_______________Detailed Description

The MAX941/MAX942/MAX944 single-supply compara-

tors feature internal hysteresis, high speed, and low

power. Their outputs are guaranteed to pull within 0.4V

of either supply rail without external pullup or pulldown

circuitry. Rail-to-rail input voltage range and low-volt-

age single-supply operation make these devices ideal

for portable equipment. The MAX941/MAX942/

MAX944 interface directly to CMOS and TTL logic.

Timing

Most high-speed comparators oscillate in the linear

region because of noise or undesired parasitic feed-

back. This tends to occur when the voltage on one

input is at or equal to the voltage on the other input. To

counter the parasitic effects and noise, the MAX941/

MAX942/MAX944 have internal hysteresis.

The hysteresis in a comparator creates two trip points:

one for the rising input voltage and one for the falling

input voltage (Figure 1). The difference between the trip

points is the hysteresis. When the comparator’s input

voltages are equal, the hysteresis effectively causes

one comparator input voltage to move quickly past the

other, thus taking the input out of the region where

oscillation occurs. Standard comparators require hys-

teresis to be added with external resistors. The

MAX941/MAX942/MAX944’s fixed internal hysteresis

eliminates these resistors and the equations needed to

determine appropriate values.

Figure 1 illustrates the case where IN- is fixed and IN+

is varied. If the inputs were reversed, the figure would

look the same, except the output would be inverted.

The MAX941 includes an internal latch that allows stor-

—

A

—

T

—

C

—

H

–

pin has a high

—

A

—

T

—

C

—

H

–

is high, the latch is transpar-

ent (i.e., the comparator operates as though the latch is

not present). The comparator's output state is stored

—

A

—

T

—

C

—

H

–

is pulled low. All timing constraints must

be met when using the latch function (Figure 2).

Shutdown Mode (MAX941 Only)

The MAX941 shuts down when

–

S

—

H

—

D

—

N

–

is low. When shut

down, the supply current drops to less than 60µA, and

the three-state output becomes high impedance. The

–

S

—

H

—

D

—

N

–

pin has a high input impedance. Connect

–

S

—

H

—

D

—

N

–

to V+ for normal operation. Exit shutdown with

L

—

A

—

T

—

C

—

H

–

high; otherwise, the output will be indeterminate.

Input Stage Circuitry

The MAX941/MAX942/MAX944 include internal protec-

tion circuitry that prevents damage to the precision

input stage from large differential input voltages. This

protection circuitry consists of two back-to-back diodes

between IN+ and IN- as well as two 4.1k

Ω resistors

(Figure 3). The diodes limit the differential voltage

applied to the internal circuitry of the comparators to be

of the diode (about 0.7V at +25°C).

this protection circuitry increases the input bias current

at IN+ (source) and IN- (sink).

Input Current =

2 x 4.1k

Ω

Input current with large differential input voltages

also allows for the input common-mode range of the

MAX941/MAX942/MAX944 to extend beyond both

power-supply rails. The output is in the correct logic

state if one or both inputs are within the common-mode

range.

High-Speed, Low-Power, 3V/5V, Rail-to-Rail,

Single-Supply Comparators

_______________________________________________________________________________________

7

COMPARATOR

OUTPUT

2

Figure 1. Input and Output Waveform, Noninverting Input

Varied