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參數(shù)資料
| 型號(hào)： | MIC2779 |
| 廠商： | Micrel Semiconductor,Inc. |
| 英文描述： | Voltage Monitor with Adjustable Hysteresis Preliminary Information |
| 中文描述： | 電壓監(jiān)視器可調(diào)滯回的初步信息 |
| 文件頁(yè)數(shù)： | 5/8頁(yè) |
| 文件大小： | 52K |
| 代理商： | MIC2779 |
June 2000
5
MIC2779
MIC2779
Applications Information
Programming the Thresholds
The low-voltage threshold is calculated using:
Micrel
V
V
R1 R2 R3
R2 R3
BAT(lo)
REF
=
The high-voltage threshold is calculated using:
V
V
R1 R2 R3
R3
BAT(hi)
REF
=
where, for both equations:
V
1.240V
REF
=
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 1M
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 3M
.
V
BATT
HTH
/RST
RST
VDD
LTH
GND
MIC2779
604k
1%
56k
1%
340k
1%
R1
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 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:
V
1.241M
R3
BAT(hi)
=
R3
344k
=
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.
V
3.1V
1.24
1M
+
R2
344k
BAT(lo)
=
=
R2
R1 1M
R1 600k
56k
=
R2 R3
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.
0
20
40
60
80
100
120
140
1
10
100
1000
M
μ
s
RESET COMP. OVERDRIVE, V
REF
–
V
LTH
(mV)
Input Transient
Response
Figure 2. Input Transient Response
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