MAX6339
Quad Voltage µP Supervisory Circuit
in SOT Package
_______________________________________________________________________________________ 5
Detailed Description
The MAX6339 is a very small, low-power, quad voltage
µP supervisory circuit designed to maintain system
integrity in multi-supply systems (Figure 1). The device
offers several internally trimmed undervoltage threshold
options that minimize or eliminate the need for external
components. Preset voltage monitoring options for
+5.0V, +3.3V, +3.0V, +2.5V, +1.8V, and -5.0V make it
ideal for telecommunications, desktop and notebook
computers, high-end printers, data storage equipment,
and networking equipment applications.
The quad monitor/reset includes an accurate bandgap
reference, four precision comparators, and a series of
internal trimmed resistor-divider networks to set the fac-
tory-fixed reset threshold options. The resistor networks
scale the specified IN_ reset voltages to match the
internal bandgap reference/comparator voltage. User-
adjustable threshold options bypass the internal resis-
tor networks and connect directly to one of the
comparator inputs (an external resistor-divider network
is required for threshold matching). All threshold volt-
age options, fixed and adjustable, are indicated
through a single-letter code in the product number (see
the Selector Guide).
Each of the internal comparators has a typical hystere-
sis of 0.3% with respect to its reset threshold. This built-
in hysteresis improves the monitor’s immunity to
ambient noise without significantly reducing threshold
accuracy when an input sits at its specified reset volt-
age. The MAX6339 is also designed to ignore short IN_
transients. See the Typical Operating Characteristics for
a glitch immunity graph.
Applications Information
Reset Output
The MAX6339 RESET output is asserted low when any
of the monitored IN_ voltages drop below its specified
reset threshold (or above for -5V option) and remain
low for the reset timeout period (140ms minimum) after
all inputs exceed their thresholds (Figure 2). The output
is open drain with a weak internal pullup to the moni-
tored IN2 supply (10µA typ). For many applications no
external pullup resistor is required to interface with
other logic devices. An external pullup resistor to any
voltage from 0 to +5.5V can overdrive the internal
pullup if interfacing to different logic supply voltages
(Figure 3). Internal circuitry prevents reverse current
flow from the external pullup voltage to IN2.
The MAX6339 is normally powered from the monitored
IN2 supply when all input voltages are above their
specified thresholds. When any supply drops below its
threshold, the reset output is asserted and guaranteed
to remain low while either IN1 or IN2 is above +1.0V.
User-Adjustable Thresholds
The MAX6339 offers several monitor options with user-
adjustable reset thresholds. The threshold voltage at
each adjustable IN_ input is typically 1.23V. To monitor
a voltage > 1.23V, connect a resistor-divider network to
the circuit as shown in Figure 4.
V
INTH
= 1.23V x (R1 + R2) / R2
or, solved in terms of R1:
R1 = R2 ((VI
NTH
/ 1.23V) - 1)
Because the MAX6339 has a guaranteed input current
of ±0.1µA on its adjustable inputs, resistor values up to
100kΩ can be used for R2 with < 1% error.
Unused Inputs
If some monitor inputs are to be unused, they must be
tied to a supply voltage greater in magnitude than their
specified threshold voltages. For unused IN3 or IN4
options with positive thresholds (fixed or adjustable),
the inputs can be connected directly to the IN2 supply.
For unused IN4 options with negative thresholds, the
input must be tied to a more negative supply. The IN2
input must always be used for normal operation (device
power-supply pin). Unused pins cannot be connected
to ground or allowed to float.
Negative Voltage Monitoring Beyond -5V
The MAX6339 is offered with options to monitor -5V sup-
plies with internally fixed thresholds. To monitor supplies
more negative than -5V, a low-impedance resistor-
divider network can be used external to the MAX6339 as
shown in Figure 5. The current through the external resis-
tor-divider should be greater than the input current for
the -5V monitor options. For an input monitor current
error of < 1%, the resistor-divider current should ≥ 2mA
(for I
IN4
= 20µA max). Set R2 = 2.5kΩ. Calculate R1
based on the desired V
IN_
reset threshold voltage, using
the following formula:
R1 = R2
✕
[(V
INTH
/ V
TH
) -1]
where R2 ≤ 2.49kΩ, V
INTH
= desired threshold voltage
and V
TH
is the internal threshold voltage.
