ETC A5016 Standby current for rs-422 repeater is less than3a Datasheet

DESIGN SHOWCASE
Standby current for RS-422 repeater is
less than 3µA
The RS-422 interface is an excellent choice for communicating in noisy environments and over a distance.
However, when the distance exceeds the RS-422 capability for reliable data transfer, you must add a repeater.
The repeater circuit in Figure 1, which must operate
from batteries when no power supply is available,
draws less than 3µA of current from a 3V supply.
machine asserts a high level at U2’s driver-enable pin,
causing the incoming data byte to be retransmitted
from U2 at full RS-422 levels.
The state machine watches for transitions on RO.
When a data-byte transfer is complete (as indicated by
no falling edges within a time delay internal to the state
machine), the state machine resets itself in anticipation
of the next data byte from either side of the interface.
Low standby power and true fail-safe operation are the
key features in this application. U1 and U2 drive their
receiver outputs (RO) high when the RS-422 inputs
are open circuited or terminated and undriven. An
incoming data byte on the differential inputs A1 and
B1 forces a transition on RO of U1, and a state
machine (right half of the Figure 1 schematic) is
latched ON by the falling edge of RO. The state
An incoming data burst (Figure 2, top) is retransmitted as outputs A2 (bottom trace) and B2 (middle
trace). U2 de-asserts those outputs 700µs after the
final transition. Other delays can be implemented by
adjusting R1/C1 and R2/C2 as shown in Figure 1.
VCC
D1
1N4148
R1
4.7MΩ
C1
220pF
8
B1
R3
120Ω
A1
VCC
7
B
6
5
U1
RO
1
RE
2
A
DE
3
GND
DI
4
MAX3471
6
5
1
1
2
3
U3A
74HC00
3
2
CLK
CLR
C3
100nF
U3B
74HC00
4
Q
D
PRE
VCC
Q
VCC
4
6
5
U4A
74HC74
GND1
C4
100nF
B2
R4
120Ω
A2
8
VCC
7
B
6
5
U2
RO
1
RE
2
A
DE
3
GND
DI
4
MAX3471
8
12
D
PRE
GND2
9
10
U3C
74HC00
Q
CLK
CLR
10
VCC
Q
11
13
VCC
D2
1N4148
R2
12
4.7MΩ
13
C2
220pF
Figure 1. This RS-422 repeater draws only 3µA of standby current.
16
U3D
74HC00
11
9
8
U4B
74HC74
(dB)
Figure 2. An input data byte causes the repeater to transmit the byte, then go to standby. Vertical scale is 1V per division; horizontal scale is
400µs per division.
17
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