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參數(shù)資料
| 型號： | MIC2778 |
| 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 |
| 文件頁數(shù)： | 5/8頁 |
| 文件大?。?/td> | 65K |
| 代理商： | MIC2778 |
October 1999
5
MIC2778
MIC2778
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
dominate. 470k
is the maximum recommended value.
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 signifi-
cant 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
VDD
LTH
GND
MIC2778
604k
1%
56k
1%
340k
1%
470k
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 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 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
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 4.
HTH
RST
VDD
LTH
GND
MIC2778
604k
1%
56k
1%
340k
1%
V
BATT
R1
R2
R3
Microprocessor
Supply In
Reset In
GND
100k
Figure 4. Interfacing to Bidirectional Reset Pin
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