undesired operation. (In most cases, however, it is expected
鈮?/div>
1.5V.)
If a given application requires that /RST be valid below V
IN
=
1.5V, this can be accomplished by adding a pull-down resis-
tor to the /RST output. A value of 100k鈩?is recommended as
this is usually an acceptable compromise of leakage current
and pull-down current. The resistor鈥檚 value is not critical,
however.
The statements above also apply to the MIC2753鈥檚 RST
output. That is, to ensure valid RST signal levels at V
IN
< 1.5V,
a pull-up resistor (as opposed to a pull-down) should be
added to the RST output. A value of 100k鈩?is typical for this
application as well. See Figure 2.
Interfacing to Processors with Bidirectional Reset Pins
Some microcontrollers and microprocessors utilize reset
signal pins that are bidirectional in nature, rather than simply
being input only. The Motorola 68HC11 family is one ex-
ample. To use the MIC2753/4 with these processors, it is
necessary to insert a resistor into the signal path between the
MIC2753/4鈥檚 reset output and the microprocessor鈥檚 reset
input. This prevents excessive current from flowing due to
contention between the two drivers. The signal present at the
processor鈥檚 reset pin will now be of limited drive capability, so
it is necessary to buffer it in order to drive other circuits. This
technique is shown in Figure 3.
Application Information
Supply Transients
The MIC2753/4 are inherently immune to very short 鈥済litches鈥?/div>
on V
IN
. In the case of
very brief
transients, V
IN
may drop
below the reset threshold or exceed the overvoltage thresh-
old without activating the RST, /RST or /HV output. As shown
in the graphs of Figure 1, the narrower the transient, the
deeper the threshold overdrive that will be ignored by the
MIC2753/4. The lines on the graph represent the typical
allowable transient duration for a given amount of threshold
overdrive that will not generate a reset or overvoltage indica-
tion. The data from which Figure 1 is derived was taken by
adding negative-going square-wave pulses to a dc V
IN
set at
0.5V above or below the actual measured threshold for the
part being characterized.
In the case of the MIC2754L/M for example (refer to Figure 1),
a transient on V
IN
which goes below the reset threshold by
100mV and lasts no more than 8.4碌s will typically not cause
a reset to occur. Bypass capacitance placed as close as
physically possible to the device鈥檚 VIN and GND pins will
increase the transient immunity (0.1碌F, for example).
Ensuring Proper Operation at Low Supply
At levels of V
IN
below 1.5V, the MIC2754鈥檚 /RST output driver
cannot turn on sufficiently to sink current and produce a valid
logic-low on the /RST output. In this situation, other CMOS
circuits driven by /RST could be allowed to float, causing
40
Reset Comparator Overdrive
vs. Duration
8
7
Overvoltage Comparator
Overdrive vs. Duration
T
A
= 25擄C
DURATION (碌s)
DURATION (碌s)
T
A
= 25擄C
10
100
OVERDRIVE (mV)
1000
30
6
5
4
3
2
1
0
1
10
100
OVERDRIVE (mV)
1000
20
10
0
1
A. Reset
B. Overvoltage
Figure 1. MIC2753/2754 Transient Immunity Behavior
June 1999
9
MIC2753/2754
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