TL7700CPWR datasheet(10/22 Pages) TI

10 / 22 page

VCC

GND

RESET

VCC

RESET

GND

SENSE

TL7700

SLVS220F

– JULY 1999 – REVISED AUGUST 2011

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DETAILED DESCRIPTION

Sense-Voltage Setting

The SENSE terminal input voltage, Vs, of the TL7700 typically is 500 mV. By using two external resistors, the

circuit designer can obtain any sense voltage over 500 mV. In Figure 19, the sensing voltage, Vs

', is calculated

as:

Vs' = Vs × (R1 + R2)/R2

Where:

Vs = 500 mV typ at TA = 25°C

At room temperature, Vs has a variation of 500 mV ± 5 mV. In the basic circuit shown in Figure 19, variations of

[

±5 ± (R1 + R2)/R2] mV are superimposed on Vs.

Figure 19.

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

Vhys = Is × R1

Where:

Is = 2.5 μA typ at TA = 25°C

At room temperature, Is has variations of 2.5 mA

± 0.5 mA. Therefore, in the circuit shown in Figure 19, Vhys has

variations of (

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

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

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