tpo

tpo

RESET

Vhys

VCC

1.5 V

T

T

Vs’

VCC

Vs

GND

CT

RESET

RL

R1

R2

Ct

VCC

RESET

GND

SENSE

11

TL7700

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SLVS220G – JULY 1999 – REVISED AUGUST 2016

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Feature Description (continued)

Figure 18. Setting the Sense Voltage

8.3.2 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 is unstable. Hysteresis is added to the sense voltage to prevent such problems. As shown in

Figure 19, the hysteresis (Vhys) is added, and the value is determined as:

Vhys = Is × R1

where

•

Is = 2.5 μA typ at TA = 25°C

(2)

At room temperature, Is has variations of 2.5 µA ± 0.5 µA. Therefore, in the circuit shown in Figure 18, 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.

The sense voltage, Vs', is different from the SENSE terminal input voltage, Vs. Vs normally is 500 mV for triggering.

Figure 19. VCC-RESET Timing Chart