HCPL-J312 datasheet(21/24 Pages) AVAGO

21 / 24 page

TJE = LED junction temperature

TJD = detector IC junction temperature

TC = case temperature measured at the center of the package bottom

qLC = LED-to-case thermal resistance

qLD = LED-to-detector thermal resistance

qDC = detector-to-case thermal resistance

qCA = case-to-ambient thermal resistance

qCA will depend on the board design and the placement of the part.

Figure 28. Thermal model.

HCPL-3120 fig 28

θLD = 442 °C/W

TJE

TJD

θLC = 467 °C/W

θDC = 126 °C/W

θCA = 83 °C/W*

TJE = LED JUNCTION TEMPERATURE

TJD = DETECTOR IC JUNCTION TEMPERATURE

TC = CASE TEMPERATURE MEASURED AT THE

CENTER OF THE PACKAGE BOTTOM

θLC = LED-TO-CASE THERMAL RESISTANCE

θLD = LED-TO-DETECTOR THERMAL RESISTANCE

θDC = DETECTOR-TO-CASE THERMAL RESISTANCE

θCA = CASE-TO-AMBIENT THERMAL RESISTANCE

θCA 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

CLEDP

CLEDN

HCPL-3120 fig 30

CLEDP

CLEDN

SHIELD

CLEDO1

CLEDO2

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 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.

HCPL-J312 Datasheet(PDF) 21 Page - AVAGO TECHNOLOGIES LIMITED

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