MIC2779
Micrel
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
of the two remaining resistor values.
1M
Ω
V
BAT(lo)
=
3.1V
=
1.24
R2
+
344k
Applications Information
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
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 MIC2779 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 MIC2779.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not gener-
ate a reset.
MAX. TRANSIENT DURATION (µs)
140
120
100
80
60
40
20
0
1
10
100
1000
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 mea-
surable effect on accuracy.
When working with large resistors, a small amount of leakage
current can cause voltage offsets that degrade system accu-
racy. The maximum recommended total resistance from
V
BAT
to ground is 3MΩ.
V
BATT
Input Transient
Response
R1
604k
1%
56k
1%
340k
1%
MIC2779
VDD
LTH
HTH
/RST
RST
GND
R2
R3
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
RESET COMP. OVERDRIVE, V
REF
–V
LTH
(mV)
Figure 2. Input Transient Response
June 2000
5
MIC2779
MICREL [ MICREL SEMICONDUCTOR ]
