

    Thanks,
    Ricardo
  • 0
    TI__Genius 12630 points

    Takahiro Nishizawa,

    The CT capacitor charges to an internal reference voltage of 1.5V. The capacitor begins charging as soon as the voltage at the SENSE pin reaches 0.5V. The capacitor charges with constant current source of 15uA typical. The delay equation comes from the charge and capacitor relationship I = C dv/dt.

    When you solve for dt which is the delay, you get:

    delay = C * dv/dt and when you substitute the correct values, you get:

    delay = C * (1.5V / 15uA)

    I hope this answers your question. Please let us know if you have any more questions. Thanks!

    -Michael