Datasheet

16
line powered 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.
Figure 27. Example printed circuit board layout.
TOP LAYER
BOTTOM LAYER