HCPL-788J Datasheet, PDF (13/20 Pages) AVAGO TECHNOLOGIES LIMITED – Isolation Amplifier with Short Circuit and Overload Detection SO-16 Package

Applications Information
Production Description
Figure 21 shows the internal block diagram of the HCPL-
788J. The analog input (V ) is converted to a digital signal
IN
using a sigma-delta (∑-Δ) analog to digital (A/D) convert-
er. 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 trans-
mitted 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) converter. Finally
a low pass filter and output buffer drive the output signal
(V ) which linearly represents the analog input. The out-
OUT
put signal full-scale range is determined by the external
reference voltage (V ). By sharing this reference voltage
REF
(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 ) to both the negative and positive full-scale values.
IN
If the input exceeds the full-scale range, the short-circuit
fault output (FAULT) is activated quickly. This feature op-
erates independently of the ∑-Δ A/D converter in order
to provide the high-speed response (typically 3 μs) need-
ed to protect power transistors. 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 out-
put pulls the common output high. When three sinu-
soidal motor phases are combined, the rectified output
(ABSVAL) is essentially a DC signal representing the rms
motor current. This single DC signal and a threshold
comparator can indicate motor overload conditions be-
fore damage to the motor or drive occur. Figure 22 shows
the ABSVAL output when 3 HCPL-788Js are used to mon-
itor 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 gal-
vanic isolation between the analog input and the ana-
log output. An internal voltage reference determines the
full-scale analog input range of the modulator (approxi-
mately ±256 mV); an input range of ±200 mV is recom-
mended to achieve optimal performance.
4.0
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TIME – SECONDS
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TIME – SECONDS
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TIME – SECONDS
Figure 22. ABSVAL with 3 phases, wired-ORed
together.
Figure 23. ABSVAL with 2 phases, wired-ORed
together.
Figure 24. ABSVAL with 1 phase.
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