Broadcom

ACPL-C787U-DS100

14

ACPL-C787U Data Sheet

Optically Isolated Sigma-Delta Modulator with

Low Offset Drift and Wide Operating Temperature

Application Information

Typical Application Circuit

Figure 16 shows a typical application circuit for motor control phase current sensing. By choosing the appropriate shunt

resistance, a wide range of current can be monitored from less than 1A to more than 100A.

Figure 16: Typical Application Circuit in Motor Phase Current Setting

HV+

GATE

FLOATING

POSITIVE SUPPLY

R1

ISOLATION

NON

MOTOR

+–

R SENSE

DRIVE

CIRCUIT

R2 43

C2

10nF

D1

5.1V

C1b

10 F

C1a

0.1 F

BARRIER

C3a

0.1 F

ISOLATED

5V/3.3V

C3b

10 F

HV –

GND1

ACPL -C787U

GND2

VDD2

MCLK

MDAT

GND2

VDD1

VIN+

VIN–

GND1

Shunt Resistors

The current-sensing shunt 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 shunt is usually a compromise between

minimizing power dissipation and maximizing accuracy.

Smaller shunt resistances decrease power dissipation,

while larger shunt resistances can improve circuit accuracy

by utilizing the full input range of the isolated modulator.

The first step in selecting a shunt is determining how much

current the shunt will be sensing. The graph in Figure 17

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 shunt is determined by the current

being measured and the maximum recommended input

voltage of the isolated modulator. The maximum shunt

resistance can be calculated by taking the maximum

recommended input voltage and dividing by the peak

current that the shunt should see during normal operation.

For example, if a motor will have a maximum rms current of

35 Arms and can experience up to 50% overloads during

normal operation, the peak current is 75A (= 35 × 1.414 ×

1.5). Assuming a maximum input voltage of 200 mV without

overload condition, the maximum value of shunt resistance

in this case would be about 4 mΩ. Under overload

conditions, the maximum input voltage will then be 300 mV

(75A × 4 mΩ), well within the ±320 mV FSR.

Figure 17: Motor Output Horsepower vs. Motor Phase Current

and Supply

110

100

90

80

70

60

50

40

30

20

10

0

0

10

20

30

40

50

60

70

80

90 100

MOTOR PHASE CURRENT - A (rms)

440 Vac

380 Vac

220 Vac

120 Vac