HCPL-7101 datasheet(14/14 Pages) HP

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HCPL-7100/7101

Application Information

The HCPL-7100/7101 is

extremely easy to use. Because

the optocoupler uses high-speed

CMOS IC technology, the inputs

and output are fully compatible

with all +5 V TTL and CMOS

logic. TTL or CMOS logic can be

connected directly to the inputs

and output; no external interface

circuitry is required.

As shown in Figure 1, the only

external components required for

proper operation are two ceramic

bypass capacitors. Capacitor

values should be between 0.01

µF

and 0.1

µF. For each capacitor,

the total lead length between both

ends of the capacitor and the

power-supply pins should not

exceed 20 mm. Figure 2

illustrates the recommended

printed circuit board layout for

the HCPL-7100/7101.

Propagation Delay, Pulse-

Width Distortion, and

Propagation Delay Skew

Propagation delay is a figure of

merit which describes how

quickly a logic signal propagates

through a system. The propaga-

tion delay from low to high (tPLH)

is the amount of time required for

an input signal to propagate to

the output, causing the output to

change from low to high.

Similarly, the propagation delay

from high to low (tPHL) is the

amount of time required for the

input signal to propagate to the

output, causing the output to

change from high to low (see

Figure 7).

Pulse-width distortion (PWD)

results when tPLH and tPHL differ in

value. PWD is defined as the

difference between t

PLH and tPHL

and often determines the maxi-

mum data rate capability of a

transmission system. PWD can be

expressed in percent by dividing

the PWD (in ns) by the minimum

pulse width (in ns) being

transmitted. Typically, PWD on

the order of 20-30% of the

minimum pulse width is tolerable;

the exact figure depends on the

particular application (RS232,

RS422, T-1, etc.).

Propagation delay skew, t

PSK, is

an important parameter to con-

sider in parallel data applications

where synchronization of signals

on parallel data lines is a concern.

If the parallel data is being sent

through a group of optocouplers,

differences in propagation delays

will cause the data to arrive at the

outputs of the optocouplers at

different times. If this difference

in propagation delays is large

enough, it will determine the

maximum rate at which parallel

data can be sent through the

optocouplers.

Propagation delay skew is defined

as the difference between the

minimum and maximum propaga-

tion delays, either t

PLH or tPHL, for

any given group of optocouplers

which are operating under the

same conditions (i.e., the same

supply voltage, output load, and

operating temperature). As illus-

trated in Figure 10, if the inputs

of a group of optocouplers are

switched either ON or OFF at the

same time, t

PSK is the difference

between the shortest propagation

delay, either t

PLH or tPHL, and the

longest propagation delay, either

PLH or tPHL.

As mentioned earlier, t

PSK can

determine the maximum parallel

data transmission rate. Figure 11

is the timing diagram of a typical

parallel data application with both

the clock and the data lines being

sent through optocouplers. The

figure shows data and clock

signals at the inputs and outputs

of the optocouplers. To obtain the

maximum data transmission rate,

both edges of the clock signal are

being used to clock the data; if

only one edge were used, the

clock signal would need to be

twice as fast.

Propagation delay skew repre-

sents the uncertainty of where an

edge might be after being sent

through an optocoupler. Figure

11 shows that there will be

uncertainty in both the data and

the clock lines. It is important

that these two areas of uncertainty

not overlap, otherwise the clock

signal might arrive before all of

the data outputs have settled, or

some of the data outputs may

start to change before the clock

signal has arrived. From these

considerations, the absolute

minimum pulse width that can be

sent through optocouplers in a

parallel application is twice t

PSK. A

cautious design should use a

slightly longer pulse width to

ensure that any additional

uncertainty in the rest of the

circuit does not cause a problem.

The HCPL-7101 optocoupler offers

the advantages of guaranteed

specifications for propagation

delays, pulse-width distortion and

propagation delay skew over the

recommended temperature, and

power supply ranges.

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