MIC2779
Micrel, Inc.
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.
1MΩ
V
BAT(lo)
= 3.1V = 1.24
R2 + 344k
R2 = 56k Ω
R1 = 1MΩ − 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 dura-
tion for a given amount of threshold overdrive that will not
generate a reset.
Input Transient
Response
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
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
measurable 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
R 1
604k
1%
R 2
56k
1%
R 3
340k
1%
MIC2779
VDD / R S T
LTH
R ST
HTH
GND
MAX. TRANSIENT DURATION (µs)
140
120
100
80
60
40
20
0
1
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:
1MΩ
V
BAT(hi)
= 1.24
R3
R3 = 344kΩ
10
100
1000
RESET COMP. OVERDRIVE, V
REF
–V
LTH
(mV)
Figure 2. Input Transient Response
January 2006
5
MIC2779
MICREL [ MICREL SEMICONDUCTOR ]
