MIC2777
Micrel
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
resistor to the /RST output. A value of 100kΩ is recom-
mended 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 MIC2777’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
MICROPROCESSOR
VCC
MIC2777-XX
VDD /RST
IN
RST
GND
Application Information
V
CORE
1.0V
±5%
V
I/O
2.5V
±5%
R1
676k
1%
R2
324k
1%
MIC2777-23
VDD /RST
IN
RST
GND
MICROPROCESSOR
VCORE
VI/O
/RESET GND
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 3a and 3b, 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.
Typical INPUT
Transient Response
MAX. TRANSIENT DURATION (µs)
R1
R2
/RESET GND
100k
Rpull-down
40
35
30
25
20
15
10
5
0
0
100
200
300
Figure 4. MIC2777 Valid /RST Below 1.2V
V
CC
MICROPROCESSOR
VCC
R1
MIC2777-XX
RST
/RST
GND
100k
Rpull-up
RESET
GND
VDD
IN
R2
RESET COMP. OVERDRIVE, V
REF
–V
IN
(mV)
Figure 3a. Typical INPUT Transient Response
Typical V
MAX. TRANSIENT DURATION (µs)
Figure 5. MIC2777 Valid RST Below 1.2V
DD
100
80
60
40
20
Transient Response
0
0
500
1000
1500
2000
RESET COMP. OVERDRIVE, V
REF
–V
DD
(mV)
Figure 3b. 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,
September 29, 2000
7
MIC2777
MICREL [ MICREL SEMICONDUCTOR ]
