ACPL-P456-500E データシートの表示（PDF） - Broadcom Corporation

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ACPL-P456-500E

High CMR Intelligent Power Module and Gate Drive Interface Optocoupler

Broadcom Corporation 

ACPL-P456 and ACPL-W456

Data Sheet

Figure 11 Propagation Delay vs. Input Current

500

VCC = 15V

CL = 100 pF

RL = 20 kΩ

tPLH

tPHL

400

TA = 25°C

Figure 13 Optocoupler Input to Output Capacitance Model for

Unshielded Optocouplers

1 CLEDP

6

2

5

300

3 CLEDN

4

200

100

0

5

10

15

20

IF – FORWARD LED CURRENT – mA

Applications Information

LED Drive Circuit Considerations For Ultra High

CMR Performance

Without a detector shield, the dominant cause of optocoupler

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 capacitive 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 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 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

+5V

1

2

3

CMOS

SHIELD

6

5

VOUT +

-

VCC = 15V

CL*

4

* 100 pF TOTAL

CAPACITANCE

Another cause of CMR failure for a shielded optocoupler is

direct coupling to the optocoupler output pins through

CLEDO1 in Figure 14. Many factors influence the effect and

magnitude of the direct coupling including: the position of the

LED current setting resistor and the value of the capacitor at

the optocoupler output (CL).

Figure 14 Optocoupler Input to Output Capacitance Model for

Shielded Optocouplers

1 CLEDP

2

6

CLED01

5

3 CLEDN

SHIELD

4

CMR With The LED On (CMRL)

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 ITH of 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 +dVCM/dt

in Figure 16, the current available at the LED anode (Itotal) is

limited by the series resistor. The LED current (IF) is reduced

from its DC value by an amount equal to the current that flows

through CLEDP and CLEDO1. The situation is made worse

because the current through CLEDO1 has the effect of 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

Broadcom

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