MAX6887/MAX6888
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-
plied V
CC
. A virtual diode-ORing scheme selects the
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.
V
CC
powers the analog circuitry and is the bypass con-
nection for the MAX6887/MAX6888 internal supply.
Bypass V
CC
to GND with a 1µF ceramic capacitor
installed as close to the device as possible. The inter-
nal supply voltage, measured at V
CC
, equals the maxi-
mum of IN1–IN4. If V
CC
is externally supplied, V
CC
must be at least 200mV higher than any voltage
applied to IN1–IN4 and V
CC
must be brought up first.
V
CC
always powers the device when all IN_ are factory
set as “Adj.” Do not use the internally generated V
CC
to
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-
