Hysteresis

AN4071

18/27

Doc ID 022939 Rev 1

used:

Equation 6

Example of real design of comparator using external hysteresis network:

Task: find resistor values and voltage reference in order to implement 400 mV hysteresis on

1.2 V threshold voltage. Use push-pull comparator TS3021 powered by +5 V source.

Solution: knowing the output voltage level swing and required hysteresis, calculate the

resistor ratio and choose the appropriate resistor (in the range of hundreds of k

Ω). Then,

substituting resistor values and trip point voltage in the second formula, calculate the

reference voltage.

For 5 V output swing and hysteresis 400 mV, the ratio of feedback and input resistor is:

Equation 7

calculated:

Equation 8

Figure 21 shows the final circuit. A reference voltage is generated by the voltage divider

supplied from the +5 V source of the comparator. With resistors 68 k

Ω and 24 kΩ from the

R

FB

R

I

----------

V

–

V

HYST

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

=

V

REF

R

I

2R

I

R

FB

+

()

•

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

V

OH

V

OL

+

()

R

FB

R

I

R

FB

+

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

V

TRI P

•

+

•

=

R

FB

R

I

----------

V

OH

V

OL

–

V

HYST

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

5V

0.4V

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

12.5

==

=

V

REF

R

1

2R

I

R

FB

+

()

•

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

V

OH

V

OL

+

()

R

FB

R

I

R

FB

+

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

V

TRIP

•

+

•

=

=

2.4

10

4

•

22.4

10

4

310

5

•

+

•

()

•

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

5

310

5

•

2.4

10

4

310

5

•

+

•

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

1.2

•

+

•

0.185

1.111V

1.296V

=

+

==