MIC841_05 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – Comparator with Reference

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_DD. Thus, the maxi-
mum output voltage of the “H” and “L” versions is V_DD. See
“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_IN to ground is 3MΩ. The accuracy of
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:
V_IN(LO)
=

V_{REF }
R1+ R2 + R3^
R2 + R3 
The high-voltage threshold is calculated using:
V_IN(HI)
=
V_REF


R1+
R2 +
R3
R3 ^

where, for both equations:
V_REF = 1.240V
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_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.
V_IN V_DD
R1
604k
1%
R2
56k
1%
MIC841N
5
4
VDD OUT
3
LTH
1
2
HTH GND
470k
V_OUT
R3
340k
1%
Micrel
Once the desired trip points are determined, set the V_IN(HI)
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:
V_IN(HI)
=
3.6V
=
1.24


1MΩ
R3


R3 = 344kΩ
Once R3 is determined, the equation for V_IN(LO) can be used
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_IN(LO) threshold allows
calculation of the two remaining resistor values.
V_IN(LO)
=
3.1V
=

1.24 _
1MΩ ^
R2 + 344k 
R2 = 56kΩ
1MΩ − (R2 − R3) = R1
R1= 600kΩ
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:
V_IN(LO)
=
V_REF


R1+ R2
R2


where:
V_REF = 1.240V
V_IN
MIC842N
470k
R1
VDD OUT
5
4
V_OUT
INP GND
1
2
R2
Figure 2. 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.
Figure 1. MIC841 Example Circuit
MIC841/2
6
January 2005