TL7700CPWR datasheet(10/17 Pages) TI

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TL7700

SUPPLY VOLTAGE SUPERVISOR

SLVS220C − JULY 1999 − REVISED NOVEMBER 2004

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

sense-voltage hysteresis setting

If the sense voltage, Vs’, does not have hysteresis in it, and the voltage on the sensing line contains ripples, the

resetting of TL7700 will be unstable. Hysteresis is added to the sense voltage to prevent such problems. As

shown in Figure 20, the hysteresis, Vhys, is added, and the value is determined as:

Vhys = Is × R1

Where:

Is = 2.5 µA, typically at TA = 25°C

At room temperature, Is has variations of 2.5 µA ± 0.5 µA. Therefore, in the circuit shown in Figure 19, Vhys has

variations of (

±0.5 × R1) µV. In circuit design, it is necessary to consider the voltage-dividing resistor tolerance

and temperature coefficient in addition to variations in Vs and Vhys.

tpo

RESET

Vhys

VCC

1.5 V

Vs’

NOTE A: The sense voltage, Vs’, is different from the SENSE terminal input voltage, Vs. Vs normally is 500 mV for

triggering.

Figure 20. VCC-RESET Timing Chart

output pulse-duration setting

Constant-current charging starts on the timing capacitor when the sensing-line voltage reaches the TL7700

sense voltage. When the capacitor voltage exceeds the threshold level of the output drive comparator, RESET

changes from a low to a high level. The output pulse duration is the time between the point when the sense-pin

voltage exceeds the threshold level and the point when the RESET output changes from a low level to a high

level. When the TL7700 is used for system power-on reset, the output pulse duration, tpo, must be set longer

than the power rise time. The value of tpo is:

tpo = Ct × 105 seconds

Where:

Ct is the timing capacitor in farads

There is a limit on the device response speed. Even if Ct = 0, tpo is not 0, but approximately 5 µs to 10 µs.

Therefore, when the TL7700 is used as a comparator with hysteresis, without connecting Ct, switching speeds

(tr/tf, tpo/tpd, etc.) must be considered.

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