
Detailed Description
The MAX941/MAX942/MAX944 single-supply compara-
tors feature internal hysteresis, high speed, and low
power. Their outputs are guaranteed to pull within 0.4V
of either supply rail without external pullup or pulldown
circuitry. Rail-to-rail input voltage range and low-voltage
single-supply operation make these devices ideal for por-
table equipment. The MAX941/MAX942/MAX944 inter-
face directly to CMOS and TTL logic.
Timing
Most high-speed comparators oscillate in the linear
region because of noise or undesired parasitic feedback.
This tends to occur when the voltage on one input is at
or equal to the voltage on the other input. To counter
the parasitic effects and noise, the MAX941/MAX942/
MAX944 have internal hysteresis.
The hysteresis in a comparator creates two trip points:
one for the rising input voltage and one for the falling input
voltage (Figure 1). The difference between the trip points
is the hysteresis. When the comparator’s input voltages
are equal, the hysteresis effectively causes one com-
parator input voltage to move quickly past the other, thus
taking the input out of the region where oscillation occurs.
Standard comparators require hysteresis to be added
with external resistors. The MAX941/MAX942/MAX944’s
fixed internal hysteresis eliminates these resistors and the
equations needed to determine appropriate values.
Figure 1 illustrates the case where IN- is fixed and IN+ is
varied. If the inputs were reversed, the figure would look
the same, except the output would be inverted.
The MAX941 includes an internal latch that allows stor-
age of comparison results. The LATCH pin has a high
input impedance. If LATCH is high, the latch is transpar-
ent (i.e., the comparator operates as though the latch
is not present). The comparator’s output state is stored
when LATCH is pulled low. All timing constraints must be
met when using the latch function (Figure 2).
Shutdown Mode (MAX941 Only)
The MAX941 shuts down when SHDN is low. When shut
down, the supply current drops to less than 60μA, and the
three-state output becomes high impedance. The SHDN
pin has a high input impedance. Connect SHDN to V+ for
normal operation. Exit shutdown with LATCH high; other-
wise, the output will be indeterminate.
Input Stage Circuitry
The MAX941/MAX942/MAX944 include internal protec-
tion circuitry that prevents damage to the precision input
stage from large differential input voltages. This protection
circuitry consists of two back-to-back diodes between IN+
and IN- as well as two 4.1kΩ resistors (Figure 3). The
diodes limit the differential voltage applied to the internal
circuitry of the comparators to be no more than 2VF,
where VF is the forward voltage drop of the diode (about
0.7V at +25°C).
For a large differential input voltage (exceeding 2VF), this
protection circuitry increases the input bias current at IN+
(source) and IN- (sink).
F
Input Current 2 x 4.1k
=
Ω
Input current with large differential input voltages should
not be confused with input bias current (IB). As long as the
differential input voltage is less than 2VF, this input cur-
rent is equal to IB. The protection circuitry also allows for
the input common-mode range of the MAX941/MAX942/
MAX944 to extend beyond both power-supply rails. The
output is in the correct logic state if one or both inputs are
within the common-mode range.
Figure 1. Input and Output Waveform, Noninverting Input
Varied
VTRIP+
VHYST
VTRIP-
COMPARATOR
OUTPUT
VOH
VOL
VTRIP+ + VTRIP-
2
VOS =
VIN- = 0V
VIN+
MAX941/MAX942/
MAX944
High-Speed, Low-Power, 3V/5V, Rail-to-Rail,
Single-Supply Comparators
www.maximintegrated.com Maxim Integrated
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