HCPL788J Datasheet, PDF (14/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

14
transformer or a high-frequency
DC-DC converter.
An inexpensive 78L05 three-
terminal regulator can also be
used to reduce the floating 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 25, 0.1 µF
bypass capacitors (C1, C3, C4,
and C6) should be located as
close as possible to the pins of
the HCPL-788J. The bypass
capacitors are required because
of the high-speed digital nature of
the signals inside the HCPL-788J.
A 0.01 µF bypass capacitor (C2)
is also recommended at the input
due to the switched-capacitor
nature of the input circuit. The
input bypass capacitor also
forms part of the anti-aliasing
filter, which is recommended to
prevent high-frequency noise
from aliasing down to lower
frequencies and interfering with
the input signal. The input filter
also performs an important
reliability function — it reduces
transient spikes from ESD events
flowing through the current
sensing resistor.
PC Board Layout
The design of the printed circuit
board (PCB) should follow good
layout practices, such as keeping
bypass capacitors close to the
supply pins, keeping output
signals away from input signals,
the use of ground and power
planes, etc. In addition, the layout
of the PCB can also affect the
isolation transient immunity
(CMTI) of the HCPL-788J, due
primarily to stray capacitive
coupling between the input and
the output circuits. To obtain
optimal CMTI performance, the
layout of the PC board should
minimize any stray coupling
by maintaining the maximum
possible distance between the
input and output sides of the
circuit and ensuring that any
ground or power plane on the
PC board does not pass directly
below or extend much wider than
the body of the HCPL-788J.
TOP LAYER
BOTTOM LAYER
Figure 27. Example Printed Circuit Board Layout.