Rainbow Electronics MAX996 - Manual (Page 8)

Detailed Description
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 propaga-
tion 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.
The output stage employs a unique design that mini-
mizes supply-current surges while switching, virtually
eliminating the supply glitches typical of many other
comparators. The MAX987/MAX991/MAX995 have a
push/pull output structure that sinks as well as sources
current. The MAX988/MAX992/MAX996 have an open-
drain output stage that can be pulled beyond V
CC to an
absolute maximum of 6V above V EE .
Input Stage Circuitry
The devices’ input common-mode range extends from
-0.25V to (V
CC + 0.25V). These comparators may oper-
ate at 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 pro-
tected from overvoltage by internal body diodes con-
nected to the supply rails. As the input voltage exceeds
the supply rails, these body diodes become forward
biased and begin to conduct. Consequently, bias cur-
rents increase exponentially as the input voltage
exceeds the supply rails.
Output Stage Circuitry
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 oper-
ation. However, with this family of comparators, the
supply-current change during an output transition is
extremely small. The Typical Operating Characteristics
graph Supply Current vs. Output Transition Frequency
shows the minimal supply-current increase as the out-
put switching frequency approaches 1MHz. This char-
acteristic 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.
Applications Information
Additional Hysteresis
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 calcu-
late 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 current through R3 at the trip point is (V
REF -
V OUT ) / R3. Considering the two possible output
states and solving for R3 yields two formulas: R3 =
V
REF / 1µA or R3 = (V REF - V CC ) / 1µA. Use the
smaller of the two resulting resistor values. For
example, if V
REF = 1.2V and V CC = 5V, then the two
R3 resistor values are 1.2M Ω and 3.8MΩ. Choose a
1.2M
Ω standard value for R3.
2) Choose the hysteresis band required (V HB ). For this
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = R3 x (V HB / V CC )
For this example, insert the values R1 = 1.2M
Ω x
(50mV / 5V) = 12k Ω.
4) Choose the trip point for V IN rising (V THR ; V THF is
the trip point for V IN falling). This is the threshold
voltage at which the comparator switches its output
from low to high as V
IN rises above the trip point. For
this example, choose 3V.
MAX987/MAX988/MAX991/MAX992/MAX995/MAX996
High-Speed, Micropower, Low-Voltage,
SOT23, Rail-to-Rail I/O Comparators
V CC
MAX987
MAX991
MAX995
OUT
0.1 µF
R3
R1
R2
V REF
V
EE
V IN
V CC
Figure 1. Additional Hysteresis (MAX987/MAX991/MAX995)
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