HCPL-0466 datasheet(20/21 Pages) AVAGO

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LED Drive Circuit Considerations for Ultra High CMR Performance

Without a detector shield, the dominant cause of op-

tocoupler CMR failure is capacitive coupl-ing from the

input side of the optocoupler, through the package, to

the detector IC as shown in Figure 16. The HCPL-4506

series 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 elimi-

nate the capacitive coupling between the LED and the

optocoupler output pins and output ground as shown in

Figure 17. 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 oroff) during common mode transients. For ex-

ample, the recommended application circuit (Figure 15),

can achieve 15 kV/µs CMR while minimizing component

complexity. Note that a CMOS gate is recommended in

Figure 15 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 17. Many factors influence

the effect and magnitude of the direct coupling includ-

ing: the use of an internal or external output pull-up re-

sistor, 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 mini-

mize 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 dur-

ing common mode transients. This is achieved by over-

driving the LED current beyond the input threshold so

that it is not pulled below the threshold during a tran-

sient. The recommended minimum LED current of 10 mA

provides adequate margin over the maximum ITH of

5.0 mA (see Figure 1) to achieve 15 kV/µs CMR. Capacitive

coupling is higher when the internal load resistor is used

(due to CLEDO2) and an IF = 16mA is required to obtain

10 kV/µs CMR.

The placement of the LED current setting resistor effects

the ability of the drive circuit to keep the LED on dur-

ing transients and interacts with the direct coupling to

the optocoupler output. For example, the LED resistor in

Figure 18 is connected to the anode. Figure 19 shows the

AC equivalent circuit for Figure 18 during common mode

transients. During a +dVcm/dt in Figure 19, 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 15) places the

current setting resistor in series with the LED cathode.

Figure 20 is the AC equivalent circuit for Figure 15 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 sup-

plied 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 perfor-

mance 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 per-

turbations in the LED on current, as it is for the recom-

mended drive circuit (Figure 15), 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 uncon-

nected.

CMR with the LED Off (CMRH)

A high CMR LED drive circuit must keep the LED off

(VF ≤VF(OFF)) during common mode transients. For ex-

ample, during a +dVcm/dt transient in Figure 20, the

current flowing through CLEDN is supplied by the parallel

combination of the LED and series resistor. As long as the

voltage developed across the resistor is less than VF(OFF)

the LED will remain off and no common mode failure will

occur. Even if the LED momentarily turns on, the 100 pF

capacitor from pins 6-5 will keep the output from dip-

ping below the threshold. The recommended LED drive

circuit (Figure 15) provides about 10V of margin between

the lowest optocoupler output voltage and a 3V IPM

threshold during a 15 kV/µs transient with VCM=1500V.

Additional margin can be obtained by adding a diode

in parallel with the resistor, as shown by the dashed line

connection in Figure 20, to clamp the voltage across the

LED below VF(OFF).

Since the open collector drive circuit, shown

in Figure 21, cannot keep the LED off during

a +dVcm/dt transient, it is not desirable for applications

requiring ultra high CMRH performance. Figure 22 is the

AC equivalent circuit for Figure 21 during common mode

transients. Essentially all the current flowing through

CLEDN during a +dVcm/dt transient must be supplied by

the LED. CMRH failures can occur at dV/dt rates where

the current through the LED and CLEDN exceeds the input

threshold. Figure 23 is an alternative drive circuit which

does achieve ultra high CMR performance by shunting

the LED in the off state.

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