November 2005 7 MIC2777
MIC2777 Micrel, Inc.
IN
/RST
RST
VDD
GND
MIC2777-23
R1
676k
1%
R2
324k
1%
/RESET
VCORE
GND
V
CORE
1.0V 5%
MICROPROCESSOR
VI/O
V
I/O
2.5V 5%
Figure 1. MIC2777 Example Design
Transient response
The MIC2777 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 2a and 2b, in general the narrower the
transient, the deeper the threshold overdrive that will be
ignored by the MIC2777. The graph represents the typical
allowable transient duration for a given amount of threshold
overdrive that will not generate a reset.
0
5
10
15
20
25
30
35
40
0 100 200 300
RESET COMP. OVERDRIVE, V
REF
–V
IN(mV)
Typical INPUT
Transient Response
Figure 2a. Typical INPUT Transient Response
0
20
40
60
80
100
0 500 1000 1500 2000
RESET COMP. OVERDRIVE, V
REF
–V
DD
(mV)
Typical V
DD
Transient Response
Figure 2b. Typical V
DD
Transient Response
Ensuring Proper Operation at Low Supply
At levels of V
DD
below 1.2V, the MIC2777’s /RST output
cannot turn on sufficiently to produce a valid logic-low on
/RST. In this situation, other 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
MIC2777 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 3.
The statements above also apply to the MIC2777’s RST out-
put. 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 4.
IN
/RST
RST
VDD
GND
MIC2777-XX
R1
R2
/RESET
VCC
GND
V
CC
MICROPROCESSOR
100k
Rpull-down
Figure 3. MIC2777 Valid /RST Below 1.2V
IN
RST
/RST
VDD
GND
MIC2777-XX
R1
R2
RESET
VCC
GND
V
CC
MICROPROCESSOR
100k
Rpull-up
Figure 4. MIC2777 Valid RST Below 1.2V
