ADM2486 datasheet(17/20 Pages) AD

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ADM2486

Rev. C | Page 17 of 20

ISOLATED POWER SUPPLY CIRCUIT

The ADM2486 requires isolated power capable of 5 V at 100 mA

to be supplied between the VDD2 and GND2 pins. If no suitable

integrated power supply is available, a discrete circuit, such as

the one in Figure 31, can be used. A center-tapped transformer

provides electrical isolation. The primary winding is excited

with a pair of square waveforms that are 180° out of phase with

each other. A pair of Schottky diodes and a smoothing capacitor

are used to create a rectified signal from the secondary winding.

The ADP667 linear voltage regulator provides a regulated

power supply to the ADM2486’s bus-side circuitry.

To create the pair of square waves, a D-type flip-flop with

complementary Q/Q outputs is used. The flip-flop can be

connected so that output Q follows the clock input signal. If no

local clock signal is available, a simple digital oscillator can be

implemented with a hex-inverting Schmitt trigger and a resistor

and capacitor. In this case, values of 3.9 kΩ and 1 nF generate a

364 kHz square wave. A pair of discrete NMOS transistors,

switched by the Q/Q flip-flop outputs, conduct current through

the center tap of the primary transformer, winding in an

alternating fashion.

ISOLATION

BARRIER

VCC

78253

SD103C

µF

BS107A

µF

OUT

SET GND SHDN

VCC

VDD1

VDD2

GND1

GND2

CLK

CLR

74HC74A

100nF

VCC

100nF

74HC14A

3.9k

Ω

1nF

ADP667

ADM2486

Figure 31. Isolated Power Supply Circuit

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