HCPL-0302-500E View Datasheet(PDF) - Avago Technologies
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HCPL-0302-500E Datasheet PDF : 15 Pages
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LED Drive Circuit Considerations for Ultra High CMR
Performance
Without a detector shield, the dominant cause of optocou-
pler CMR failure is capacitive coupling from the input side
of the optocoupler, through the package, to the detector
IC as shown in Figure 19. The HCPL-3020 and HCPL-0302
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 optocoupler pins
5-8 as shown in Figure 20. 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 tran-
sients. For example, the recommended application circuit
(Figure 17), can achieve 10 kV/µs CMR while minimizing
component complexity.
Techniques to keep the LED in the proper state are dis-
cussed in the next two sections.
1
8
CLEDP
2
7
3
6
CLEDN
4
5
Figure 19. Optocoupler input to output capacitance model for
unshielded optocouplers.
1
CLEDO1
8
CLEDP
2
3
CLEDN
7
CLEDO2
6
4
5
SHIELD
Figure 20. Optocoupler Input to output capacitance model for
shielded optocouplers.
+5 V
+
VSAT
–
1
CLEDP
2
ILEDP
3
CLEDN
4
SHIELD
8
0.1
7
µF
+
–
VCC = 18 V
6
•••
Rg
5
•••
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING –dVCM/dt.
+–
VCM
Figure 21. Equivalent circuit for figure 15 during common mode
transient.
1
8
+5 V
CLEDP
2
7
3
6
Q1
CLEDN
ILEDN
4
5
SHIELD
Figure 22. Not recommended open collector drive circuit.
1
8
+5 V
CLEDP
2
7
3
6
CLEDN
4
5
SHIELD
Figure 23. Recommended LED drive circuit for ultra-high CMR IPM
dead time and propagation delay specifications.
13
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