±15kV ESD-Protected, Isolated, 3.3V

RS-485/RS-422 Data Interfaces

_______________________________________________________________________________________

9

_______________Detailed Description

The MAX3480EA/MAX3480EB 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 (nonisolated 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.

Figure 1. Detailed Block Diagram

AC1 (MAKE NO CONNECTION)

AC2 (MAKE NO CONNECTION)

SHIELD (OPTIONAL)

EXTERNAL RS-485/RS-422 WIRING

B

D2

D1

D1, D2

1N914

ISO RO DRV

A

B

A

SH

ISO DI IN

ISO DE IN

SD

FS

GND1

ISO COM1

ISO DI DRV

ISO DE DRV

GND2

DE

DI

ISO COM2

ISO RO LED

RO

R4*

R5*

R8

100

Ω

BOOSTED V+

C3

0.01

µF

C4

270pF

4kV

R6*

R7*

R1*

R2*

DI

´

DE

´

R3*

RO

LOGIC

I/O

+3.0V TO +3.6V

C1

22

µF

C2

0.1

µF

MAX3480EA/EB

LOGIC GROUND

ISOLATION BARRIER

ISOLATION COMMON

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

MAX3480EA: MAX485E

MAX3480EB: MAX487E

RE

ISO RO DRV

ISO DE IN

ISO DI IN

ISO COM1

A

B

R

D

N

Q

Q

TF/F

FS

OSC

1.1MHz/

1.6MHz

SD

GND1

D2

D1

SHIELD (OPTIONAL)

NOTE: RESISTOR R8 PROTECTS THE

MAX3480E FROM TRANSIENT

CURRENTS BETWEEN SHIELD AND

A AND B.

TWISTED PAIR

TO OTHER TRANSCEIVERS

TERMINATING RESISTOR

(ONE RESISTOR ON EACH END)

TWISTED PAIR

TO OTHER TRANSCEIVERS

MAX485E

MAX487E

*SEE TABLE 1.

Table 1. Pullup and LED Drive Resistors

PART

R1 (

Ω)

R2 (

Ω)

R3 (

Ω)

R4 (

Ω)

R5 (

Ω)

R6 (

Ω)

MAX3480EA

100

100

680

3600

1000

200

MAX3480EB

100

100

2000

3600

3600

200

R7 (

Ω)

Open

430