MIC2778
Micrel
of the two remaining resistor values.
1M
Ω
V
BAT(lo)
=
3.1V
=
1.24
R2
+
344k
R2
=
56k
Ω
R1
=
1M
Ω −
R2
−
R3
R1
=
600k
Ω
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Input Transients
The MIC2778 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the V
BAT(lo)
threshold without tripping the output.
As shown in Figure 2, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2778.
The graph represents the typical allowable transient dura-
tion for a given amount of threshold overdrive that will not
generate a reset.
140
Applications Information
Output
Since the MIC2778 output is an open-drain MOSFET, most
applications will require a pull-up resistor. The value of the
resistor should not be too large or leakage effects may domi-
nate. 470kΩ is the maximum recommended value.
Programming the Thresholds
The low-voltage threshold is calculated using:
R1
+
R2
+
R3
V
BAT(lo)
=
V
REF
R2
+
R3
The high-voltage threshold is calculated using:
R1
+
R2
+
R3
V
BAT(hi)
=
V
REF
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 battery current but has no
significant effect on accuracy.
When working with large resistors, a small amount of leak-
age current can cause voltage offsets that degrade system
accuracy. The maximum recommended total resistance from
V
BAT
to ground is 3MΩ.
V
BATT
Input T rans ient
R es pons e
MAX. TRANSIENT DURATION (µs)
120
100
80
60
40
20
0
1
10
100
1000
R1
R2
R3
604k
1%
56k
1%
340k
1%
MIC2778
VDD
LTH
HTH
R ST
GND
470k
RESET COMP. OVERDRIVE, V
REF
–V
LTH
(mV)
Figure 2. Input Transient Response
Interfacing to Processors
With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, rather than input only. The Motorola 68HC11 family
is one example. Because the MIC2778’s output is open drain,
it can be connected directly to the processor’s reset pin using
only the pull-up resistor normally required. See Figure 3.
V
BATT
Microprocessor
Figure 1. Example Circuit
Once the desired trip points are determined, set the V
BAT(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:
1M
Ω
V
BAT(hi)
=
1.24
R3
R3
=
344k
Ω
Once R3 is determined, the equation for V
BAT(lo)
can be
used to determine R2. A single lithium-ion cell should not be
discharged below 2.5V. Many applications limit the drain to
3.1V. Using 3.1V for the V
BAT(lo)
threshold allows calculation
R1
R2
R3
604k
1%
56k
1%
340k
1%
MIC2778
VDD
LTH
HTH
R ST
GND
Supply In
100k
Reset In
GND
Figure 3. Interfacing to Bidirectional Reset Pin
January 2005
5
M9999-011405
MICREL [ MICREL SEMICONDUCTOR ]
