ACPL-C780 datasheet(12/17 Pages) AVAGO

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Application Information

Power Supplies and Bypassing

The recommended supply connections are shown in

Figure 17. A floating power supply (which in many ap-

plications could be the same supply that is used to drive

the high-side power transistor) is regulated to 5 V using

a simple zener diode (D1); the value of resistor R4 should

be chosen to supply sufficient current from the existing

floating supply. The voltage from the current sensing

resistor (Rsense) is applied to the input of the ACPL-C78A/

C780/C784 through an RC anti-aliasing filter (R2 and C2).

Although the application circuit is relatively simple, a few

recommendations should be followed to ensure optimal

performance.

The power supply for the ACPL-C78A/C780/C784 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

windingonanexistingtransformer.Otherwise,somesortof

simple isolated supply can be used, such as a line powered

transformer or a high-frequency DC-DC converter.

An inexpensive 78L05 three-terminal regulator can also be

used to reduce the floating 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.

As shown in Figure 18, 0.1

μF bypass capacitors (C1, C2)

should be located as close as possible to the pins of the

Figure 16. CMTI Test Circuits.

0.1 µF

VDD2

VOUT

ACPL-C78A

ACPL-C780

ACPL-C784

2 K

78L05

+15 V

0.1 µF

-15 V

–

+ MC34081

150

IN OUT

0.1

µF

0.1

µF

9 V

PULSE GEN.

VCM

–

10 K

150 pF

ACPL-C78A/C780/C784. The bypass capacitors are

required because of the high-speed digital nature of

the signals inside the ACPL-C78A/C780/C784. A 0.01

μF

bypass capacitor (C2) is also recommended at the input

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-C78A/C780/C784,

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-C78A/C780/C784.

ACPL-C780 Datasheet(PDF) 12 Page - AVAGO TECHNOLOGIES LIMITED

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