HCPL2400 datasheet(13/14 Pages) HP

13 / 14 page

1-312

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 (t

PLH)

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 (t

PHL) 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 5).

Pulse-width distortion (PWD)

results when t

PLH and tPHL differ in

value. PWD is defined as the

difference between t

PLH and tPHL

and often determines the

maximum 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

consider in parallel data applica-

tions 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

drive current, supply voltage,

output load, and operating tem-

perature). As illustrated in

Figure 15, 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, tPSK can

determine the maximum parallel

data transmission rate. Figure 16

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 signals

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 16 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

PHZ.

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-2400/30 optocouplers

offer the advantages of guaran-

teed specifications for propaga-

tion delays, pulse-width distortion,

and propagation delay skew over

the recommended temperature,

input current, and power supply

ranges.

Application Circuit

A recommended LED drive circuit

is shown in Figure 13. This circuit

utilizes several techniques to

minimize the total pulse-width

distortion at the output of the

optocoupler. By using two

inverting TTL gates connected in

series, the inherent pulse-width

distortion of each gate cancels the

distortion of the other gate. For

best results, the two series-

connected gates should be from

the same package.

The circuit in Figure 13 also uses

techniques known as prebias and

peaking to enhance the

performance of the optocoupler

LED. Prebias is a small forward

voltage applied to the LED when

the LED is off. This small prebias

voltage partially charges the

junction capacitance of the LED,

allowing the LED to turn on more

quickly. The speed of the LED is

further increased by applying

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