HCPL-7520-300 Datasheet, PDF(14/16 Page) AVAGO TECHNOLOGIES LIMITED

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HCPL-7520-300 Datasheet, PDF (14/16 Pages) AVAGO TECHNOLOGIES LIMITED – Isolated Linear Sensing IC

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MOTOR PHASE CURRENT â A (rms)

Figure 18. Motor output horsepower vs. motor phase current and supply

voltage.

When laying out a PC board for the current sensing

resistors, a couple of points should be kept in mind. The

Kelvin connections to the resistor should be brought

together under the body of the resistor and then run

very close to each other to the input of the HCPL-7520;

this minimizes the loop area of the connection and

reduces the possibility of stray magnetic fields from in-

terfering with the measured signal. If the sense resistor

is not located on the same PC board as the HCPL-7520

circuit, a tightly twisted pair of wires can accomplish

the same thing. Also, multiple layers of the PC board

can be used to increase current carrying capacity.

Numerous plated-through vias should surround each

non-Kelvin terminal of the sense resistor to help dis-

tribute the current between the layers of the PC board.

The PC board should use 2 or 4 oz. copper for the layers,

resulting in a current carrying capacity in excess of 20

A. Making the current carrying traces on the PC board

fairly large can also improve the sense resistorâs power

dissipation capability by acting as a heat sink. Liberal

use of vias where the load current enters and exits the

PC board is also recommended.

Sense Resistor Connections

The recommended method for connecting the HCPL-

7520 to the current sensing resistor is shown in Figure

17. VIN+ (pin 2 of the HPCL-7520) is connected to the

positive terminal of the sense resistor, while VIN- (pin

3) is shorted to GND1 (pin 4), with the powersupply

return path functioning as the sense line to the negative

terminal of the current sense resistor. This allows a

single pair of wires or PC board traces to connect the

HCPL-7520 circuit to the sense resistor. By referenc-

ing the input circuit to the negative side of the sense

resistor, any load current induced noise transients on

the resistor are seen as a common- mode signal and

will not interfere with the current-sense signal. This

is important because the large load currents flowing

through the motor drive, along with the parasitic in-

ductances inherent in the wiring of the circuit, can

generate both noise spikes and offsets that are relative-

ly large compared to the small voltages that are being

measured across the current sensing resistor. If the same

power supply is used both for the gate drive circuit and

for the current sensing circuit, it is very important that

the connection from GND1 of the HCPL-7520 to the

sense resistor be the only return path for supply current

to the gate drive power supply in order to eliminate

potential ground loop problems. The only direct con-

nection between the HCPL-7520 circuit and the gate

drive circuit should be the positive power supply line.

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