MIC2774_05 Datasheet, PDF (7/8 Pages) Micrel Semiconductor – Dual Micro-Power Low Voltage Supervisor

MIC2774
V_CORE
1.0V 5%
V_I/O
2.5V 5%
R1
676k
1%
R2
324k
1%
Manual
Reset
MIC2774L-23
VDD /RST
IN
/MR GND
MICROPROCESSOR
VCORE
VI/O
/RESET GND
Figure 1. MIC2774 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 MIC2774N’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.
V_CC
MICROPROCESSOR
VCC
MIC2774N-XX
VDD /RST
R1
IN
R2 _{/MR GND}
100k
/RESET GND
Figure 2. Interfacing to Bidirectional Reset Pin
Transient response
The MIC2774 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
thresholds without tripping the output.
As shown in Figure 3a and 3b, in general the narrower the
transient, the deeper the threshold overdrive that will be
ignored by the MIC2774. The graphs represent the typical
allowable transient duration for a given amount of threshold
overdrive that will not generate a reset.
Typical IN
Trans ient R es pons e
40
35
30
25
20
15
10
5
0
0
100
200
300
RESET COMP. OVERDRIVE, V_REF –V _IN (mV)
Figure 3a. Typical INPUT Transient Response
Typical V_DD
100 ^{T rans ient R es pons e}
80
60
40
20
00
500 1000 1500 2000
RESET COMP. OVERDRIVE, V_REF –V _DD(mV)
Micrel
Figure 3b. Typical V_DD Transient Response
Ensuring Proper Operation at Low Supply
At levels of V_DD below 1.2V, the MIC2774’s reset output can-
not turn on sufficiently to produce a valid logic-low on /RST.
In this situation, circuits driven by /RST could be allowed to
float, causing undesired operation. (In most cases, however,
it is expected that the circuits driven by the MIC2774L will be
similarly inoperative at V_DD ≤ 1.2V.)
If a given application requires that /RST be valid below V_DD
= 1.2V, this can be accomplished by adding a pull-down re-
sistor to the /RST output. A value of 100kΩ is recommended
as this is usually an acceptable compromise of quiescent
current and pull-down current. The resistor’s value is not
critical, however. See Figure 4.
The statements above also apply to the MIC2774H’s RST
output. That is, to ensure valid RST signal levels at V_DD <
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.
V_CC
R1
R2
Manual
Reset
MICROPROCESSOR
VCC
MIC2774L-XX
VDD /RST
/RESET GND
IN
100k
Rpull-down
/MR GND
Figure 4. MIC2774L Valid /RST Below 1.2V
V_CC
R1
R2
Manual
Reset
MIC2774H-XX
VDD RST
IN
MICROPROCESSOR
VCC
100k
Rpull-up
RESET GND
/MR GND
October 2005
Figure 5. MIC2774H Valid RST Below 1.2V
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M9999-102605