Micropower, Low-Voltage, UCSP/SC70,

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 out-

put 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 MAX985/MAX989/MAX993 have a

push-pull output structure that sinks as well as sources

current. The MAX986/MAX990/MAX994 have an open-

Input Stage Circuitry

The devices’ input common-mode range extends from

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. Another advantage realized in high-

speed, 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 calcu-

late 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 cur-

R3. Considering the two possible output states in

are 1.2M

Ω and 3.8MΩ. Choose a 1.2MΩ standard

value for R3.

example, choose 50mV.

3) Calculate R1 according to the following equation:

For this example, insert the values R1 = 1.2M

Ω x

(50mV / 5V) = 12k

Ω.

voltage at which the comparator switches its output

this example, choose 3V.

MAX985

MAX989

MAX993

OUT

R3

R1

R2

Figure 1. Additional Hysteresis (MAX985/MAX989/MAX993)