Datasheet

Figure 24. CMOS gate drive circuit for HCPL-261A/-261N families.Figure 23. TTL open-collector/open drain gate drive circuit for
HCPL-261A/-261N families.
820 Ω
HCPL-�261A fig 22
1
3
2
4
V
CC
74HC00
(OR ANY
OPEN-COLLECTOR/
OPEN-DRAIN
LOGIC GATE)
HCPL-261X
LED
750 Ω
HCPL-�261A fig 23
1
3
2
4
V
CC
74HC04
(OR ANY
TOTEM-POLE
OUTPUT LOGIC
GATE)
HCPL-261A/261N
1N4148
LED
Propagation Delay, Pulse-Width Distortion and Propa-
gation Delay Skew
Propagation delay is a gure 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 propa-
gate 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 9).
Pulse-width distortion (PWD) results when t
PLH
and t
PHL
dier in value. PWD is dened as the dierence between
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 gure depends on the par-
ticular application (RS232, RS422, T-1, etc.).
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, dierences in propagation delays will cause
the data to arrive at the outputs of the optocouplers at
dierent times. If this dierence 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 dened as the dierence 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 25, if the in-
puts of a group of optocouplers are switched either ON
or OFF at the same time, t
PSK
is the dierence 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 26 is the timing
diagram of a typical parallel data application with both
the clock and the data lines being sent through opto-
couplers.
Table 1. Eects of Common Mode Pulse Direction on Transient I
LED
If |I
LP
| < |I
LN
|, If |I
LP
| > |I
LN
|,
LED I
F
Current LED I
F
Current
If dV
CM
/dt Is: then I
LP
Flows: and I
LN
Flows: Is Momentarily: Is Momentarily:
positive (>0) away from LED away from LED increased decreased
anode through C
LA
cathode through C
LC
negative (<0) toward LED toward LED decreased increased
anode through C
LA
cathode through C
LC