EL5287C 查看數據表（PDF） - Elantec -> Intersil

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EL5287C

Dual and Window 4ns High-Speed Comparators

Elantec -> Intersil 

EL5287C - Preliminary

Dual and Window 4ns High-Speed Comparators

Applications Information

Power Supplies and Circuit Layout

The EL5287C comparator operates with single and dual

supply with 5V to 12V between VS+ and VS-. The out-

put side of the comparator 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 recom-

mends 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 mini-

mize 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 sta-

bility of the comparators.

Input Voltage Considerations

The EL5287C’s input range is specified from 0.1V

below VS- to 2.25V below VS+. The criterion for the

input 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 propa-

gation delay will increase. When both inputs are outside

the input voltage range, the output becomes unpredict-

able. Large differential voltages greater than the supply

voltage should be avoided to prevent damages to the

input stage.

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 min-

imum slew rate requirements. In some applications, it

may be helpful to apply some positive feedback (hyster-

esis) 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

EL5287C, 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 EL5287C 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 con-

nected 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).

Power Dissipation

When switching at high speeds, the comparator's drive

capability is limited by the rise in junction temperature

caused by the internal power dissipation. For reliable

operation, the junction temperature must be kept below

TJMAX (125°C).

An approximate equation for the device power dissipa-

tion is as follows. Assume the power dissipation in the

load is very small:

PDISS = (VS × IS + VSD × ISD )

where:

VS is the analog supply voltage from VS+ to VS-

IS is the analog quiescent supply current per comparator

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