Datasheet

MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
_______________________________________________________________________________________ 9
_______________Detailed Description
The MAX3480A/MAX3480B are electrically isolated,
RS-485/RS-422 data-communications interface solu-
tions. Transceivers, optocouplers, a power driver, and a
transformer are in one standard 28-pin DIP package.
Signals and power are internally transported across the
isolation barrier (Figure 1). Power is transferred from the
logic side (non-isolated side) to the isolated side of the
barrier through a center-tapped transformer. Signals
cross the barrier through high-speed optocouplers. A
single +3.3V supply on the logic side powers both
sides of the interface.
The MAX3480B features reduced-slew-rate drivers that
minimize EMI and reduce reflections caused by
improperly terminated cables, allowing error-free trans-
mission at data rates up to 250kbps. The MAX3480A’s
driver slew rates are not limited, allowing transmission
rates up to 2.5Mbps.
The frequency-select FS is connected to GND_ in normal
operation, which selects a switching frequency of
approximately 600kHz. Connect to high for a higher
900kHz switching frequency.
Drivers are short-circuit current limited and are protect-
ed against excessive power dissipation by thermal
Figure 1. Detailed Block Diagram
AC1 (MAKE NO CONNECTION)
AC2 (MAKE NO CONNECTION)
SHIELD (OPTIONAL)
EXTERNAL RS-485/RS-422 WIRING
ISO V
CC1
B
D2
D1
D1, D2
1N914
V
CC2
V
CC1
ISO RO DRV
A
B
A
SH
ISO DI IN
ISO DE IN
V
CC3
SD
FS
GND1
ISO COM1
ISO DI DRV
ISO V
CC2
ISO DE DRV
GND2
DE
V
CC4
DI
ISO COM2
ISO RO LED
V
CC5
RO
R
L
R
L
R4
R5
R8
100Ω
BOOSTED V+
C3
0.01uF

R6 
200Ω
R7
R1
R2
DI
´
DE´
R3
RO
LOGIC
I/O
V
IN
3.0V TO 3.6V
22µF 0.1µF
C1 C2
MAX3480A/B
LOGIC GROUND
ISOLATION BARRIER ISOLATION COMMON
MAX485
MAX487
MAX845
1
2
28
27
3
4
5
6
7
8
9
10
11
12
13
14
26
25
24
23
22
21
20
19
18
17
16
15
MAX845
N
MAX3480A: MAX485
MAX3480B: MAX487
RE
ISO RO DRV
ISO DE IN
ISO DI IN
ISO COM1
ISO V
CC1
A
B
R
D
N
Q
Q
TF/F
V
CC3
FS
OSC
1.1MHz/
1.6MHz
SD
GND1
D2
D1
SHIELD (OPTIONAL)
NOTE: RESISTOR R8 PROTECTS THE
MAX3480 FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
“A” & “B”.
TWISTED PAIR
TO OTHER TRANSCEIVERS
TERMINATING RESISTOR
(ONE RESISTOR ON EACH END)
TWISTED PAIR
TO OTHER TRANSCEIVERS