Broadcom

- 13 -

ACPL-785E, HCPL-7850, HCPL-7851, 5962-97557

Data Sheet

Applications Information

Applications Information

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

Supplies and Bypassing

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

Figure 27 Bottom Layer of a Printed Circuit Board Layout

0.1 F

+5 V

8

7

6

1

3

U2

5

2

4

R1

2.00 K

+5 V

C8

0.1 F

–

+

MC34071

R3

10.0 K

HCPL-785x

C4

R4B

20.0 K

C6

150 pF

U3

R4A

20.0 K

+5 V

C5

150 pF

R2

2.00 K

C3

C2

C4

R5