8

FN3182.8

April 5, 2006

Either detector may be used alone, as well as both together,

in any of the circuits shown here.

variations and noise may cause it to wander back and forth

across this level, leading to erratic output ON and OFF

conditions. The addition of hysteresis, making the trip points

slightly different for rising and falling inputs, will avoid this

condition.

Threshold Detector with Hysteresis

Figure 10A shows how to set up such hysteresis, while

Figure 10B shows how the hysteresis around each trip point

produces switching action at different points depending on

of change. The HYST outputs are basically switches which

point. Thus if the input voltage rises from a low value, the trip

point is reached. As this value is passed, the detector

The input will then have to fall to this new point to restore the

initial comparator state, but as soon as this occurs, the trip

point will be raised again.

An alternative circuit for obtaining hysteresis is shown in

Figure 11. In this configuration, the HYST pins put the extra

resistor in parallel with the upper setting resistor. The values

of the resistors differ, but the action is essentially the same.

The governing equations are given in Table 1. These ignore

the effects of the resistance of the HYST outputs, but these

can normally be neglected if the resistor values are above

about 100k

Ω.

FIGURE 9A. CIRCUIT CONFIGURATION

FIGURE 9B. TRANSFER CHARACTERISTICS

FIGURE 9. SIMPLE THRESHOLD DETECTOR

OUT1

SET1

SET2

OUT2

V+

OFF

ON

DETECTOR 2

DETECTOR 1

FIGURE 10A. CIRCUIT CONFIGURATION

FIGURE 10B. TRANSFER CHARACTERISTICS

FIGURE 10. THRESHOLD DETECTOR WITH HYSTERESIS

HYST1

SET1

SET2

HYST2

V+

OUT1

OUT2

OVERVOLTAGE

OVERVOLTAGE

ON

OUT

OFF

DETECTOR 2

DETECTOR 1

+

()

----------------------------------

1.3

+

()

---------------------------------- for detector 2

=

=

ICL7665S