MAX987/MAX988/MAX991/
MAX992/MAX995/MAX996
High-Speed, Micropower, Low-Voltage,
Rail-to-Rail I/O Comparators
Detailed Description
Applications Information
Additional Hysteresis
The
MAX987/MAX988/MAX991/MAX992/MAX995/
MAX996 are single/dual/quad low-power, low-voltage
comparators. They have an operating supply voltage
range between +2.5V and +5.5V and consume only 48µA
per comparator, while achieving 120ns propagation delay.
Their common-mode input voltage range extends 0.25V
beyond each rail. Internal hysteresis ensures clean output
switching, even with slow-moving input signals. Large
internal output drivers allow rail-to-rail output swing with
up to 8mA loads.
MAX987/MAX991/MAX995
The MAX987/MAX991/MAX995 have ±2.5mV internal
hysteresis. Additional hysteresis can be generated
with three resistors using positive feedback (Figure 1).
Unfortunately, this method also slows hysteresis response
time. Use the following procedure to calculate resistor
values for the MAX987/MAX991/MAX995.
1) Select R3. Leakage current at IN is under 10nA; therefore,
the current through R3 should be at least 1µA to
minimize errors caused by leakage current. The
The output stage employs a unique design that minimizes
supply-current surges while switching, virtually eliminating
the supply glitches typical of many other comparators.
current through R3 at the trip point is (V
- V
)
REF
OUT
/ R3. Considering the two possible output states and
solving for R3 yields two formulas: R3 = V / 1µA or
The MAX987/MAX991/MAX995 have
output structure that sinks as well as sources current.
The MAX988/MAX992/MAX996 have an open-drain output
a
push-pull
REF
R3 = (V
- V ) / 1µA. Use the smaller of the two
REF
CC
resulting resistor values. For example, if V
= 1.2V
REF
stage that can be pulled beyond V to an absolute maximum
CC
and V
= 5V, then the two R3 resistor values are
CC
of 6V above V
EE.
1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard value
for R3.
Input Stage Circuitry
The devices’ input common-mode range extends from
2) Choose the hysteresis band required (V ). For this
HB
-0.25V to (V + 0.25V). These comparators may operate at
CC
example, choose 50mV.
any differential input voltage within these limits. Input bias
current is typically 1.0pA if the input voltage is between
the supply rails. Comparator inputs are protected from
overvoltage by internal body diodes connected to the
supply rails. As the input voltage exceeds the supply rails,
these body diodes become forward biased and begin to
conduct. Consequently, bias currents increase exponentially
as the input voltage exceeds the supply rails.
3) Calculate R1 according to the following equation:
R1 = R3 x (V
/ V
)
HB
CC
For this example, insert the values R1 = 1.2MΩ x
(50mV / 5V) = 12kΩ.
4) Choose the trip point for V rising (V
; V
is
THF
IN
THR
the trip point for V falling). This is the threshold
IN
voltage at which the comparator switches its output
from low to high as V rises above the trip point. For
this example, choose 3V.
Output Stage Circuitry
IN
These comparators contain a unique output stage
capable of rail-to-rail operation with up to 8mA loads.
Many comparators consume orders of magnitude more
current during switching than during steady-state operation.
However, with this family of comparators, the supply-
current change during an output transition is extremely
small. The Supply Current vs. Output Transition Frequency
graph in the Typical Operating Characteristics section
shows the minimal supply-current increase as the output
switching frequency approaches 1MHz. This characteristic
eliminates the need for power-supply filter capacitors to
reduce glitches created by comparator switching currents.
Battery life increases substantially in high-speed, battery-
powered applications.
V
CC
R3
R1
0.1µF
V
IN
V
CC
OUT
R2
V
EE
MAX987
MAX991
MAX995
V
REF
Figure 1. Additional Hysteresis (MAX987/MAX991/MAX995)
Maxim Integrated
│ 8
www.maximintegrated.com
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