HCNW4506 Avago Technologies US Inc., HCNW4506 Datasheet - Page 20

• Manufacturer Part Number: HCNW4506
• Description: Optocoupler
• Manufacturer: Avago Technologies US Inc.

Specifications of HCNW4506

• No. Of Channels: 1
• Isolation Voltage: 5kV
• Optocoupler Output Type: Gate Drive
• Input Current: 20mA
• Output Voltage: 30V
• Opto Case Style: DIP
• No. Of Pins: 8
• Propagation Delay Low-high: 0.55µs
• Ctr Min: 44%
• Voltage - Isolation: 3750Vrms
• Number Of Channels: 1, Unidirectional
• Current - Output / Channel: 15mA
• Data Rate: 1MBd
• Propagation Delay High - Low @ If: 200ns @ 10mA
• Current - Dc Forward (if): 25mA
• Input Type: DC
• Output Type: Open Collector
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.400", 10.16mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: 516-1041-5

LED Drive Circuit Considerations for Ultra High CMR Performance
Without a detector shield, the dominant cause of op-
tocoupler CMR failure is capacitive coupl-ing from the
input side of the optocoupler, through the package, to
the detector IC as shown in Figure 16. The HCPL-4506
series improve CMR performance by using a detector
IC with an optically transparent Faraday shield, which
diverts the capacitively coupled current away from the
sensitive IC circuitry. However, this shield does not elimi-
nate the capacitive coupling between the LED and the
optocoupler output pins and output ground as shown in
Figure 17. This capacitive coupling causes perturbations
in the LED current during common mode transients and
becomes the major source of CMR failures for a shielded
optocoupler. The main design objective of a high CMR
LED drive circuit becomes keeping the LED in the proper
state (on or off ) during common mode transients. For ex-
ample, the recommended application circuit (Figure 15),
can achieve 15 kV/µs CMR while minimizing component
complexity. Note that a CMOS gate is recommended in
Figure 15 to keep the LED off when the gate is in the high
state.
Another cause of CMR failure for a shielded optocoupler
is direct coupling to the optocoupler output pins through
C
the effect and magnitude of the direct coupling includ-
ing: the use of an internal or external output pull-up re-
sistor, the position of the LED current setting resistor, the
connection of the unused input package pins, and the
value of the capacitor at the optocoupler output (C
Techniques to keep the LED in the proper state and mini-
mize the effect of the direct coupling are discussed in the
next two sections.
CMR with the LED On (CMR
A high CMR LED drive circuit must keep the LED on dur-
ing common mode transients. This is achieved by over-
driving the LED current beyond the input threshold so
that it is not pulled below the threshold during a tran-
sient. The recommended minimum LED current of 10 mA
provides adequate margin over the maximum I
5.0 mA (see Figure 1) to achieve 15 kV/µs CMR. Capacitive
coupling is higher when the internal load resistor is used
(due to C
10 kV/µs CMR.
The placement of the LED current setting resistor effects
the ability of the drive circuit to keep the LED on dur-
ing transients and interacts with the direct coupling to
the optocoupler output. For example, the LED resistor in
Figure 18 is connected to the anode. Figure 19 shows the
AC equivalent circuit for Figure 18 during common mode
transients. During a +dVcm/dt in Figure 19, the current
available at the LED anode (Itotal) is limited by the series
resistor. The LED current (I
by an amount equal to the current that flows through
20
LEDO1
and C
LEDO2
LEDO2
) and an I
in Figure 17. Many factors influence
L
)
F
F
= 16 mA is required to obtain
) is reduced from its DC value
L
TH
).
of
C
the current through C
the output high (toward a CMR failure) at the same time
the LED current is being reduced. For this reason, the
recommended LED drive circuit (Figure 15) places the
current setting resistor in series with the LED cathode.
Figure 20 is the AC equivalent circuit for Figure 15 during
common mode transients. In this case, the LED current
is not reduced during a +dVcm/dt transient because the
current flowing through the package capacitance is sup-
plied by the power supply. During a -dVcm/dt transient,
however, the LED current is reduced by the amount of
current flowing through C
mance is achieved since the current flowing in C
during a negative transient acts to keep the output low.
Coupling to the LED and output pins is also affected by
the connection of pins 1 and 4. If CMR is limited by per-
turbations in the LED on current, as it is for the recom-
mended drive circuit (Figure 15), pins 1 and 4 should be
connected to the input circuit common. However, if CMR
performance is limited by direct coupling to the output
when the LED is off, pins 1 and 4 should be left uncon-
nected.
CMR with the LED Off (CMR
A high CMR LED drive circuit must keep the LED off
(V
ample, during a +dVcm/dt transient in Figure 20, the
current flowing through C
combination of the LED and series resistor. As long as the
voltage developed across the resistor is less than V
the LED will remain off and no common mode failure will
occur. Even if the LED momentarily turns on, the 100 pF
capacitor from pins 6-5 will keep the output from dip-
ping below the threshold. The recommended LED drive
circuit (Figure 15) provides about 10 V of margin between
the lowest optocoupler output voltage and a 3 V IPM
threshold during a 15 kV/µs transient with V
Additional margin can be obtained by adding a diode
in parallel with the resistor, as shown by the dashed line
connection in Figure 20, to clamp the voltage across the
LED below V
Since the open collector drive circuit, shown
in Figure 21, cannot keep the LED off during
a +dVcm/dt transient, it is not desirable for applications
requiring ultra high CMR
AC equivalent circuit for Figure 21 during common mode
transients. Essentially all the current flowing through
C
the LED. CMR
the current through the LED and C
threshold. Figure 23 is an alternative drive circuit which
does achieve ultra high CMR performance by shunting
the LED in the off state.
LEDP
LEDN
F
≤ V
and C
during a +dVcm/dt transient must be supplied by
F(OFF)
) during common mode transients. For ex-
LEDO1
F(OFF)
H
failures can occur at dV/dt rates where
. The situation is made worse because
.
LEDO1
H
)
H
LEDN
performance. Figure 22 is the
LEDN
has the effect of trying to pull
is supplied by the parallel
. But, better CMR perfor-
LEDN
exceeds the input
CM
= 1500 V.
LEDO1
F(OFF)