AN4071

Hysteresis

Doc ID 022939 Rev 1

17/27

4.2

External hysteresis

If the device doesn't include built-in hysteresis, or if a large hysteresis is required, a positive

feedback network can be implemented. Figure 19 shows a non-inverting and Figure 20 an

inverting comparator with hysteresis.

Figure 19.

Non-inverting comparator with hysteresis

Figure 20.

Inverting comparator with hysteresis

For a non-inverting comparator circuit, neglecting input offset voltage and the effect of input

biasing current, the input threshold voltages are:

Equation 4

Equation 5

In Equation 5 the influence of reference voltage on the trigger voltage level can be seen. The

V

TH +

R

I

R

FB

----------

V

REF

V

OL

–

() V

REF

+

•

=

V

TH –

R

I

R

FB

----------

V

REF

V

OH

–

() V

REF

+

•

=

V

HYST

V

TH +

V

TH –

–

R

I

R

FB

----------

V

OH

V

OL

–

()

•

==

V

TRIP

V

TH +

V

TH –

+

2

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

V

REF

R

I

R

FB

----------

V

REF

V

OL

V

OH

+

2

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

–

⎝⎠

⎛⎞

•

+

==