ICL7665S

FN3182 Rev 10.00

Page 8 of 15

August 10, 2015

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

in any of the circuits shown here.

When VIN is very close to one of the trip voltage, normal

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

whether VIN is rising or falling (the arrows indicated direction

of change. The HYST outputs are basically switches which

short out R31 or R32 when VIN is above the respective trip

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

point will be controlled by R1N, R2N, and R3N, until the trip

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

changes state, R3N is shorted out, and the trip point

becomes controlled by only R1N and R2N, a lower value.

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

.

FIGURE 9A. CIRCUIT CONFIGURATION

FIGURE 9B. TRANSFER CHARACTERISTICS

FIGURE 9. SIMPLE THRESHOLD DETECTOR

OUT1

SET1

SET2

OUT2

V+

R21

RP2

VIN

RP1

R22

R11

R12

OFF

VOUT

ON

VTR2

VNOM

VTR1

DETECTOR 2

DETECTOR 1

FIGURE 10A. CIRCUIT CONFIGURATION

FIGURE 10B. TRANSFER CHARACTERISTICS

FIGURE 10. THRESHOLD DETECTOR WITH HYSTERESIS

HYST1

SET1

SET2

HYST2

V+

VIN

OUT1

OUT2

R31

R32

R12

R11

R21

R22

OVERVOLTAGE

OVERVOLTAGE

VL2 VU2

VL1 VU1

ON

OUT

OFF

VNOM

DETECTOR 2

DETECTOR 1

VIN

VTR2

VSET2

R12 R22

+



R12

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

1.3

R12 R22

+



R12

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

=

=