MIC2779 Micrel Semiconductor, MIC2779 Datasheet - Page 5

• Manufacturer Part Number: MIC2779
• Description: Voltage Monitor with Adjustable Hysteresis Preliminary Information
• Manufacturer: Micrel Semiconductor

Applications Information
Programming the 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 1M
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
Once the desired trip points are determined, set the V
threshold first.
For example, use a total of 1M = 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:
June 2000
MIC2779
BAT
to ground is 3M .
R3 344k
V
V
V
V
BAT(lo)
BAT(hi)
REF
BAT(hi)
1.240V
Figure 1. Example Circuit
1.24
V
V
REF
REF
R1
R2
R3
V
BATT
604k
1%
56k
1%
340k
1%
1M
R1 R2 R3
R1 R2 R3
R3
for R
R2 R3
R3
VDD
LTH
HTH
TOTAL
MIC2779
/RST
GND
RST
is a reasonable value
TOTAL
BAT(hi)
.
5
Once R3 is determined, the equation for V
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
of the two remaining resistor values.
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
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.
R2 56k
R1 1M
R1 600k
V
BAT(lo)
Figure 2. Input Transient Response
RESET COMP. OVERDRIVE, V
140
120
100
80
60
40
20
3.1V 1.24
0
1
R2 R3
Input Transient
10
BAT(lo)
Response
R2 344k
1M
threshold allows calculation
100
REF
–V
1000
BAT(lo)
LTH
(mV)
can be used
MIC2779
BAT(lo)
Micrel