MIC2775

Micrel

MIC2775

6

September 29, 2000

Application Information

Ensuring Proper Operation at Low Supply

At levels of V

cannot turn on sufficiently to produce a valid logic-low on

/RST. In this situation, 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 MIC2775

will be similarly inoperative at V

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 100k

Ω 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 1.

/RST

RST

VDD

/MR

Manual

Reset

GND

MIC2775-XX

/RESET

VCC

GND

MICROPROCESSOR

100k

Figure 1. MIC2775 Valid /Reset Below 1.2V

The statements above also apply to the MIC2775’s RST

output. That is, to ensure valid RST signal levels at V

1.2V, 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.

/RST

RST

VDD

/MR

GND

MIC2775-XX

/RESET

VCC

GND

Manual

Reset

MICROPROCESSOR

100k

Rpull-down

Figure 2. MIC2775 Valid Reset Below 1.2V

Transient response

The MIC2775 is inherently immune to very short negative-

going “glitches.” Very brief transients may exceed the voltage

threshold without tripping the output.

As shown in Figure 3, in general the narrower the transient,

the deeper the threshold overdrive that will be ignored by the

MIC2775. The graph represents the typical allowable tran-

sient duration for a given amount of threshold overdrive that

will not generate a reset.

0

20

40

60

80

100

0

500

1000

1500

2000

RESET COMP. OVERDRIVE, V

Typical V

DD

Transient Response

Figure 3. Typical V