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-

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

Supply Current vs. Output Transition Frequency graph

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.

states and solving for R3 yields two formulas: R3 =

smaller of the two resulting resistor values. For

R3 resistor values are 1.2MΩ 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 = 1.2MΩ x

(50mV / 5V) = 12kΩ.

voltage at which the comparator switches its output

this example, choose 3V.

High-Speed, Micropower, Low-Voltage,

SOT23, Rail-to-Rail I/O Comparators

MAX987

MAX991

MAX995

OUT

0.1µF

R3

R1

R2

Figure 1. Additional Hysteresis (MAX987/MAX991/MAX995)

8

_______________________________________________________________________________________