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Intelligent Power Module and Gate Drive Interface Hermetically Sealed Optocouplers

Avago Technologies 

LED Drive Circuit Considerations For Ultra High CMR

Performance

Without a detector shield, the dominant cause of opto-

coupler 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 opto-

coupler. 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/s CMR while minimizing component com-

plexity. 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 connec-

tion 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 overdriv-

ing 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/s 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/s 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 rec-

ommended 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 perturba-

tions 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 perfor-

mance is limited by direct coupling to the output when the

LED is off, pins 1 and 4 should be left unconnected.

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