_______________Detailed Description

The MAX975/MAX977 single/dual comparators have

three operating modes, and use a +2.7V to +5.25V

single supply. The operating modes are as follows:

high speed, high speed with auto-standby, and low

power. Propagation delay is typically 28ns in high-

speed mode, while typical supply current is 250µA. In

low-power mode, propagation delay is typically 480ns

and power consumption is only 3µA. The auto-standby

feature switches into low-power standby for each

comparator with unchanging outputs in high-speed

mode. The timeout period, or the time that OUT_

must be idle (unchanged state) for the MAX975/

MAX977 to enter auto-standby, is adjustable by means

of an external capacitor. All inputs and outputs can tol-

erate a continuous short-circuit fault condition to either

rail. Internal hysteresis in high-speed mode ensures

clean output switching, even with slow-moving input

signals.

The MAX975 functional diagram shows two paralleled

comparators, a timing circuit, a transition detector, and

logic gates. The upper comparator is high speed, while

the lower comparator is a slower low-power compara-

tor. The dual MAX977 features independent timeout

adjustment. The following sections discuss the details

of operation.

Hysteresis (High-Speed Mode Only)

Most high-speed comparators can oscillate in the linear

operating region because of noise or undesired para-

sitic feedback. This tends to occur when the voltage on

one input is equal to or very close to the voltage on the

other input. The MAX975/MAX977 have internal hys-

teresis to counter parasitic effects and noise.

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 comparators’ input

voltages are equal, the hysteresis effectively causes

one comparator input voltage to move quickly past the

other, taking the input out of the region where oscilla-

tion occurs.

Figure 1 illustrates the case where IN- has a fixed volt-

age applied and IN+ is varied. If the inputs were

reversed, the figure would be the same, except with an

inverted output.

Auto-Standby Mode

The MAX975/MAX977’s auto-standby function operates

only in high-speed mode. The device enters auto-

standby when OUT_ remains unchanged for a prepro-

grammed timeout period. In auto-standby mode, the

low-power comparator is enabled while the high-speed

comparator is disabled and STAT_ goes high. The logic

state and sink/source capabilities of OUT_ remain

unchanged, but propagation delay increases to 480ns.

In this mode, the timing circuitry is powered down, and

the transition detector monitors the low-power com-

parator for a transition. When an output transition

occurs (OUT_ changes state), the timing circuitry is

Single/Dual, +3V/+5V Dual-Speed

Comparators with Auto-Standby

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11

INPUTS

LP

STO_

IDLE TIME

MODE

STAT

OUTPUT

L

High speed

(Auto-standby enabled)

L

L

Auto-standby

H

↓

(falling edge)

L

X

High speed

(Auto-standby mode disabled)

L

H

X

X

Low power

H

Table 1. Programming

2

COMPARATOR

OUTPUT

Figure 1. Input and Output Waveforms, Noninverting Input

Varied