1-61

Figure 30. Waveforms for Dead Time Calculation.

Figure 29. Minimum LED Skew for Zero Dead Time.

VOUT1

VOUT2

ILED2

tPLH MAX.

PDD* MAX. =

(tPLH-tPHL) MAX. = tPLH MAX. - tPHL MIN.

tPHL

MIN.

ILED1

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

*PDD = PROPAGATION DELAY DIFFERENCE

NOTE: THE PROPAGATION DELAYS USED TO CALCULATE

PDD ARE TAKEN AT EQUAL TEMPERATURES.

VOUT1

VOUT2

ILED2

tPLH

MIN.

= PDD* MAX. - PDD* MIN.

tPHL

MIN.

ILED1

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

*PDD = PROPAGATION DELAY DIFFERENCE

tPLH

MAX.

tPHL

MAX.

PDD*

MAX.

MAX.

DEAD TIME

NOTE: THE PROPAGATION DELAYS USED TO CALCULATE THE MAXIMUM

DEAD TIME ARE TAKEN AT EQUAL TEMPERATURES.

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 opto-

coupler, through the package, to

the detector IC as shown in

Figure 20. The HCPL-4506,

HCPL-0466 and HCNW4506

improve CMR performance by

using a detector IC with an optic-

ally 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 21. 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 keep-

ing the LED in the proper state

(on or off) during common mode

transients. For example, the

recommended application circuit

(Figure 19), can achieve 15 kV/

µs

CMR while minimizing component

complexity. Note that a CMOS

gate is recommended in Figure 19

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

influence the effect and magni-

tude of the direct coupling includ-

ing: 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

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

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

Figure 5) to achieve 15 kV/

µs

CMR. Capacitive coupling is

higher when the internal load

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

during transients and interacts with

the direct coupling to the

optocoupler output. For example,

the LED resistor in Figure 22 is

connected to the anode. Figure 23

shows the AC equivalent circuit for

Figure 22 during common mode

transients. During a +dVcm/dt in

Figure 23, the current available at

the LED anode (Itotal) is limited by

the series resistor. The LED current

an amount equal to the current that

The situation is made worse