acpl-p456 Avago Technologies, acpl-p456 Datasheet - Page 10

• Manufacturer Part Number: acpl-p456
• Description: Intelligent Power Module And Gate Drive Interface Optocouplers
• Manufacturer: Avago Technologies

Figure 11. Propagation Delay vs. Input Current.
Applications Information
LED Drive Circuit Considerations For Ultra High CMR Per-
formance
Without a detector shield, the dominant cause of opto-
coupler CMR failure is capacitive coupling from the input
side of the optocoupler, through the package, to the
detector IC as shown in Figure 13. The ACPL-P456/W456
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 eliminate the ca-
pacitive coupling between the LED and the optocoupler
output pin and output ground as shown in Figure 14.
This capacitive coupling causes perturbations in the LED
current during common mode transients and becomes
the major source of CMR failures for a shielded optocou-
pler. 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 example, the
recommended application circuit (Figure 12), can achieve
15 kV/µs CMR while minimizing component complexity.
Note that a CMOS gate is recommended in Figure 12 to
keep the LED off when the gate is in the high state.
Figure 12. Recommended LED Drive Circuit.
Figure 13. Optocoupler Input to Output Capacitance Model for Unshielded
Optocouplers.
0
500
400
300
200
100
CMOS
+5 V
0
310 Ω
1
2
3
V
C
T
R
CC
L
A
L
= 100 pF
= 25 C
1
2
3
= 20 kΩ
= 15 V
C
C
LEDP
LEDN
I
5
F
– FORWARD LED CURRENT – mA
SHIELD
10
0.1 µF
6
5
4
6
5
4
* 100 pF TOTAL
CAPACITANCE
15
t
t
PLH
PHL
20 kΩ
C
L
*
V
OUT
20
+
+
-
-
V
CC
= 15 V
Another cause of CMR failure for a shielded optocoupler
is direct coupling to the optocoupler output pins through
C
magnitude of the direct coupling including: the position
of the LED current setting resistor and the value of the
capacitor at the optocoupler output (C
Figure 14. Optocoupler Input to Output Capacitance Model for Shielded
Optocouplers.
CMR With The LED On (CMR
A high CMR LED drive circuit must keep the LED on
during common mode transients. This is achieved by
overdriving the LED current beyond the input threshold
so that it is not pulled below the threshold during a
transient. The recommended minimum LED current of 10
mA provides adequate margin over the maximum I
4.0 mA (see Figure 1) to achieve 15 kV/µs CMR.
The placement of the LED current setting resistor effects
the ability of the drive circuit to keep the LED on during
transients and interacts with the direct coupling to the
optocoupler output. For example, the LED resistor in
Figure 15 is connected to the anode. Figure 16 shows
the AC equivalent circuit for Figure 15 during common
mode transients. During a +dV
current available at the LED anode (Itotal) is limited by
the series resistor. The LED current (I
DC value by an amount equal to the current that flows
through C
because the current through C
trying to pull 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 12)
places the current setting resistor in series with the LED
cathode. Figure 17 is the AC equivalent circuit for Figure
12 during common mode transients. In this case, the
LED current is not reduced during a +dV
because the current flowing through the package capaci-
tance is supplied by the power supply. During a -dV
dt transient, however, the LED current is reduced by the
amount of current flowing through C
CMR performance is achieved since the current flowing
in C
output low.
Figure 15. LED Drive Circuit with Resistor Connected to LED Anode (Not
Recommended).
LEDO1
+5 V
+5 V
LEDO1
1
2
3
CMOS
in Figure 14. Many factors influence the effect and
310 Ω
C
C
LEDP
LEDN
during a negative transient acts to keep the
LEDP
1
2
3
and C
SHIELD
LEDO1
C
SHIELD
LED01
L
)
. The situation is made worse
6
5
4
CM
LEDO1
0.1 µF
/dt in Figure 16, the
* 100 pF TOTAL
CAPACITANCE
6
5
4
F
) is reduced from its
has the effect of
L
).
LEDN
CM
20 kΩ
C
. But, better
L
/dt transient
*
V
OUT
+
-
V
CC
TH
= 15 V
CM
of
/