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HCPL-788J

Isolation Amplifier with Short Circuit and Overload Detection

HP => Agilent Technologies 

2

Description

The HCPL-788J isolation amplifier

is designed for current sensing in

electronic motor drives. In a

typical implementation, motor

currents flow through an external

resistor and the resulting analog

voltage drop is sensed by the

HCPL-788J. A larger analog

output voltage is created on the

other side of the HCPL-788J’s

optical isolation barrier. The

output voltage is proportional

to the motor current and can be

connected directly to a single-

supply A/D converter. A digital

over-range output (FAULT) and

an analog rectified output

(ABSVAL) are also provided. The

wire OR-able over-range output

(FAULT) is useful for quick

detection of short circuit con-

ISOLATION BOUNDARY

ditions on any of the motor

phases. The wire-OR-able rectified

output (ABSVAL), simplifies

measurement of motor load since

it performs polyphase rectification.

Since the common-mode voltage

swings several hundred volts in

tens of nanoseconds in modern

electronic motor drives, the

HCPL-788J was designed to

ignore very high common-mode

transient slew rates (10 kV/µs).

INPUT

+

CURRENT RSHUNT

0.02 Ω

39 Ω

.01 µF

0.1 µF

ISOLATED +5 V

0.1 µF

1 VIN+

HCPL-788J

GND2 16

2 VIN-

VDD2 15

3 CH

FAULT 14

4 CL

ABSVAL 13

5 VDD1

VOUT 12

6 VLED+

VREF 11

7 VDD1

VDD2 10

8 GND1

GND2 9

µC

0.1 µF

4.7 kΩ

TO OTHER

PHASE

OUTPUTS

A/D

VREF

GND

Figure 1. Current Sensing Circuit.

Pin Descriptions

Symbol

Description

VIN+ Positive input voltage (± 200 mV

recommended).

VIN- Negative input voltage (normally

connected to GND1).

CH

CL

VDD1

Internal Bias Node. Connections to or

between CH and CL other than the re-

quired 0.1 µF capacitor shown, are not

recommended.

Supply voltage input (4.5 V to 5.5 V).

VLED+

LED anode. This pin must be left uncon-

nected for guaranteed data sheet perfor-

mance. (For optical coupling testing only.)

VDD1 Supply voltage input (4.5 V to 5.5 V).

GND1 Ground input.

+5 V

Symbol

Description

GND2

VDD2

FAULT

Ground input.

Supply voltage input (4.5 V to 5.5 V).

Short circuit fault output. FAULT changes

from a high to low output voltage within

6 µs after VIN exceeds the FAULT Detec-

tion Threshold. FAULT is an open drain

output which allows outputs from all the

HCPL-788Js in a circuit to be connected

together (“wired-OR”) forming a single

fault signal for interfacing directly to the

micro-controller.

ABSVAL

Absolute value of VOUT output. ABSVAL

is 0 V when VIN=0 and increases toward

VREF as VIN approaches +256 mV or

-256 mV. ABSVAL is “wired-OR” able

and is used for detecting overloads.

VOUT

VREF

Voltage output. Swings from 0 to VREF.

The nominal gain is VREF /504 mV.

Reference voltage input (4.0 V to VDD2).

This voltage establishes the full scale

output ranges and gains of VOUT and

ABSVAL.

VDD2 Supply voltage input (4.5 V to 5.5 V).

GND2 Ground input.

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