µP Power-Supply Monitor

with Reset

4

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__________Applications Information

Replacing the TL7705 with a MAX7705

in an Existing Design

and

RESET. When using a TL7705 with a µP that has

a

RESET input, simply plug the MAX7705 into the

same socket and omit the

RESET pull-up resistor, reset

timing capacitor, and reference bypass capacitor (see

the

Typical Operating Circuit).

The reset power-up delay is created by an internal

fixed oscillator. This delay is 100% tested and guaran-

teed over the full temperature range. The

RESET out-

put both sources and sinks current (see

RESET Output

Voltage in the

Electrical Characteristics).

Negative-Going VCC Transients

The MAX7705 asserts

RESET during power-up,

power-down, and brownout conditions. However, it is

transients (glitches).

Figure 1 shows typical transient duration vs. reset com-

parator overdrive for which the MAX7705 does not gen-

erate a reset pulse. The graph was generated using a

1.5V above the actual reset threshold and ending below

the reset threshold by the magnitude indicated (reset

comparator overdrive). It indicates the typical maxi-

have without causing a reset pulse to be issued. As

the magnitude of the transient increases (goes farther

below the reset threshold), the maximum allowable

goes 100mV below the reset threshold and lasts 40µs

or less will not cause a reset pulse to be issued.

A 0.1µF bypass capacitor mounted as close as possible

Ensuring a Valid

RESET Output

Down to VCC = 0V

no longer sinks current; it becomes high impedance.

Therefore, high-impedance CMOS logic inputs con-

nected to the

RESET output can drift to indeterminate

voltages. In most applications this presents no prob-

lem, as µP and other circuitry is generally inoperative

output must be valid down to 0V, adding a pull-down

resistor to the

RESET pin (as shown in Figure 2) will

cause any stray leakage currents to flow to ground,

holding

RESET low. The resistance value of R1 is not

critical. It should be about 100k

Ω, which is large

enough not to load

RESET and small enough to pull

RESET to ground.

150

0

10

100

10k

50

100

1k

10ns

R1

GND

MAX7705

RESET

Figure 1. Maximum Transient Duration without Causing a

Reset Pulse vs. Reset Comparator Overdrive

Figure 2.