HCPL7820 datasheet(13/15 Pages) HP

13 / 15 page

1-245

the HCPL-7820/7825 through an

RC anti-aliasing filter (R5, C3).

And finally, the differential output

of the isolation amplifier is con-

verted to a ground-referenced

single-ended output voltage with a

simple differential amplifier

circuit (U3 and associated

components). Although the

application circuit is relatively

simple, a few recommendations

should be followed to ensure

optimal performance.

Supplies and Bypassing

As mentioned above, an inexpen-

sive 78L05 three-terminal regula-

tor can be used to reduce the

gate-drive power supply voltage

to 5 V. To help attenuate high-

frequency power supply noise or

ripple, a resistor or inductor can

be used in series with the input of

the regulator to form a low-pass

filter with the regulator’s input

bypass capacitor.

As shown in Figure 24, 0.1

µF

bypass capacitors (C2, C4) should

be located as close as possible to

the input and output power supply

pins of the HCPL-7820/7825. The

bypass capacitors are required

because of the high-speed digital

nature of the signals inside the

isolation amplifier. A 0.01

µF

bypass capacitor (C3) is also rec-

ommended at the input pin(s) due

to the switched-capacitor nature

of the input circuit. The input

bypass capacitor should be at

least 1000 pF to maintain gain

accuracy of the isolation amplifier.

Inductive coupling between the

input power-supply bypass

capacitor and the input circuit,

which includes the input bypass

capacitor and the input leads of

the HCPL-7820/7825, can

introduce additional DC offset in

the circuit. Several steps can be

taken to minimize the mutual

coupling between the two parts of

the circuit, thereby improving the

offset performance of the design.

Separate the two bypass capaci-

tors C2 and C3 as much as

possible (even putting them on

opposite sides of the PC board),

while keeping the total lead

lengths, including traces, of each

bypass capacitor less than 20

mm. PC board traces should be

made as short as possible and

placed close together or over

ground plane to minimize loop

area and pickup of stray magnetic

fields. Avoid using sockets, as

they will typically increase both

loop area and inductance. And

finally, using capacitors with

small body size and orienting

them perpendicular to each other

on the PC board can also help.

For more information concerning

inductive coupling, see the

Application Note Designing with

Hewlett-Packard Isolation

Amplifiers.

Shunt Resistor Selection

The current-sensing shunt resistor

should have low resistance (to

minimize power dissipation), low

inductance (to minimize di/dt

induced voltage spikes which

could adversely affect operation),

and reasonable tolerance (to

maintain overall circuit accuracy).

The value of the shunt should be

chosen as a compromise between

minimizing power dissipation by

making the shunt resistance

smaller and improving circuit

accuracy by making it larger and

using more of the input range of

the HCPL-7820/7825. Hewlett-

Packard recommends 4 different

shunts which can be used to sense

average currents in motor drives

up to 35 A and 35 hp. Table 1

shows the maximum current and

horsepower range for each of the

LVR-series shunts from Dale.

Even higher currents can be

sensed with lower value shunts

available from vendors such as

Dale, IRC, and Isotek (Isabellen-

huette). When sensing currents

large enough to cause significant

heating of the shunt, the tempera-

ture coefficient of the shunt can

introduce nonlinearity due to the

amplitude dependent temperature

rise of the shunt. Using a heat

sink for the shunt or using a shunt

with a lower tempco can help

minimize this effect. The

Application Note Designing with

Hewlett-Packard Isolation

Amplifiers

contains additional

information on designing with

current shunts.

The recommended method for

connecting the isolation amplifier

to the shunt resistor is shown in

Table 1. Current Shunt Summary

Shunt Resistor

Shunt

Maximum

Part Number

Resistance

Power Dissipation

RMS Current

Horsepower Range

LVR-3.05-1%

50 m

Ω

3 W

3 A

0.8-3.0 hp

LVR-3.02-1%

20 m

Ω

3 W

8 A

2.2-8.0 hp

LVR-3.01-1%

10 m

Ω

3 W

15 A

4.1-15 hp

LVR-5.005-1%

5 m

Ω

5 W

35 A

9.6-35 hp

HCPL7820 Datasheet(PDF) 13 Page - Agilent(Hewlett-Packard)

HCPL7820 Datasheet(HTML) 13 Page - Agilent(Hewlett-Packard)

Html Pages

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

Datasheet Download

Link URL