UCSP, 1.8V, Nanopower, Beyond-the-Rails

Comparators With/Without Reference

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11

THRESHOLDS

OUT

IN+

IN-

HYSTERESIS

BAND

Figure 2. Threshold Hysteresis Band

MAX9027

OUT

VIN

Figure 3. MAX9025/MAX9027 External Hysteresis

Simplest Circuit

The simplest circuit for adding external hysteresis is

shown in Figure 3. In this example, the hysteresis is

defined by:

back resistance. Because the comparison threshold is

put and lack of reference. This provides symmetrical

hysteresis around the threshold.

Output Considerations

In most cases, the push-pull outputs of the

MAX9025/MAX9027 are best for external hysteresis.

The open-drain output of the MAX9026/MAX9028 can

be used, but the effect of the feedback network on the

actual output high voltage must be considered.

Component Selection

Because the MAX9025–MAX9028 are intended for very

low power-supply systems, the highest impedance cir-

cuits should be used wherever possible. The offset

error due to input-bias current is proportional to the

total impedance seen at the input. For example, select-

ing components for Figure 3, with a target of 50mV hys-

the MAX9025–MAX9028 is 2nA; therefore, the error due

to source impedance is less than 400µV.

Asymmetrical Hysteresis

teresis added to the input threshold will not be symmet-

rical. This is typical of the MAX9025/MAX9026 where

the internal reference is usually used as the threshold.

If the asymmetry is unacceptable, it can be corrected

by adding resistors to the circuit.

Board Layout and Bypassing

Power-supply bypass capacitors are not typically need-

ed, but use 100nF bypass capacitors close to the

device’s supply pins when supply impedance is high,

supply leads are long, or excessive noise is expected

on the supply lines. Minimize signal trace lengths to

reduce stray capacitance. A ground plane and surface-

mount components are recommended. If the REF pin is

decoupled, use a new low-leakage capacitor.

Zero-Crossing Detector

Figure 4 shows a zero-crossing detector application.

The MAX9027’s inverting input is connected to ground,

signal source. As the signal at the noninverting input

crosses 0V, the comparator’s output changes state.

Logic-Level Translator

The Typical Application Circuit shows an application

that converts 5V logic to 3V logic levels. The MAX9028

is powered by the +5V supply voltage, and the pullup

resistor for the MAX9028’s open-drain output is con-

nected to the +3V supply voltage. This configuration

allows the full 5V logic swing without creating overvolt-

age on the 3V logic inputs. For 3V to 5V logic-level

translations, simply connect the +3V supply voltage to

Hysteresis

R

R

V

S

FB

CC

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