mic842nbc5 Micrel Semiconductor, mic842nbc5 Datasheet - Page 6

• Manufacturer Part Number: mic842nbc5
• Description: Mic841/mic842 Comparator With Reference
• Manufacturer: Micrel Semiconductor

MIC841/2
Applications Information
Output
The MIC841N and MIC842N outputs are an open-drain
MOSFET, so most applications will require a pull-up resistor.
The value of the resistor should not be too large or leakage
effects may dominate. 470kΩ is the maximum recommended
value. Note that the output of “N” version may be pulled up as
high as 6V regardless of the ICs supply voltage. The “H” and
“L” versions of the MIC841 and MIC842 have a push-pull
output stage, with a diode clamped to V
mum output voltage of the “H” and “L” versions is V
“Electrical Characteristics.”
When working with large resistors on the input to the devices,
a small amount of leakage current can cause voltage offsets
that degrade system accuracy. The maximum recommended
total resistance from V
the resistors can be chosen based upon the accuracy re-
quired by the system. The inputs may be subjected to
voltages as high as 6V steady-state without adverse effects
of any kind regardless of the ICs supply voltage. This applies
even if the supply voltage is zero. This permits the situation
in which the IC’s supply is turned off, but voltage is still present
on the inputs. See “Electrical Characteristics.”
Programming the MIC841 Thresholds
The low-voltage threshold is calculated using:
The high-voltage threshold is calculated using:
where, for both equations:
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 + R3 = R
A value such as 1MΩ for R
because it draws minimum current but has no significant
effect on accuracy.
MIC841/2
V
V
V
IN(LO)
IN(HI)
REF
Figure 1. MIC841 Example Circuit
=
=
R1
R2
R3
1.240V
=
V
V
V
REF
IN
REF
604k
1%
56k
1%
340k
1%
V






DD
R1 R2 R3
R1 R2 R3
5
3
1
IN
VDD
LTH
HTH
R2 R3
+
+
to ground is 3MΩ. The accuracy of
MIC841N
R3
+
+
GND
OUT
+
TOTAL
4
2






470k
is a reasonable value
DD
V
. Thus, the maxi-
OUT
DD
TOTAL
. See
.
6
Once the desired trip points are determined, set the V
threshold first.
For example, use a total of 1MΩ = R1 + R2 + R3. For a typical
single-cell lithium ion battery, 3.6V is a good “high threshold”
because at 3.6V the battery is moderately charged. Solving
for R3:
Once R3 is determined, the equation for V
to determine R2. A single lithium-ion cell, for example, should
not be discharged below 2.5V. Many applications limit the
drain to 3.1V. Using 3.1V for the V
calculation of the two remaining resistor values.
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Programming the MIC842 Thresholds
The voltage threshold is calculated using:
where:
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
value such as 1MΩ for R
it draws minimum current but has no significant effect on
accuracy.
R3 344k
R2 56k
1M
R1 600k
V
V
V
V
IN(HI)
IN(LO)
IN(LO)
REF
Ω −
=
=
=
Figure 2. MIC842 Example Circuit
=
=
(
R1
R2
1.240V
=
R2 R3
=
3.6V 1.24
Ω
V
3.1V 1.24
V
Ω
IN
Ω
REF
−
=
=



)
R1 R2
5
1
=
TOTAL
VDD
INP
R2
R1
+
MIC842N






1M
R2 344k
R3



GND
OUT
is a reasonable value because
Ω
1M
+



4
2
Ω
IN(LO)
470k



IN(LO)
V
threshold allows
OUT
January 2005
can be used
TOTAL
Micrel
IN(HI)
. A