HCPL-7710-000E Datasheet, PDF (11/18 Pages) AVAGO TECHNOLOGIES LIMITED – 40 ns Propagation Delay, CMOS Optocoupler

Pulse-width distortion (PWD) is the difference between
t_PHL and t_PLH 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 PWD specification for the
HCPL-x710 is 8 ns (10%) maximum across recommend-
ed operating condi­tions. 10% maximum is dictated
by the most stringent of the three fieldbus standards,
PROFIBUS.
Propagation delay skew, t_PSK, 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 op-
tocouplers, differences in propagation delays will cause
the data to arrive at the outputs of the optocouplers at
different 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 optocou-
plers.
Propagation delay skew is defined as the differ-
ence between the minimum and maximum propa­
gation 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 operating temperature). 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 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 16 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.
V_I
50%
V_O
2.5 V,
CMOS
t_PSK
V_I
50%
V_O
2.5 V,
CMOS
Figure 15. Propagation delay skew waveform.
DATA
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t_PSK
t_PSK
Figure 16. Parallel data transmission example.
HCPL-0710 fig 14
Propagation delay skew repre­sents the uncertainty of
where an edge might be after being sent through an
optocoupler. Figure 16 shows that there will be uncer-
tainty 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 considerations, the absolute
minimum pulse width that can be sent through op-
HCPL-0710 fig 15
tocouplers 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-x710 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.
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