ACPL-C87B datasheet(13/14 Pages) AVAGO

13 / 14 page

Figure 22. Recommended Power Supply and Bypassing

Power Supplies and Bypassing

A power supply of 5 V is required to power the ACPL-C87x

input side VDD1. In many motor drive DC bus voltage

sensing applications, this 5 V supply is most often obtained

from the same supply used to power the power transistor

gate drive circuit using an inexpensive 78L05 three-ter-

minal regulator. 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.

In some other applications a dedicated supply might be

required to supply the VDD1. These applications include

photovoltaic (PV) inverter voltage tracking and measure-

ment, temperature sensor signal isolation. In these 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 transformer or

a high-frequency DC-DC converter module.

As shown in Figure 22, 100 nF bypass capacitors (C2, C3)

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 100 pF bypass capacitor

(Cin) is also recommended at the input pins due to the

switched-capacitor nature of the input circuit. The input

bypass capacitor Cin also forms part of the anti-aliasing

filter, which is recommended to prevent high-frequency

noise from aliasing down to lower frequencies and inter-

fering with the input signal. When R1 is far greater than R2,

the low-pass anti-aliasing filter corner frequency can be

calculated by 1/(2

R2Cin). The input filter also performs

an important reliability function – it reduces transient

spikes from ESD events flowing through the high voltage

rails.

VDD1

SHDN

GateDrive

Circuit

78L05

ACPL-C87A

Cin

0.1nF

0.1

μF

0.1

μF

GND1

VIN

OUT

Floating

PositiveSupply

VOUT+

VDD2

VOUT-

GND2

0.1

μF

HV+

HV-

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