HCPL-0630 datasheet(21/22 Pages) AVAGO

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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 sys-

tem. The propagation 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

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 8).

Pulse-width distortion (PWD) results when t

PLH

and t

PHL

diﬀer in value. PWD is deﬁned as the diﬀerence be-

tween t

PLH

and t

PHL

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. Typi-

cally, PWD on the order of 20-30% of the minimum pulse

width is tolerable; the exact ﬁgure depends on the par-

ticular application (RS232, RS422, T-l, etc.).

Propagation delay skew, t

PSK

, is an important parameter to

consider in parallel data applications where synchroniza-

tion of signals on parallel data lines is a concern. If the

parallel data is being sent through a group of optocou-

plers, diﬀerences in propagation delays will cause the

data to arrive at the outputs of the optocouplers at diﬀer-

ent 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 be-

tween the minimum and maximum propagation delays,

either t

PLH

or t

PHL

, for any given group of optocouplers

which are operating under the same conditions (i.e., the

same drive current, supply voltage, output load, and op-

erating temperature). As illustrated in Figure 19, if the in-

puts 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 t

PHL

, and the

longest propagation delay, either t

PLH

or t

PHL

As mentioned earlier, t

PSK

can determine the maximum

parallel data transmission rate. Figure 20 is the timing

diagram of a typical parallel data application with both

the clock and the data lines being sent through opto-

couplers. 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 represents the uncertainty of

where an edge might be after being sent through an

optocoupler. Figure 20 shows that there will be uncer-

tainty in both the data and the clock lines. It is important

that these two areas of uncertainty not overlap, other-

wise 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 appli-

cation is twice t

PSK

. A cautious design should use a slightly

longer pulse width to ensure that any additional uncer-

tainty in the rest of the circuit does not cause a problem.

The t

PSK

speciﬁed optocouplers oﬀer the advantages of

guaranteed speciﬁcations for propagation delays, pulse-

width distortion and propagation delay skew over the

recommended temperature, input current, and power

supply ranges.

Figure 19. Illustration of propagation delay skew - t

PSK.

Figure 20. Parallel data transmission example.

50%

1.5 V

I F

50%

I F

tPSK

1.5 V

DATA

t PSK

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

t PSK

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