1.8V to 4.25V-Powered,

True RS-232 Dual Transceiver

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5

_______________Detailed Description

The MAX218 line driver/receiver is intended for battery-

powered EIA/TIA-232 and V.28/V.24 communications

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 volt-

age range permits direct operation from a variety of

batteries without the need for a voltage regulator. For

example, the MAX218 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 MAX218.

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. A shutdown mode extends battery life by reduc-

ing supply current to 0.04µA. While shut down, all

receivers can either remain active or be disabled under

logic control. With this feature, the MAX218 can be in

low-power shutdown mode and still monitor activity on

external devices. Three-state drivers are provided on

both receiver outputs.

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.

Diode Selection

Key diode specifications are fast recovery time (<10ns),

average current rating (>100mA), and peak current rat-

ing (>350mA). Inexpensive fast silicon diodes, such as

the 1N6050, are generally recommended. More expen-

sive Schottky diodes improve efficiency and give slightly

lists suppliers of both surface-mount and through-hole

diodes. 1N914s are usually satisfactory, but specifica-

tions and performance vary widely with different manu-

facturers.

Capacitor Selection

Use capacitors with values at least as indicated in

Figure 1. Capacitor C2 determines the ripple on V+,

but not the absolute voltage. Capacitors C1 and C3

determine both the ripple and the absolute voltage of

elsewhere (e.g., at the power supply), increase C4 to

4.7µF.

You may use ceramic or polarized capacitors in all

locations. If you use polarized capacitors, tantalum

types are preferred because of the high operating fre-

quency of the power supplies (about 250kHz). If alu-

minum electrolytics are used, higher capacitance val-

ues may be required.

™ LapLink is a trademark of Traveling Software, Inc.

R1

R2

T1

T2

T1OUT

T2OUT

T1IN

T2IN

R1OUT

R2OUT

R1IN

R2IN

GND

EN

SHDN

LX

C1+

C1-

V-

6

3

7

8

9

10

11

12

13

14

16

18

15

4

5, 17, 20

MAX218

ENABLE

ON/OFF

1.8V

TO

4.25V

1

V+

19

1

µF

C4

1

µF

C2

1

µF

C3

0.47

µF

C1

15

µH

D1

1N6050

Figure 1. Single-Supply Operation