MAX985/MAX986/MAX989/
MAX990/MAX993/MAX994
Micropower, Low-Voltage, UCSP/SC70,
Rail-to-Rail I/O Comparators
_______________Detailed Description
V
CC
The MAX985/MAX986/MAX989/MAX990/MAX993/
MAX994 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 11µA.
Their common-mode input voltage range extends 0.25V
beyond each rail. Internal hysteresis ensures clean out-
put switching, even with slow-moving input signals.
Large internal output drivers allow rail-to-rail output
swing with up to 8mA loads.
R3
R1
V
IN
V
CC
OUT
R2
V
EE
MAX985
MAX989
MAX993
V
REF
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 MAX985/MAX989/MAX993 have a
push-pull output structure that sinks as well as sources
current. The MAX986/MAX990/MAX994 have an open-
Figure 1. Additional Hysteresis (MAX985/MAX989/MAX993)
drain output stage that can be pulled beyond V
to an
CC
absolute maximum of 6V above V
.
EE
__________Applications Information
Input Stage Circuitry
The devices’ input common-mode range extends from
Additional Hysteresis
-0.25V to (V
+ 0.25V). These comparators may oper-
CC
MAX985/MAX989/MAX993
The MAX985/MAX989/MAX993 have 3mV 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 MAX985/MAX989/MAX993.
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.
1) Select R3. Leakage current at IN is under 10nA, so
the current through R3 should be at least 1µA to
minimize errors caused by leakage current. The cur-
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. Another advantage realized in high-
speed, battery-powered applications is a substantial
increase in battery life.
rent through R3 at the trip point is (V
- V
) /
OUT
REF
R3. Considering the two possible output states in
solving for R3 yields two formulas: R3 = V / 1µA
REF
or R3 = (V
- V ) / 1µA. Use the smaller of the
REF
CC
two resulting resistor values. For example, if V
=
REF
1.2V and V
= 5V, then the two R3 resistor values
CC
are 1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard
value for R3.
2) Choose the hysteresis band required (V ). For this
HB
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = R3 x (V / V
)
CC
HB
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
IN
this example, choose 3V.
Maxim Integrated
7
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