LTC6754ISC6#TRMPBF datasheet(12/22 Pages) AD

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LTC6754

6754f

For more information www.linear.com/LTC6754

applicaTions inForMaTion

Input Protection

The input stage is protected against damage from condi-

tions where the voltage on either pin exceeds the supply

voltage (VEE to VCCI) without external protection. External

input protection circuitry is only needed if input currents

can exceed the absolute maximum rating. For example,

if an input is taken beyond 300mV of either the positive

or negative supply, an internal ESD protection diode will

conduct and an external series resistor should be used to

limit the current to less than 10mA.

Outputs

The LTC6754 has been designed for driving a 100Ω load

connected between the output pins to standard LVDS

levels, with a differential output voltage of 360mV and a

commonmodevoltageof1.26V,asdescribedintheCircuit

Description Section. See the section on High Speed Board

Design Techniques for information on connecting the load

to the output pins. The outputs should not be used as

CMOS or TTL level outputs, and should not be used for

sourcing or sinking excessive load currents.

ESD

The LTC6754 has reverse-biased ESD protection diodes

on pins as shown in Figure 1.

There are additional clamps between the positive and

negative supplies that further protect the device during

ESD strikes. Hot-plugging of the device into a powered

socketisnotrecommendedsincethiscantriggertheclamp

resultinginlargecurrentsflowingbetweenthesupplypins.

Hysteresis

Comparators have very high open-loop gain. With slow

input signals that are close to each other, input noise can

cause the output voltage to switch randomly. Hysteresis

is positive feedback that increases the trip point in the

direction of the input signal transition when the output

switches. This pulls the inputs away from each other, and

prevents continuous switching back and forth. The addi-

tion of positive feedback also has the effect of making the

small signal gain infinite around the trip points. Hysteresis

is designed into most comparators and the LTC6754 has

adjustable hysteresis with a default hysteresis of 4.5mV.

The input-output transfer characteristic is illustrated in

Figure 3 showing the definitions of VOS and HYST based

upon the two measurable trip points.

In some cases, additional noise immunity is required

above what is provided by the nominal 4.5mV hysteresis.

Conversely, when processing small or fast differential

signals, hysteresis may need to be eliminated.

VOH

VOL

VOS =

6754 F03

VOUT

VOS

VTRIP+

VHYST

(= VTRIP+ – VTRIP–)

∆VIN = VIN+ – VIN–

VTRIP+ + VTRIP–

VTRIP–

FOR VTRIP+ = 3mV,

VTRIP– = –2mV,

VOS = 0.5mV,

VHYST = 5mV

Figure 3.

The LTC6754UD (QFN package) provides a hysteresis

pin, LE/HYST, that can be used to increase the internal

hysteresis, completely remove it, or enable the output

to latch. The internal hysteresis is disabled when the LE/

HYST pin voltage is above 1.7V. Although eliminating

hysteresis does reduce the voltage gain of the compara-

tor to a finite value, in many cases it will be high enough

(typically 450V/V) to amplify small input signals to valid

LVDS levels. The output will latch when the LE/HYST pin

voltage is below 0.4V. The internal hysteresis will increase

as the voltage of the pin is adjusted downward from its

default open circuit value of 1.25V to 800mV.

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