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MAX987
(Rev.:1997)

High-Speed, Micropower, Low-Voltage, SOT23, Rail-to-Rail I/O Comparators

Maxim Integrated 

High-Speed, Micropower, Low-Voltage,

SOT23, Rail-to-Rail I/O Comparators

5) Calculate R2 as shown. For this example, choose an

8.2kΩ standard value:

R2 =

1





VTHR

VREF x



R1

−

1−

R1

1

R3

R2 =

1

= 8.03kΩ

 3.0V 

 1.2 x 12kΩ 

−

1−

12kΩ

1

2.2MΩ

6) Verify trip voltages and hysteresis as follows:

VIN

rising:

VTHR

=

VREF

x R1 x

1

 R1

+

1

R2

+

1

R3 

VIN falling:

VTHF =

VTHR

−

 R1



x VCC

R3





Hysteresis = VTHR − VTHF

MAX988/MAX992/MAX996

The MAX988/MAX992/MAX996 have ±2.5mV internal

hysteresis. They have open-drain outputs and require

an external pull-up resistor (Figure 2). Additional hys-

teresis can be generated using positive feedback, but

the formulas differ slightly from those of the

MAX987/MAX991/MAX995.

R1

VIN

VCC

R3

R2

VREF

VCC

OUT

VEE

MAX988

MAX992

MAX996

0.1µF

R4

Figure 2. Additional Hysteresis (MAX988/MAX992/MAX996)

Use the following procedure to calculate resistor

values:

1) Select R3 according to the formulas R3 = VREF / 1µA

or R3 = (VREF - VCC) / 1µA - R4. Use the smaller of

the two resulting resistor values.

2) Choose the hysteresis band required (VHB). For this

example, choose 50mV.

3) Calculate R1 according to the following equation:

R1 = (R3 + R4) x (VHB / VCC)

4) Choose the trip point for VIN rising (VTHR; VTHF is

the trip point for VIN falling). This is the threshold

voltage at which the comparator switches its output

from low to high as VIN rises above the trip point.

5) Calculate R2 as follows:

R2 =

1





VTHR

VREF x



R1

−

1−

R1

1

R3 + R4

6) Verify trip voltages and hysteresis as follows:

VIN rising: VTHR = VREF x R1 x

1

 R1

+

1

R2

+

R3

1

+ R4

VIN falling:

VTHF =

VTHR

−





R1

R3

x

+

VCC

R4





Hysteresis = VTHR − VTHF

Circuit Layout and Bypassing

These comparators’ high-gain bandwidth requires

design precautions to maximize their high-speed capa-

bility. The recommended precautions are:

1) Use a printed circuit board with an unbroken, low-

inductance ground plane.

2) Place a decoupling capacitor (a 0.1µF ceramic

capacitor is a good choice) as close to VCC as

possible.

3) On the inputs and outputs, keep lead lengths short

to avoid unwanted parasitic feedback around the

comparators.

4) Solder the devices directly to the printed circuit

board instead of using a socket.

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