MAX985/MAX986/MAX989/MAX990/MAX993/MAX994
Micropower, Low-Voltage, SOT23,
Rail-to-Rail I/O Comparators
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_______________Detailed Description
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 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 minimizes 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 opendrain output stage that can be pulled beyond VCCto an
absolute maximum of 6V above VEE.
Input Stage Circuitry
The devices’ input common-mode range extends from
-0.25V to (VCC+ 0.25V). These comparators may operate 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 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.
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 operation. 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 output switching frequency approaches 1MHz. This characteristic eliminates the need for power-supply filter
capacitors to reduce glitches created by comparator
switching currents. Another advantage realized in highspeed, battery-powered applications is a substantial
increase in battery life.
__________Applications Information
Additional Hysteresis
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 calculate resistor values for the MAX985/MAX989/MAX993.
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 current through R3 at the trip point is (V
REF
- V
OUT
) /
R3. Considering the two possible output states in
solving for R3 yields two formulas: R3 = V
REF
/ 1µA
or R3 = (V
REF
- VCC) / 1µA. Use the smaller of the
two resulting resistor values. For example, if V
REF
=
1.2V and VCC= 5V, then the two R3 resistor values
are 1.2MΩ and 3.8MΩ. Choose a 1.2MΩ standard
value for R3.
2) Choose the hysteresis band required (VHB). For this
example, choose 50mV.
3) Calculate R1 according to the following equation:
R1 = R3 x (VHB/ VCC)
For this example, insert the values R1 = 1.2MΩ x
(50mV / 5V) = 12kΩ.
4) Choose the trip point for VINrising (V
THR
; V
THF
is
the trip point for VINfalling). This is the threshold
voltage at which the comparator switches its output
from low to high as VINrises above the trip point. For
this example, choose 3V.