Analog Interfacing
drive circuit. If a dedicated
supply is required, in many cases
it is possible to add an additional
winding on an existing trans-
former. Otherwise, some sort of
simple isolated supply can be
used, such as a line powered
transformer or a high-frequency
DC-DC converter.
capacitor (C2) is also recom-
mended 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.
Power Supplies and
Bypassing
The recommended application
circuit is shown in Figure 21. A
floating power supply (which in
many applications could be the
same supply that is used to drive
the high-side power transistor) is
regulated to 5 V using a simple
zener diode (D1); the value of
resistor R1 should be chosen to
supply sufficient current from the
existing floating supply. The
voltage from the current sensing
resistor or shunt (Rsense) is
applied to the input of the HCPL-
7860 (U2) through an RC anti-
aliasing filter (R2 and C2). And
finally, the output clock and data
of the isolated modulator are
connected to the digital interface
IC. Although the application
circuit is relatively simple, a few
recommendations should be
followed to ensure optimal
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
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
(CMR) of the isolated modulator,
due primarily to stray capacitive
coupling between the input and
the output circuits. To obtain
optimal CMR performance, the
layout of the PC board should
minimize any stray coupling by
maintaining the maximum
capacitor.
As shown in Figure 21, 0.1 µF
bypass capacitors (C1 and C3)
should be located as close as
possible to the input and output
power-supply pins of the isolated
modulator (U2). The bypass
capacitors are required because
of the high-speed digital nature of
the signals inside the isolated
modulator. A 0.01 µF bypass
performance.
The power supply for the isolated
modulator is most often obtained
from the same supply used to
power the power transistor gate
possible distance between the
input and output sides of the
circuit and ensuring
FLOATING
POSITIVE
SUPPLY
+ 5 V
HV+
GATE DRIVE
CIRCUIT
R1
CCLK
CLAT
V
DD
C1
D1
5.1 V
CHAN
SCLK
SDAT
CS
0.1 µF
V
V
V
V
CDAT
MCLK1
MDAT1
MCLK2
MDAT2
GND
DD1
IN+
IN-
DD2
R2 39 Ω
MCLK
MDAT
MOTOR
+
-
THR1
OVR1
RESET
C3
0.1 µF
GND1 GND2
HCPL-7860
C2
0.01 µF
R
SENSE
TO
CONTROL
CIRCUIT
HCPL-X870
HV-
Figure 21. Recommended Application Circuit.
1-284
HP [ HEWLETT-PACKARD ]
