HCPL-0721 datasheet(11/18 Pages) AVAGO

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Figure 13. Propagation delay skew waveform.

Figure 14. Parallel data transmission example.

Propagation delay skew represents the uncertainty of

where an edge might be after being sent through an op-

tocoupler. Figure 14 shows that there will be uncertainty

in both the data and 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 consider-

ations,theabsoluteminimumpulsewidththatcanbesent

throughoptocouplersinaparallelapplicationistwicetPSK.

Pulse-width distortion (PWD) is the difference between

tPHL and tPLH 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. Typical-

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

width is tolerable.

Propagation delay skew, tPSK, is an important parameter

to consider in parallel data applications where synchro-

nization of signals on parallel data lines is a concern. If

the parallel data is being sent through a group of opto-

couplers, 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 delay 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 tPLH 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 op-

erating temperature). As illustrated in Figure 13, if the in-

puts of a group of optocouplers are switched either ON

or OFF at the same time, tPSK is the difference between

the shortest propagation delay, either tPLH or tPHL, and

the longest propagation delay, either tPLH or tPHL.

As mentioned earlier, tPSK can determine the maximum

parallel data transmission rate. Figure 14 is the timing

diagram of a typical parallel data application with both

the clock and data lines being sent through the opto-

couplers. The figure shows data and clock signals at the

inputs and outputs of the optocouplers. In this case the

data is assumed to be clocked off of the rising edge of

the clock.

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-772X/072X optocouplers offer the advantage

of guaranteed specifications for propagation delays,

pulse-width distortion, and propagation delay skew

over the recommended temperature and power supply

ranges.

50%

tPSK

2.5 V,

CMOS

2.5 V,

CMOS

DATA

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

tPSK

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