_______________Detailed Description

The MAX3218 line driver/receiver is intended for bat-

tery-powered EIA/TIA-232 and V.28/V.24 communica-

tions interfaces that require two drivers and two

receivers. The operating voltage extends from 1.8V to

4.25V, yet the device maintains true RS-232 and

EIA/TIA-562 transmitter output voltage levels. This wide

supply voltage range permits direct operation from a

variety of batteries without the need for a voltage regu-

lator. For example, the MAX3218 can be run directly

from a single lithium cell or a pair of alkaline cells. It

can also be run directly from two NiCd or NiMH cells

from full-charge voltage down to the normal 0.9V/cell

end-of-life point. The 4.25V maximum supply voltage

allows the two rechargeable cells to be trickle- or fast-

charged while driving the MAX3218.

The circuit comprises three sections: power supply,

transmitters, and receivers. The power-supply section

converts the supplied input voltage to 6.5V, providing the

voltages necessary for the drivers to meet true RS-232

levels. External components are small and inexpensive.

The transmitters and receivers are guaranteed to oper-

ate at 120kbps data rates, providing compatibility with

LapLink™ and other high-speed communications soft-

ware.

The MAX3218 is equipped with Maxim’s new propri-

etary AutoShutdown™ circuitry. This achieves a 1µA

supply current by shutting down the device when the

RS-232 cable is disconnected or when the connected

peripheral transmitters are turned off. While shut down,

both receivers can remain active or can be disabled

under logic control. With this feature, the MAX3218 can

be in low-power shutdown mode and still monitor activi-

ty on external devices. Three-state drivers are provid-

ed on both receiver outputs.

Three-state drivers on both receiver outputs are provid-

ed so that multiple receivers, generally of different inter-

face standards, can be wire-ORed at the UART.

Switch-Mode Power Supply

The switch-mode power supply uses a single inductor

with one diode and three small capacitors to generate

±6.5V from an input voltage in the 1.8V to 4.25V range.

Inductor Selection

Use a 15µH inductor with a saturation current rating of at

least 350mA and less than 1

Ω resistance. Table 1 lists

suppliers of inductors that meet the 15µH/350mA/1

Ω

specifications.

1µA Supply Current, 1.8V to 4.25V-Powered

RS-232 Transceiver with AutoShutdown™

_______________________________________________________________________________________

5

Receiver Inputs

R2IN, R1IN

11, 12

Transmitter Outputs, swing between V+ and V-

T2OUT, T1OUT

13, 14

Negative Supply generated on-board

V-

15

Terminal for Charge-Pump Capacitor

C1-, C1+

16, 18

Positive Supply generated on-board

V+

19

Ground

GND

5, 17, 20

Supply Voltage Input, 1.8V to 4.25V. Bypass to GND with at least 1µF.

6

Transmitter Inputs

T1IN, T2IN

7, 8

Receiver Outputs

R1OUT, R2OUT

9, 10

Drive low to shut down transmitters and on-board power supply, overriding all automatic

circuitry and FORCEON.

FORCEOFF

4

Drive high when FORCEOFF = high to override automatic circuitry, keeping transmitters on.

FORCEON

3

PIN

Output of Invalid Signal Detector. Low if invalid RS-232 levels are present on all receiver inputs,

otherwise high.

INVALID

2

Inductor/Diode Connection Point

LX

1

FUNCTION

NAME

______________________________________________________________Pin Description

™ LapLink is a trademark of Traveling Software, Inc.

AutoShutdown is a trademark of Maxim Integrated Products.