acpl-p456 Avago Technologies, acpl-p456 Datasheet - Page 11

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

Figure 16. AC Equivalent Circuit for Figure 15 during Common Mode Tran-
sients.
Figure 17. AC Equivalent Circuit for Figure 12 during Common Mode Tran-
sients.
CMR With The LED Off (CMR
A high CMR LED drive circuit must keep the LED off (V
≤ V
during a +dV
flowing through C
bination 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 5-4 will keep the output from dipping
below the threshold. The recommended LED drive circuit
(Figure 12) provides about 10 V of margin between
the lowest optocoupler output voltage and a 3 V IPM
threshold during a 15kV/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 17, to clamp the voltage across the
LED below V

F(OFF)
+ V
I
300 Ω
TOTAL
300 Ω
* THE ARROWS INDICATE THE DIRECTION OF CURRENT
FLOW FOR + dV
** OPTIONAL CLAMPING DIODE FOR IMPROVED CMH
PERFORMANCE. V
R
** -
*
-
) during common mode transients. For example,
1
2
3
* THE ARROWS INDICATE THE DIRECTION
1
2
3
OF CURRENT FLOW DURING + dV
F(OFF)
I
CLEDP
C
I
F
I
CM
LEDN
CLEDN
C
LEDP
CM
/dt transient in Figure 17, the current
* C
R
/dt TRANSIENTS.
.
< V
LEDN
LEDN
F (OFF)
SHIELD
SHIELD
I
CLED01
V
SHIELD
DURING + dV
is supplied by the parallel com-
CM
V
CM
H
C
)
LED01
C
LED01
CM
CM
/dt
/dt
6
5
4
6
5
4
V
OUT
V
OUT
100 pF
20 kΩ
CM
20 kΩ
100 pF
= 1500 V.
pF
F(OFF)
F
Since the open collector drive circuit, shown in Figure 18,
cannot keep the LED off during a +dV
not desirable for applications requiring ultra high CMR
performance. Figure 19 is the AC equivalent circuit for
Figure 18 during common mode transients. Essentially
all the current flowing through C
transient must be supplied by the LED. CMR
occur at dv/dt rates where the current through the LED
and C
ternative drive circuit which does achieve ultra high CMR
performance by shunting the LED in the off state.
Figure 18. Not Recommended Open Collector LED Drive Circuit.
Figure 19. AC Equivalent Circuit for Figure 18 during Common Mode Tran-
sients.
Figure 20. Recommended LED Drive Circuit for Ultra High CMR.
+5 V
Q1
+5 V
Q1
Q1
LEDN
300 Ω
* THE ARROWS INDICATE THE DIRECTION OF CURRENT
FLOW FOR + dV
exceeds the input threshold. Figure 20 is an al-
1
2
3
1
2
3
1
2
3
I
CLEDN
C
C
LEDP
LEDP
* C
CM
LEDN
/dt TRANSIENTS.
SHIELD
SHIELD
SHIELD
V
CM
C
LED01
LEDN
6
5
4
during a +dV
CM
6
5
4
6
5
4
/dt transient, it is
V
OUT
H
failures can
100 pF
20 kΩ
CM
/dt
H