June 1999

9

MIC2753/2754

MIC2753/2754

Micrel

Application Information

Supply Transients

The MIC2753/4 are inherently immune to very short “glitches”

on V

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

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

100mV and lasts no more than 8.4

µs 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 (0.1

µF, for example).

Ensuring Proper Operation at Low Supply

At levels of V

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

undesired operation. (In most cases, however, it is expected

that the circuits driven by the MIC2754 will be similarly

inoperative at V

If a given application requires that /RST be valid below V

1.5V, this can be accomplished by adding a pull-down resis-

tor to the /RST output. A value of 100k

Ω 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

a pull-up resistor (as opposed to a pull-down) should be

added to the RST output. A value of 100k

Ω 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.

0

10

20

30

40

1

10

100

1000

OVERDRIVE (mV)

Reset Comparator Overdrive

vs. Duration

T

A. Reset

0

1

2

3

4

5

6

7

8

1

10

100

1000

OVERDRIVE (mV)

Overvoltage Comparator

Overdrive vs. Duration

T

B. Overvoltage

Figure 1. MIC2753/2754 Transient Immunity Behavior