Datasheet

21
T
JE
= LED junction temperature
T
JD
= detector IC junction temperature
T
C
= case temperature measured at the center of the package bottom
q
LC
= LED-to-case thermal resistance
q
LD
= LED-to-detector thermal resistance
q
DC
= detector-to-case thermal resistance
q
CA
= case-to-ambient thermal resistance
*q
CA
will depend on the board design and the placement of the part.
Figure 28. Thermal model.
HCPL-3120 fig 28
θ
LD
= 442 °C/W
T
JE
T
JD
θ
LC
= 467 °C/W θ
DC
= 126 °C/W
θ
CA
= 83 °C/W*
T
C
T
A
T
JE
= LED JUNCTION TEMPERATURE
T
JD
= DETECTOR IC JUNCTION TEMPERATURE
T
C
= CASE TEMPERATURE MEASURED AT THE
CENTER OF THE PACKAGE BOTTOM
θ
LC
= LED-TO-CASE THERMAL RESISTANCE
θ
LD
= LED-TO-DETECTOR THERMAL RESISTANCE
θ
DC
= DETECTOR-TO-CASE THERMAL RESISTANCE
θ
CA
= CASE-TO-AMBIENT THERMAL RESISTANCE
* θ
CA
WILL DEPEND ON THE BOARD DESIGN AND
THE PLACEMENT OF THE PART.
LED Drive Circuit Considerations for Ultra High CMR Per-
formance. (Discussion applies to HCPL-3120, HCPL-J312,
and HCNW3120)
Without a detector shield, the dominant cause of op-
tocoupler CMR failure is capacitive coupling from the
input side of the optocoupler, through the package, to
the detector IC as shown in Figure 29. The HCPL-3120
improves CMR perform-ance 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 30. This capacitive coupling causes
Figure 29. Optocoupler input to output capacitance model for unshielded
optocouplers.
Figure 30. Optocoupler input to output capacitance model for shielded
optocouplers.
HCPL-3120 fig 29
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
HCPL-3120 fig 30
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
SHIELD
C
LEDO1
C
LEDO2
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 o) during common mode
transients. For example, the recommended application
circuit (Figure 25), can achieve 25 kV/µs CMR while mini-
mizing component complexity.
Techniques to keep the LED in the proper state are
discussed in the next two sections.