ad51-069-0reel Analog Devices, Inc., ad51-069-0reel Datasheet - Page 17

• Manufacturer Part Number: ad51-069-0reel
• Description: Half-duplex Icoupler-r Isolated Rs-485 Transceiver
• Manufacturer: Analog Devices, Inc.

APPLICATIONS INFORMATION
POWER_VALID INPUT
To avoid chatter on the A and B outputs caused by slow power-
up and power-down transients on V
ADM2483 features a power_valid (PV) digital input. This pin
should be driven low until V
greater than 2.0 V, the pin should be driven high. Conversely,
upon power-down, the PV should be driven low before V
reaches 2.0 V.
The power_valid input can be driven, for example, by the
output of a system reset circuit such as the ADM809Z, which
has a threshold voltage of 2.32 V.
RESET
V
DD1
2.0V
Figure 29. Driving PV with ADM809Z
ADM809Z
V
DD1
2.32V
t
POR
1nF
DD1
RESET
exceeds 2.0 V. When V
V
CC
74HC14
100nF
DD1
PV
3.9kΩ
ADM2483
(>100 µs/V), the
2.32V
GND
V
DD1
1
2.0V
D
CLK
PR
74HC74A
V
CC
Figure 30. Isolated Power Supply Circuit
CLR
DD1
100nF
Q
Q
DD1
is
Rev. B | Page 17 of 20
BS107A
BS107A
V
CC
ISOLATION
BARRIER
78253
ISOLATED POWER SUPPLY CIRCUIT
The ADM2483 requires isolated power capable of 5 V at
100 mA to be supplied between the V
suitable integrated power supply is available, a discrete circuit,
such as the one in Figure 30, 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 ADM2483’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.
SD103C
SD103C
22µF
IN
SET
ADP667
GND
V
GND
V
DD1
CC
ADM2483
1
SHDN
OUT
GND
5V
V
DD2
2
DD2
and GND
ADM2483
2
pins. If no