MIC2753 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – Power Supply Supervisors Preliminary Information

MIC2753/2754
Application Information
Supply Transients
The MIC2753/4 are inherently immune to very short “glitches”
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 8.4µs will typically not cause
a reset to occur. Bypass capacitance placed as close as
physically possible to the device’s VIN and GND pins will
increase the transient immunity (0.1µF, for example).
Ensuring Proper Operation at Low Supply
At levels of V_IN below 1.5V, the MIC2754’s /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
Micrel
undesired operation. (In most cases, however, it is expected
that the circuits driven by the MIC2754 will be similarly
inoperative at V_CC ≤ 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 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.
The statements above also apply to the MIC2753’s 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 100kΩ 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’s reset output and the microprocessor’s reset
input. This prevents excessive current from flowing due to
contention between the two drivers. The signal present at the
processor’s 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.
Reset Comparator Overdrive
vs. Duration
40
30
20
10
T_A = 25°C
01
10
100
OVERDRIVE (mV)
1000
A. Reset
Overvoltage Comparator
Overdrive vs. Duration
8
7
T_A = 25°C
6
5
4
3
2
1
01
10
100
1000
OVERDRIVE (mV)
B. Overvoltage
Figure 1. MIC2753/2754 Transient Immunity Behavior
June 1999
9
MIC2753/2754