HCPL-788J Datasheet, PDF(17/20 Page) AVAGO TECHNOLOGIES LIMITED – Isolation Amplifier with Short Circuit and Overload Detection SO-16 Package

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HCPL-788J Datasheet, PDF (17/20 Pages) AVAGO TECHNOLOGIES LIMITED – Isolation Amplifier with Short Circuit and Overload Detection SO-16 Package

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When laying out a PC board for the current sensing

resistors, a couple of points should be kept in mind.

The Kelvin connections to the resistor should be

brought together under the body of the resistor and

then run very close to each other to the input of the

HCPL-788J; this minimizes the loop area of the con-

nection and reduces the possibility of stray magnetic

fields from interfering with the measured signal. If

the sense resistor is not located on the same PC board as

the HCPL-788J circuit, a tightly twisted pair of wires can

accomplish the same thing.

Also, multiple layers of the PC board can be used to

increase current carrying capacity. Numerous plated-

through vias should surround each non-Kelvin terminal

of the sense resistor to help distribute the current be-

tween the layers of the PC board. The PC board should

use 2 or 4 oz. copper for the layers, resulting in a current

carrying capacity in excess of 20 A. Making the current

carrying traces on the PC board fairly large can also im-

prove the sense resistorâs power dissipation capability

by acting as a heat sink. Liberal use of vias where the

load current enters and exits the PC board is also recom-

mended.

Sense Resistor Connections

The recommended method for connecting the

HCPL-788J to the current sensing resistor is shown

in Figure 26. VIN+ (pin 1 of the HCPL-788J) is con-

nected to the positive terminal of the sense resis-

tor, while V (pin 2) is shorted to GND (pin 8), with

IN-

the power-supply return path functioning as the sense line

to the negative terminal of the current sense

resistor. This allows a single pair of wires or

PC board traces to connect the HCPL-788J circuit to the

sense resistor. By referencing the input circuit to the neg-

ative side of the sense resistor, any load current induced

noise transients on the resistor are seen as a common-

mode signal and will not interfere with the current-sense

signal. This is important because the large load currents

flowing through the motor drive, along with the para-

sitic inductances inherent in the wiring of the circuit, can

generate both noise spikes and offsets that are relatively

large compared to the small voltages that are being

measured across the current sensing resistor.

If the same power supply is used both for the gate drive circuit

and for the current sensing circuit, it is very important

that the connection from GND of the HCPL-788J to the

sense resistor be the only return path for supply current

to the gate drive power supply in order to eliminate po-

tential ground loop problems. The only direct connec-

tion between the HCPL-788J circuit and the gate drive

circuit should be the positive power supply line. Please

refer to Avago Technologiesâ Applications Note 1078 for

additional information on using Isolation Amplifiers.

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