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HCPL-530K

Intelligent Power Module and Gate Drive Interface Hermetically Sealed Optocouplers

Broadcom Corporation 

HCPL-5300, HCPL-5301, HCPL-530K, 5962-96852

Data Sheet

LED Drive Circuit Considerations for Ultra High CMR Performance

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 14. The HCPL-530x improves 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 pins and output ground as shown in Figure 15. 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 13), can achieve 10 kV/μs CMR while minimizing

component complexity. Note that a CMOS gate is

recommended in Figure 13 to keep the LED off when the gate

is in the high state.

Another cause of CMR failure for a shielded optocoupler is

direct coupling to the optocoupler output pins through CLEDO1

and CLEDO2 in Figure 15. Many factors influence the effect and

magnitude of the direct coupling including the use of an

internal or external output pull-up resistor, the position of the

LED current setting resistor, the connection of the unused

input package pins, and the value of the capacitor at the

optocoupler output (CL).

Techniques to keep the LED in the proper state and minimize

the effect of the direct coupling are discussed in the next two

sections.

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 5.0 mA (see Figure 1) to achieve

10 kV/μs CMR. Capacitive coupling is higher when the internal

load resistor is used (due to CLEDO2) and an IF = 16 mA is

required to obtain 10 kV/μs CMR.

The placement of the LED current setting resistor affects 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 16 is connected

to the anode. Figure 17 shows the AC equivalent circuit for

Figure 16 during common mode transients. During a +dVCM/dt

in Figure 17, 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 13) places the current

setting resistor in series with the LED cathode. Figure 18 is the

AC equivalent circuit for Figure 13 during common mode

transients. In this case, the LED current is not reduced during a

+dVCM/dt transient because the current flowing through the

package capacitance is supplied by the power supply. During a

dVCM/dt transient, however, the LED current is reduced by the

amount of current flowing through CLEDN. But better CMR

performance is achieved since the current flowing in CLEDO1

during a negative transient acts to keep the output low.

Coupling to the LED and output pins is also affected by the

connection of pins 1 and 4. If CMR is limited by perturbations in

the LED on current, as it is for the recommended drive circuit

(Figure 13), pins 1 and 4 should be connected to the input

circuit common. However, if CMR performance is limited by

direct coupling to the output when the LED is off, pins 1 and 4

should be left unconnected.

Broadcom

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