Datasheet

14
Application Information
Power Supplies and Bypassing
The recommended supply con-nections are shown
in Figure 17. A oating 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
R4 should be chosen to supply sucient current from
the existing oating supply. The voltage from the current
sensing resistor (Rsense) is applied to the input of the
HCPL-7840 through an RC anti-aliasing lter (R2 and C2).
Although the application circuit is relatively simple, a few
recommendations should be followed to ensure optimal
performance.
The power supply for the HCPL -7840 is most often ob-
tained from the same supply used to power the power
transistor gate drive circuit. If a dedicated supply is re-
quired, 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 powered transformer or a high-frequency DC-DC
converter.
An inexpensive 78L05 three-terminal regulator can also
be used to reduce the oating 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 lter with
the regulator’s input bypass capacitor.
Figure 17. Recommended supply and sense resistor connections.
HCPL-7840 fig 17
HCPL-7840
C1
0.1 µF
R2
39 Ω
GATE DRIVE
CIRCUIT
FLOATING
POWER
SUPPLY
• • •
HV+
• • •
HV–
• • •
–+
R
SENSE
MOTOR
C2
0.01 µF
D1
5.1 V
–
+
R1