MIC841_15 Datasheet, PDF (7/11 Pages) Micrel Semiconductor – Comparator with 1.25% Reference and Adjustable Hysteresis

Micrel, Inc.
Using 3.1V for the V_IN(LO) threshold allows calculation of
the two remaining resistor values:
V_IN(LO)
=
3.1V
=
1.24V


R2
1MΩ
+ 344kΩ


Eq. 4
MIC841/842
Where:
R2 = 56kΩ
R1 = 1MΩ − R2 − R3
R1 = 600kΩ
The accuracy of the resistors can be chosen based upon
the accuracy required by the system.
V_IN
VIN(HI)
VIN(LO)
0V
VHYSTERISIS
OUT
H AND N VERSIONS 0V
OUT
L VERSION
0V
Figure 2. Output Response and Hysteresis
Programming the MIC842 Thresholds
The voltage threshold is calculated using Equation 5:
Figure 3. MIC842 Example Circuit
In order to provide the additional criteria needed to solve
for the resistor values, the resistors can be selected such
that they have a given total value, that is, R1 + R2 =
R_TOTAL. A value such as 1MΩ for R_TOTAL is a reasonable
value because it draws minimum current but has no
significant effect on accuracy.
Input Transients
The MIC841/2 is inherently immune to very short
negative-going “glitches.” Very brief transients may
exceed the V_IN(LO) threshold without tripping the output.
As shown in Figure 4, the narrower the transient, the
deeper the threshold overdrive that will be ignored by the
MIC841/2. The graph represents the typical allowable
transient duration for a given amount of threshold
overdrive that will not generate an output.
V_IN(LO)
=
V_REF


R1+ R2
R2


Eq. 5
Where:
V_REF = 1.240V
Figure 4. Input Transient Response
July 24, 2015
7
Revision 5.0