Detailed Description

The MAX49140 single-supply comparator features internal hysteresis, high speed, and low power. The output is pulled

to within 300mV of either supply rail without external pullup or pulldown circuitry. Rail-to-rail input voltage range and low-

voltage, single-supply operation make the device ideal for portable equipment. The devices interface directly to CMOS

and TTL logic.

Most high-speed comparators oscillate in the linear region because of noise or undesired parasitic feedback. 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 device has an internal hysteresis of 1.2mV.

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. The average of the trip points is the offset

voltage. 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 hysteresis to be added with external resistors. The device's fixed internal hysteresis eliminates these resistors.

To increase hysteresis and noise margin even more, add positive feedback with two resistors as a voltage-divider from

the output to the non-inverting input.

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.

2

COMPARATOR

OUTPUT

Figure 1. Input and Output Waveform, Noninverting Input Varied

Input Stage Circuitry

The device includes internal protection 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 series

1kΩ resistors (Figure 2). The diodes limit the differential voltage applied to the internal circuitry of the comparator to be

(source) and IN- (sink).

InputCurrent =

(IN + − IN − ) − 2VF

2x1kΩ

both inputs are within the common-mode range.

MAX49140

30ns, Low-Power, 3V/5V, Rail-to-Rail Single-

Supply Comparator

www.maximintegrated.com

19-100740

Maxim Integrated | 9