MAX812TEUS-T 查看數據表（PDF） - Maxim Integrated

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MAX812TEUS-T

4-Pin µP Voltage Monitors with Manual Reset Input

Maxim Integrated 

4-Pin µP Voltage Monitors

with Manual Reset Input

8

TA = +25°C

7

6

MAX81 _L/M

5

4

MAX81 _R/S/T

3

2

1

0

1

10

100

1000

RESET COMPARATOR OVERDRIVE , VTH - VCC (mV)

VCC

MAX811

RESET

R1

GND

Figure 1. Maximum Transient Duration without Causing a

Reset Pulse vs. Comparator Overdrive

__________ Applications Information

Negative-Going VCC Transients

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, the MAX811/

MAX812 are relatively immune to short duration nega-

tive-going VCC transients (glitches).

Figure 1 shows typical transient durations vs. reset

comparator overdrive, for which the MAX811/MAX812

do not generate a reset pulse. This graph was generat-

ed using a negative-going pulse applied to VCC, start-

ing above the actual reset threshold and ending below

it by the magnitude indicated (reset comparator over-

drive). The graph indicates the typical maximum pulse

width a negative-going VCC transient may 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 pulse width

decreases. Typically, a VCC transient that goes 125mV

below the reset threshold and lasts 40µs or less

(MAX81_L/M) or 20µs or less (MAX81_T/S/R) will not

cause a reset pulse to be issued. A 0.1µF capacitor

mounted as close as possible to VCC provides addi-

tional transient immunity.

Figure 2. RESET Valid to VCC = Ground Circuit

Ensuring a Valid RESET Output

Down to VCC = 0V

When VCC falls below 1V, the MAX811 RESET output

no longer sinks current—it becomes an open circuit.

Therefore, high-impedance CMOS-logic inputs con-

nected to the RESET output can drift to undetermined

voltages. This presents no problem in most applica-

tions, since most µP and other circuitry is inoperative

with VCC below 1V. However, in applications where the

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

down resistor to the RESET pin will cause any stray

leakage currents to flow to ground, holding RESET low

(Figure 2). R1’s value is not critical; 100kΩ is large

enough not to load RESET and small enough to pull

RESET to ground.

A 100kΩ pull-up resistor to VCC is also recommended

for the MAX812 if RESET is required to remain valid for

VCC < 1V.

6 _______________________________________________________________________________________

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