Hex/Quad, Power-Supply Supervisory Circuits

_______________________________________________________________________________________

7

Detailed Description

The MAX6887/MAX6888 provide several supply-detector

inputs, one watchdog input, and three outputs for power-

supply monitoring applications. The MAX6887 offers six

voltage-detector inputs, while the MAX6888 offers four.

Each voltage-detector input offers both an undervoltage

and overvoltage threshold.

The undervoltage and overvoltage thresholds are facto-

ry-set for monitoring standard supply voltages (see the

Selector Guide). Inputs in the Selector Guide that con-

tain “Adj” allow an external voltage-divider to be con-

nected to set a user-defined threshold.

RESET goes low when any input voltage drops below

its undervoltage threshold or when MR is brought low.

RESET stays low for 200ms after all assertion-causing

conditions have been cleared. OV goes low when an

input voltage rises above its overvoltage threshold. OV

typically stays low for 25µs (typ) after all inputs fall

back under their overvoltage thresholds.

The MAX6887/MAX6888 offer a watchdog timer with

initial and normal timeout periods of 102.4s and 1.6s,

respectively. WDO goes low when the watchdog timer

expires and deasserts when WDI transitions from low-

to-high or high-to-low.

Powering the MAX6887/MAX6888

The MAX6887/MAX6888 derive power from the voltage-

detector inputs IN1–IN4 or through an externally sup-

positive input that supplies power to the device (see

the Functional Diagram). The highest input voltage on

IN1–IN4 supplies power to the device. One of IN1–IN4

must be at least 2.7V to ensure proper operation.

Internal hysteresis ensures that the supply input that

initially powered the device continues to power the

device when multiple input voltages are within 50mV of

each other.

nection for the MAX6887/MAX6888 internal supply.

installed as close to the device as possible. The inter-

must be at least 200mV higher than any voltage

provide power to external circuitry.

The MAX6887/MAX6888 generate a supply voltage at

BP for the internal logic circuitry. Bypass BP to GND with

a 1µF ceramic capacitor installed as close to the device

as possible. The nominal BP output voltage is +2.55V.

Do not use BP to provide power to external circuitry.

Inputs

The MAX6887 offers six voltage-detector inputs, while

the MAX6888 offers four voltage-detector inputs. Each

voltage-detector input offers an undervoltage and over-

voltage threshold set at the factory to monitor standard

supply voltages (see the Selector Guide). The 5% and

10% tolerances are based on maximum and minimum

threshold

values.

Actual

thresholds

for

the

MAX6887/MAX6888 are shown in Tables 1 and 2.

Inputs in the Selector Guide listing “Adj” allow an exter-

nal voltage-divider to be connected to set a user-

defined threshold.

Adjustable Threshold Inputs

Inputs listed in the Selector Guide containing “Adj” for

inputs allow external resistor voltage-dividers to be

connected at the voltage-detector inputs. These inputs

monitor any voltage supply higher than 0.6V (see

Figure 1). Use the following equation to set a voltage-

MAX6887

MAX6888

IN_

R1

R2

TO 0.6V ACCORDING TO THE DEVICE'S TOLERANCE

Figure 1. Adjusting the Monitored Threshold