HCPL-7850-100 データシートの表示（PDF） - Broadcom Corporation
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HCPL-7850-100 Datasheet PDF : 16 Pages
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ACPL-785E, HCPL-7850, HCPL-7851, 5962-97557
Data Sheet
Applications Information
Applications Information
Supplies and Bypassing
Functional Description
Figure 23 shows the primary functional blocks of the
HCPL-785x. In operation, the sigma-delta modulator converts
the analog input signal into a high-speed serial bit stream. The
time average of this bit stream is directly proportional to the
input signal. This stream of digital data is encoded and
optically transmitted to the detector circuit. The detected
signal is decoded and converted back into an analog signal,
which is filtered to obtain the final output signal.
Application Circuit
The recommended application circuit is shown in Figure 24. 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 5V using a simple three-terminal
voltage regulator (U1). The voltage from the current sensing
resistor, or shunt (Rsense), is applied to the input of the
HCPL-785x through an RC anti-aliasing filter (R5, C3). And
finally, the differential output of the isolation amplifier is
converted to a ground-referenced single-ended output
voltage with a simple differential amplifier circuit (U3 and
associated components). Although the application circuit is
relatively simple, a few recommendations should be followed
to ensure optimal performance.
Figure 25 Single-Supply Post-Amplifier Circuit
C5
150 pF
1
2
U2
3
+5 V
R3
+5 V
8 C4
R4A
20.0 K
0.1 PF
7
R1
2.00 K
6
R2
2.00 K
10.0 K
+5 V
C8
0.1 PF
–
U3
VOUT
+ MC34071
As mentioned, an inexpensive three-terminal regulator can be
used to reduce the gate-drive power supply voltage to 5V. 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 24, a 0.1-μF bypass capacitor (C2, C4)
should be located as close as possible to the input and output
power supply pins of the HCPL-785x. The bypass capacitors are
required because of the high-speed digital nature of the
signals inside the isolation amplifier. A 0.01-μF bypass
capacitor (C3) is also recommended at the input pin(s) due to
the switched-capacitor nature of the input circuit. The input
bypass capacitor should be at least 1000 pF to maintain gain
accuracy of the isolation amplifier.
Inductive coupling between the input power-supply capacitor
and the input circuit, including the input bypass capacitor and
the input leads of the HCPL-785x, can introduce additional DC
offset in the circuit. Several steps can be taken to minimize the
mutual coupling between the two parts of the circuit, thereby
improving the offset performance of the design. Separate the
two bypass capacitors C2 and C3 as much as possible (even
putting them on opposite sides of the PC board), while keeping
the total lead lengths, including traces, of each bypass
capacitor less than 20 mm. PC board traces should be made as
short as possible and placed close together or over ground
plane to minimize loop area and pickup of stray magnetic
fields. Avoid using sockets, as they will typically increase both
loop area and inductance. And finally, using capacitors with
small body size and orienting them perpendicular to each
other on the PC board can also help. For more information
concerning this effect, see Application Note 1078, Designing
with Avago Technologies Isolation Amplifiers.
Figure 26 Top Layer of Printed Circuit Board Layout
R5
C2 C4
C3
Figure 27 Bottom Layer of a Printed Circuit Board Layout
4
5
HCPL-785x
C6
150 pF
R4B
20.0 K
TO VDD1
TO RSENSE+
TO RSENSE–
TO VDD2
VOUT+
VOUT–
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