MIC2776 Ver la hoja de datos (PDF) - Micrel
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MIC2776 Datasheet PDF : 8 Pages
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MIC2776
VCORE
1.0V 5%
VI/O
2.5V 5%
R1
676k
1%
R2
324k
1%
MIC2776
VDD /RST
IN
/MR GND
Manual
Reset
MICROPROCESSOR
VCORE
VI/O
/RESET GND
Figure 1. MIC2776 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bi-
directional, rather than input only. The Motorola 68HC11
family is one example. Because the MIC2776N’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 2.
VCC
MICROPROCESSOR
VCC
MIC2776N
VDD /RST
R1
IN
R2 /MR GND
100k
/RESET GND
Figure 2. Interfacing to Bidirectional Reset Pin
Transient Response
The MIC2776 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
threshold without tripping the output.
As shown in Figure 3, the narrower the transient, the deeper the
threshold overdrive that will be ignored by the MIC2776. The
graph represents the typical allowable transient duration for a
given amount of threshold overdrive that will not generate a reset.
Typical INPUT
Transient Response
40
35
30
25
20
15
10
5
0
0
100
200
300
RESET COMP. OVERDRIVE, VREF–VIN (mV)
Figure 3. Typical INPUT Transient Response
Micrel, Inc.
Ensuring Proper Operation at Low Supply
At levels of VDD below 1.2V, the MIC2776L’s /RST output
driver cannot turn on sufficiently to produce a valid logic-low
on the /RST output. In this situation, other circuits driven by
/RST could be allowed to float, causing undesired opera-
tion. (In most cases, however, it is expected that the circuits
driven by the MIC2776L will be similarly inoperative at VDD
≤ 1.2V.)
If a given application requires that /RST be valid below VDD =
1.2V, this can be accomplished by adding a pull-down resis-
tor to the /RST output. A value of 100kΩ is recommended as
this is usually an acceptable compromise of leakage current
and pull-down current. The resistor’s value is not critical,
however. See Figure 4.
The statements above also apply to the MIC2776H’s RST
output. That is, to ensure valid RST signal levels at VDD <
1.2V, a pull-up resistor (as opposed to a pull-down) should
be added to the RST output. A value of 100kΩ is typical for
this application as well. See Figure 5.
VCC
R1
R2
Manual
Reset
MICROPROCESSOR
VCC
MIC2776L
VDD /RST
IN
/MR GND
/RESET GND
100k
Rpull-down
Figure 4. MIC2776L Valid /Reset Below 1.2V
VCC
R1
R2
Manual
Reset
MIC2776H
VDD RST
IN
MICROPROCESSOR
VCC
100k
Rpull-up
RESET GND
/MR GND
Figure 5. MIC2776H Valid Reset Below 1.2V
November 2005
7
MIC2776
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