HCPL4701 datasheet(15/16 Pages) HP

15 / 16 page

Data Communication and

Input/Output Interfaces

In data communication, the

HCPL-47XX can be used as a line

receiver on a RS-232-C line or

this optocoupler can be part of a

proprietary data link with low

input current, multi-drop stations

along the data path. Also, this

low-power optocoupler can be

used within equipment that

monitors the presence of high-

voltage. For example, a benefit of

the low input LED current (40

µA) helps the input sections of a

Programmable Logic Controller

(PLC) monitor proximity and limit

switches. The PLC I/O sections

can benefit from low input

current optocouplers because the

total input power dissipation

when monitoring the high voltage

(120 Vac - 220 Vac) inputs is

minimized at the I/O connections.

This is especially important when

many input channels are stacked

together.

Circuit Design Issues

Power Supply Filtering

Since the HCPL-47XX is a high-

gain, split-Darlington amplifier,

any conducted electrical noise on

the VCC power supply to this

optocoupler should be minimized.

A recommended VCC filter circuit

is shown in Figure 12 to improve

the power supply rejection (psr)

of the optocoupler. The filter

should be located near the

combination of pin 8 and pin 5 to

provide best filtering action. This

filter will drastically limit any

sudden rate of change of VCC with

time to a slower rate that cannot

interfere with the optocoupler.

Common-Mode Rejection &

LED Driver Circuits

With the combination of a high-

efficiency AlGaAs LED and a

high-gain amplifier in the HCPL-

47XX optocoupler, a few circuit

techniques can enhance the

common-mode rejection (CMR) of

this optocoupler. First, use good

high-frequency circuit layout

practices to minimize coupling of

common-mode signals between

input and output circuits. Keep

input traces away from output

traces to minimize capacitive

coupling of interference between

input and output sections. If

possible, parallel, or shunt switch

the LED current as shown in

Figure 13, rather than series

switch the LED current as

illustrated in Figure 15. Not only

will CMR be enhanced with these

circuits (Figures 13 and 14), but

the switching speed of the opto-

coupler will be improved as well.

This is because in the parallel

switched case the LED current is

current-steered into or away from

the LED, rather than being fully

turned off as in the series switched

case. Figure 13 illustrates this

type of circuit. The Schottky

diode helps quickly to discharge

and pre-bias the LED in the off

state. If a common-mode voltage

across the optocoupler suddenly

attempts to inject a current into

the off LED anode, the Schottky

diode would divert the interfering

current to ground. The combina-

tion of the Schottky diode forward

voltage and the Vol saturation

voltage of the driver output stage

(on-condition) will keep the LED

voltage at or below 0.8 V. This will

prevent the LED (off-condition)

from conducting any significant

forward current that might cause

the HCPL-47XX to turn on. Also,

if the driver stage is an active

totem-pole output, the Schottky

diode allows the active output

pull-up section to disconnect from

the LED and pull high.

As shown in Figure 14, most

active output driver integrated

circuits can source directly the

forward current needed to operate

the LED of the HCPL-47XX

optocoupler. The advantage of

using the silicon diode in this

circuit is to conduct charge out of

the LED quickly when the LED is

turned off. Upon turn-on of the

LED, the silicon diode capaci-

tance will provide a rapid

charging path (peaking current)

for the LED. In addition, this

silicon diode prevents common-

mode current from entering the

LED anode when the driver IC is

on and no operating LED current

exists.

In general, series switching the low

input current of the HCPL-47XX

LED is not recommended. This is

particularly valid when in a high

common-mode interference

environment. However, if series

switching of the LED current must

be done, use an additional pull-up

resistor from the cathode of the

LED to the input VCC as shown in

Figure 15. This helps minimize any

differential-mode current from

conducting in the LED while the

LED is off, due to a common-mode

signal occurring on the input VCC

(anode) of the LED. The common-

mode signal coupling to the anode

and cathode could be slightly

different. This could potentially

create a LED current to flow that

would rival the normal, low input

current needed to operate the

optocoupler. This additional

parallel resistor can help shunt any

leakage current around the LED

should the drive circuit, in the off

state, have any significant leakage

current on the order of 40

µA.

With the use of this parallel

resistor, the total drive current

conducted when the LED is on is

the sum of the parallel resistor and

LED currents. In the series circuit

of Figure 15 with the LED off, if a

common-mode voltage were to

couple to the LED cathode, there

can be enough imbalance of

common-mode voltage across the

LED to cause a LED current to

flow and, inadvertently, turn on the

optocoupler. This series, switching

circuit has no protection against a

negative-transition, input common-

mode signal.

HCPL4701 Datasheet(PDF) 15 Page - Agilent(Hewlett-Packard)

HCPL4701 Datasheet(HTML) 15 Page - Agilent(Hewlett-Packard)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL