MAX987EUK-T datasheet(9/13 Pages) MAXIM

Part #	MAX987EUK-T

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

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Manufacturer	MAXIM [Maxim Integrated Products]

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9 / 13 page

High-Speed, Micropower, Low-Voltage,

SOT23, Rail-to-Rail I/O Comparators

_______________________________________________________________________________________

9

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

8.2kΩ standard value:

6) Verify trip voltages and hysteresis as follows:

MAX988/MAX992/MAX996

The MAX988/MAX992/MAX996 have ±2.5mV internal

hysteresis. They have open-drain outputs and require

an external pullup resistor (Figure 2). Additional hys-

teresis can be generated using positive feedback, but

the formulas differ slightly from those of the

MAX987/MAX991/MAX995.

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:

6) Verify trip voltages and hysteresis as follows:

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 PCB 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 PCB instead of

using a socket.

V

rising: V

= V

x R1 x

1

R1

V

falling

IN

THR

REF

IN

:

++

+

⎛

⎝

⎜

⎞

⎠

⎟

=

−

+

⎛

⎝

⎜

⎞

⎠

⎟

=

−

1

2

1

34

1

34

RR

R

VV

Rx V

RR

Hysteresis

V

THF

THR

CC

THR

THF

R2 =

1

V

THR

REF xR

R

1

34

⎛

⎝

⎜

⎞

⎠

⎟ −−

+

V

rising: V

= V

x R1 x

1

R1

V

falling

IN

THR

REF

IN

:

++

⎛

⎝

⎜

⎞

⎠

⎟

=

−

⎛

⎝

⎜

⎞

⎠

⎟

=

−

1

2

1

3

1

3

RR

VV

Rx V

R

Hysteresis

V

THF

THR

CC

THR

THF

R2 =

1

V

R2 =

1

3.0V

1.2 x 12k

THR

REF

.

xR

R

kM

k

1

3

1

12

1

22

803

⎛

⎝

⎜

⎞

⎠

⎟ −−

⎛

⎝

⎜

⎞

⎠

⎟ −−

=

ΩΩ

Ω

VEE

VCC

OUT

R3

R2

R1

R4

0.1µF

VREF

VIN

VCC

MAX988

MAX992

MAX996

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

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