13
Figure 22. Recommended Power Supply and Bypassing
Power Supplies and Bypassing
A power supply of 5 V is required to power the ACPL-C87x
input side VDD1. In many motor drive DC bus voltage
sensing applications, this 5 V supply is most often obtained
from the same supply used to power the power transistor
gate drive circuit using an inexpensive 78L05 three-ter-
minal regulator. 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
lter with the regulator’s input bypass capacitor.
In some other applications a dedicated supply might be
required to supply the VDD1. These applications include
photovoltaic (PV) inverter voltage tracking and measure-
ment, temperature sensor signal isolation. In these cases
it is possible to add an additional winding on an existing
transformer. Otherwise, some sort of simple isolated
supply can be used, such as a line powered transformer or
a high-frequency DC-DC converter module.
As shown in Figure 22, 100 nF bypass capacitors (C2, C3)
should be located as close as possible to the pins of the
isolation ampli er. The bypass capacitors are required
because of the high-speed digital nature of the signals
inside the isolation ampli er. A 100 pF bypass capacitor
(Cin) is also recommended at the input pins due to the
switched-capacitor nature of the input circuit. The input
bypass capacitor Cin also forms part of the anti-aliasing
lter, which is recommended to prevent high-frequency
noise from aliasing down to lower frequencies and inter-
fering with the input signal. When R1 is far greater than R2,
the low-pass anti-aliasing lter corner frequency can be
calculated by 1/(2πR2Cin). The input lter also performs
an important reliability function – it reduces transient
spikes from ESD events owing through the high voltage
rails.
V
DD1
SHDN
Gate Drive
Circuit
78L05
ACPL-C87A
Cin
0.1nF
C1
0.1μF
C2
0.1μF
GND1
V
IN
IN
OUT
Floating
Positive Supply
V
OUT+
5V
V
DD2
V
OUT-
GND2
C3
0.1μF
R1
R2
HV+
HV-
