MAX9025 데이터 시트보기 (PDF) - Maxim Integrated

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MAX9025

UCSP, 1.8V, Nanopower, Beyond-the-Rails Comparators With/Without Reference

Maxim Integrated 

MAX9025–MAX9028

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

Comparators With/Without Reference

IN+

VTHR

IN-

VHB

VTHF

THRESHOLDS

HYSTERESIS

BAND

OUT

RS

VIN

VCC/2

RFB

VCC

MAX9027

OUT

Figure 2. Threshold Hysteresis Band

Simplest Circuit

The simplest circuit for adding external hysteresis is

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

defined by:

Hyste= resis

RS

R FB

× VCC

where RS is the source resistance and RFB is the feed-

back resistance. Because the comparison threshold is 1/2

VCC, the MAX9027 was chosen for its push-pull output

and lack of reference. This provides symmetrical hyster-

esis 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 circuits

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, selecting components for

Figure 3, with a target of 50mV hysteresis, a 5V supply,

and choosing an RFB of 10MΩ gives RS as 100kΩ. The

total impedance seen at IN+ is therefore 10MΩ || 100kΩ,

or 99kΩ. The maximum IB of the MAX9025−MAX9028 is

2nA; therefore, the error due to source impedance is less

than 400µV.

Figure 3. MAX9025/MAX9027 External Hysteresis

Asymmetrical Hysteresis

When the input threshold is not set at 1/2 VCC, the hys-

teresis added to the input threshold will not be symmetri-

cal. 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 needed,

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 sup-

ply lines. Minimize signal trace lengths to reduce stray

capacitance. A ground plane and surface-mount compo-

nents 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, and

its noninverting input is connected to a 100mVP-P 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 resis-

tor for the MAX9028’s open-drain output is connected to

the +3V supply voltage. This configuration allows the full

5V logic swing without creating overvoltage on the 3V

logic inputs. For 3V to 5V logic-level translations, simply

connect the +3V supply voltage to VCC and the +5V sup-

ply voltage to the pullup resistor.

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