8

Applications Information

Power Supplies and Circuit Layout

The EL5481 and EL5482 comparators operate with single

output side of the comparators is supplied by a single supply

from 2.7V to 5V. The rail to rail output swing enables direct

connection of the comparator to both CMOS and TTL logic

circuits. As with many high speed devices, the supplies must

be well bypassed. Elantec recommends a 4.7µF tantalum in

parallel with a 0.1µF ceramic. These should be placed as

close as possible to the supply pins. Keep all leads short to

reduce stray capacitance and lead inductance. This will also

minimize unwanted parasitic feedback around the

comparator. The device should be soldered directly to the

PC board instead of using a socket. Use a PC board with a

good, unbroken low inductance ground plane. Good ground

plane construction techniques enhance stability of the

comparators.

Input Voltage Considerations

The EL5481 and EL5482 input range is specified from 0.1V

limit is that the output still responds correctly to a small

differential input signal. The differential input stage is a pair

of PNP transistors, therefore, the input bias current flows out

of the device. When either input signal falls below the

negative input voltage limit, the parasitic PN junction formed

by the substrate and the base of the PNP will turn on,

resulting in a significant increase of input bias current. If one

of the inputs goes above the positive input voltage limit, the

output will still maintain the correct logic level as long as the

other input stays within the input range. However, the

propagation delay will increase. When both inputs are

outside the input voltage range, the output becomes

unpredictable. Large differential voltages greater than the

supply voltage should be avoided to prevent damages to the

input stage. Inputs of unused channels should not be left

floating. They should be driven to a known state. For

example, one input can be tied to ground and the other input

can be connected to some voltage reference (like ±100mV)

to avoid oscillation in the output due to unwanted output to

input feedback.

Input Slew Rate

Most high speed comparators oscillate when the voltage of

one of the inputs is close to or equal to the voltage on the

other input due to noise or undesirable feedback. For clean

output waveform, the input must meet certain minimum slew

rate requirements. In some applications, it may be helpful to

apply some positive feedback (hysteresis) between the

output and the positive input. The hysteresis effectively

causes one comparator's input voltage to move quickly past

the other, thus taking the input out of the region where

oscillation occurs. For the EL5481 and EL5482, the

propagation delay increases when the input slew rate

increases for low overdrive voltages. With high overdrive

voltages, the propagation delay does not change much with

the input slew rate.

Latch Pin Dynamics

The EL5482 contains a “transparent” latch for each channel.

The latch pin is designed to be driven with either a TTL or

CMOS output. When the latch is connected to a logic high

level or left floating, the comparator is transparent and

immediately responds to the changes at the input terminals.

When the latch is switched to a logic low level, the

comparator output remains latched to its value just before

the latch’s high-to-low transition. To guarantee data

retention, the input signal must remain the same state at

least 1ns (hold time) after the latch goes low and at least 2ns

(setup time) before the latch goes low. When the latch goes

high, the new data will appear at the output in approximately

6ns (latch propagation delay). The EL5481 has no latch pins.

Hysteresis

Hysteresis can be added externally. The following two

methods can be used to add hysteresis.

Inverting comparator with hysteresis:

greater than the input bias current of the comparator:

where:

Non inverting comparator with hysteresis:

+

-

V

TH

V

REF

R

1

R

1

R

2

+

---------------------

×

=

R

3

V

O

V

H

--------

R

1

(

R

||

×

=

R

(

V

V

TH

R

1

-----------







÷

–

V

TH

0.5V

SD

–

R

3

--------------------------------------







+

=

+

-

EL5481, EL5482