ACPL-7900 查看數據表（PDF） - Avago Technologies
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ACPL-7900 Datasheet PDF : 13 Pages
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Power Supplies and Bypassing
As mentioned above, an inexpensive 78L05 three-terminal
regulator can be used to reduce the gate-drive power
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 power supply for the isolation amplifier 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 powered trans-
former or a high-frequency DC-DC converter.
As shown in Figure 21, 0.1 PF bypass capacitors (C2, C4)
should be located as close as possible to the pins of the
isolation amplifier. The bypass capacitors are required
because of the high-speed digital nature of the signals
inside the isolation amplifier. A 47 nF bypass capacitor
(C3) is also recommended at the input pins 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 input filter also performs an
important reliability function – it reduces transient spikes
from ESD events flowing through the current sensing
resistor.
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 (CMTI) of the ACPL-790B/790A/7900,
due primarily to stray capacitive coupling between the
input and the output circuits. To obtain optimal CMTI
performance, the layout of the PC board should minimize
any stray coupling by maintaining the maximum possible
distance between the input and output sides of the circuit
and ensuring that any ground or power plane on the PC
board does not pass directly below or extend much wider
than the body of the ACPL-790B/790A/7900. Figure 22
shows an example PCB layout.
TO RSENSE+
TO GND1 TO VDD1
C2
U2
R5 C3
TO VDD2 TO GND2
C4
VOUT+
VOUT–
TO RSENSE–
Note: Drawing not to scale
ACPL-790B/790A/7900
Figure 22. Example printed circuit board layout.
12
Shunt Resistor Selection
The current sensing resistor should have low resistance (to
minimize power dissipation), low inductance (to minimize
di/dt induced voltage spikes which could adversely
affect operation), and reasonable tolerance (to maintain
overall circuit accuracy). Choosing a particular value for
the resistor is usually a compromise between minimiz-
ing power dissipation and maximizing accuracy. Smaller
sense resistance decreases power dissipation, while larger
sense resistance can improve circuit accuracy by utilizing
the full input range of the ACPL-790B/790A/7900.
The first step in selecting a sense resistor is determining
how much current the resistor will be sensing. The graph in
Figure 23 shows the RMS current in each phase of a three-
phase induction motor as a function of average motor
output power (in horsepower, hp) and motor drive supply
voltage. The maximum value of the sense resistor is deter-
mined by the current being measured and the maximum
recommended input voltage of the isolation amplifier. The
maximum sense resistance can be calculated by taking
the maximum recommended input voltage and dividing
by the peak current that the sense resistor should see
during normal operation. For example, if a motor will have
a maximum RMS current of 10 A and can experience up
to 50% overloads during normal operation, then the peak
current is 21.1 A (= 10 x 1.414 x 1.5). Assuming a maximum
input voltage of 200 mV, the maximum value of sense re-
sistance in this case would be about 10 m:.
The maximum average power dissipation in the sense
resistor can also be easily calculated by multiplying the
sense resistance times the square of the maximum RMS
current, which is about 1 W in the previous example. If the
power dissipation in the sense resistor is too high, the re-
sistance can be decreased below the maximum value to
decrease power dissipation. The minimum value of the
sense resistor is limited by precision and accuracy require-
ments of the design. As the resistance value is reduced,
the output voltage across the resistor is also reduced,
which means that the offset and noise, which are fixed,
40
35
440 V
380 V
30
220 V
120 V
25
20
15
10
5
0
0 5 10 15 20 25 30 35
MOTOR PHASE CURRENT - A (rms)
Figure 23. Motor output horsepower vs. motor phase current and supply
voltage.
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