Power Supplies and Bypassing
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.
The recommended application circuit is shown in Figure
17. 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 sup-
ply. The voltage from the current sensing resistor or shunt
(Rsense) is applied to the input of the HCPL-7860/HCPL-
786J (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 recommenda-
tions should be followed to ensure optimal performance.
As shown in Figure 17, 0.1 ꢄF 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 ꢄF bypass 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.
The power supply for the isolated modulator is most
often obtained from the same supply used to power the
power transistor gate drive circuit. If a dedicated supply is
required, in many 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 pow-
ered transformer or a high-frequency DC-DC converter.
FLOATING
POSITIVE
SUPPLY
+ 5 V
HV+
GATE DRIVE
CIRCUIT
R1
CCLK
VDD
CHAN
SCLK
SDAT
CS
C1
D1
5.1 V
CLAT
0.1 μF
VDD1
VIN+
VIN-
VDD2
MCLK
MDAT
CDAT
R2 39 Ω
MCLK1
MDAT1
MCLK2
MDAT2
GND
MOTOR
+
-
THR1
OVR1
RESET
C3
0.1 μF
GND1
GND2
C2
0.01 μF
RSENSE
HCPL-7860ꢀ
HCPL-786J
TO
CONTROL
CIRCUIT
HCPL-0872
HV-
Figure 17. Recommended Application Circuit.
14
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
