HCPL-0720-500E 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 transm­ itted. Typical-
ly, PWD on the order of 20 - 30% of the minimum pulse
width is tolerable.
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 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 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 volta­ ge, 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, 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 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.
V_I
50%
V_O
2.5 V,
CMOS
t_PSK
V_I
50%
V_O
2.5 V,
CMOS
Figure 13. Propagation delay skew waveform.
DATA
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t_PSK
t_PSK
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, 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-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.
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