HCPL788J Datasheet, PDF (12/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

12
Applications Information
Production Description
Figure 21 shows the internal
block diagram of the HCPL-788J.
The analog input (V_IN) is con-
verted to a digital signal using a
sigma-delta (∑-∆) analog to
digital (A/D) converter. This A/D
samples the input 6 million times
per second and generates a high
speed 1-bit output representing
the input very accurately. This 1
bit data stream is transmitted via
a light emitting diode (LED) over
the optical barrier after encoding.
The detector converts the optical
signal back to a bit stream. This
bit stream is decoded and drives a
1 bit digital to analog (D/A) con-
verter. Finally a low pass filter
and output buffer drive the output
signal (V_OUT) which linearly rep-
resents the analog input. The
output signal full-scale range is
determined by the external
reference voltage (V_REF). By
sharing this reference voltage
(which can be the supply
voltage), the full-scale range of
the HCPL-788J can precisely
match the full-scale range of
an external A/D converter.
In addition, the HCPL-788J
compares the analog input (V_IN)
to both the negative and positive
full-scale values. If the input
exceeds the full-scale range,
the short-circuit fault output
(FAULT) is activated quickly.
This feature operates indepen-
dently of the ∑-∆ A/D converter
in order to provide the high-
speed response (typically 3 µs)
needed to protect power tran-
sistors. The FAULT output is wire
OR-able so that a short circuit
on any one motor phase can be
detected using only one signal.
One other output is provided —
the rectified output (ABSVAL).
This output is also wire OR-able.
The motor phase having the
highest instantaneous rectified
output pulls the common output
high. When three sinusoidal
motor phases are combined, the
rectified output (ABSVAL) is
essentially a DC signal represent-
ing the rms motor current. This
single DC signal and a threshold
comparator can indicate motor
overload conditions before dam-
age to the motor or drive occur.
Figure 22 shows the ABSVAL
output when 3 HCPL-788Js are
used to monitor a sinusoidal
60 Hz current. Figures 23 and 24
show the ABSVAL output when
only 2 or 1 of the 3 phases are
monitored, respectively.
The HCPL-788J’s other main
function is to provide galvanic
isolation between the analog
input and the analog output.
An internal voltage reference
determines the full-scale analog
input range of the modulator
(approximately ± 256 mV);
an input range of ± 200 mV is
recommended to achieve optimal
performance.
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Figure 22. ABSVAL with 3 Phases,
Wired-ORed Together.
Figure 23. ABSVAL with 2 Phases,
Wired-ORed Together.
Figure 24. ABSVAL with 1 Phase.