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TL7700 CT-pin

taichi
Expert 1395 points
Other Parts Discussed in Thread: TL7700

Hi All

TL7700's datasheet is written below.

(Output Pulse-Duration Setting)

When the capacitor voltage exceeds the threshold level of the output drive comparator, RESET changes from a low to a high level.

What will be the threshold voltage on Ct pin?

and, does it have hysterisys?

Regards, 

Taichi

  • 0
    TI__Mastermind 24885 points

    Hello Taichi,

    Let me get back to you on threshold voltage.

    As for hysteresis: Yes, it does have hysteresis. Please refer to page 10 of the datasheet under Sense-Voltage Hysteresis setting.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    TI__Mastermind 24885 points

    Hello Taichi,

    The threshold voltage is the internal 1.5V temperature compensated voltage reference.

    Additionally, the threshold voltage can be calculated.

    The constant current charging starts on the timing capacitor when the sense voltage reaches 500mV and the threshold level of the output drive capacitor is when the RESET pin changes from low to high. This time is defined as the Tpo in the datasheet. Tpo = Ct * 10^5 (Figure 12 gives a more accurate representation)Additionally, the constant current charging is 15uA.

    The voltage on a capacitor is V = Q/C and Q = I * t.

    Therefore, V = I*t / C = ((15uA*(Ct*10^5))/(Ct) = (15*10^-6)*10^5 = 1.5 V

    This will vary based on the Tpo since tpo isn't always Ct*10^5 (refer to Figure 12 in the datasheet.)

    Mostly, however, this is beside the point. The important thing to know is the following:

    How long does it take for the RESET to go high after the sense voltage is exceeded? Answer: Tpo

    What affects Tpo? Answer: Ct

    I hope this helps and makes sense!
    Best,
    Michael