+3V-Powered, Low-Power,

True RS-232 Transceiver

6

_______________________________________________________________________________________

Operating Modes

–

S

—

D and EN determine the operation of the MAX212

as follows:

Shutdown

V+ and V- are disabled and the transmitters are put into

a high-impedance state when

–

S

—

D is taken to logic low.

Receiver operation is not affected, but power consump-

tion is dramatically reduced while in shutdown mode.

Supply current is minimized when the receiver inputs are

static in any of three states: floating (ground), less than

RS-232 conditions. The presence of AC signals on receiv-

From a low-power system perspective, powering-up with

non-critical or unused sections of circuitry shut down or

deselected is generally recommended. Activating these

sections only when required saves power and reduces

power-up current surges. Powering-up the MAX212 in

shutdown

mode

(holding

the

shutdown

pin,

–

S

—

Driving the MAX212 from 5V Logic

The MAX212 can interface with 5V logic while it operates

from a 3V supply, however a 200k

Ω series resistor between

–

S

—

D and the +5V logic signal is required (see Figure 2).

This protects the MAX212 by preventing

–

S

—

D from sinking

current while V+ is powering up. Also, the UART must not

be permitted to send a logic high to the MAX212’s EN or

transmitter input pins until

–

S

—

D is high (not shut down).

Mouse Driveability

The MAX212 has been specifically designed to power

serial mice while operating from low-voltage power

supplies. It has been tested with samples of ten major

mouse models from six manufacturers, including the

leading three, Logitech (5 models), Mouse Systems,

and Microsoft. The MAX212 successfully drove all seri-

al mice and met their respective current and voltage

requirements (Figure 1).

Figure 3 shows the transmitter output voltages under

increasing load current. The MAX212’s switching regu-

lator ensures the transmitters will supply at least ±5V

during worst-case load conditions.

EIA/TIA-232E and 562 Standards

Most of the power drawn by RS-232 circuits is con-

sumed because the EIA/TIA-232E standard demands

that at least ±5V be delivered by the transmitters to

impedances that can be as low as 3k

Ω. For applica-

–

S

—

D

EN

RECEIVER

OUTPUTS

DRIVER

OUTPUTS

L

L

High-Z

High-Z

Off

Minimum

L

H

Enabled

High-Z

Off

Minimum

H

L

High-Z

Enabled

On

Normal

H

H

Enabled

Enabled

On

Normal

SUPPLY

CURRENT

PART

No. OF

TRANSMITTERS/

RECEIVERS

DATA RATE

(kbps)

FEATURES

MAX212

3.0 to 3.6

3/5

5

250

True RS-232 from +3V

MAX560

3.0 to 3.6

4/5

2

120

2 receivers active in shutdown

MAX561

3.0 to 3.6

4/5

0

120

Pin compatible with MAX241

MAX562

2.7 to 5.25

3/5

5

250

Wide supply range

POWER-SUPPLY

VOLTAGE

(V)

No. OF

RECEIVERS

ACTIVE IN

SHUTDOWN

____________________________+3V Powered RS-232 Transceivers from Maxim

DC-DC

CONVERTER

MAX212

SD

+3.3V

DC-DC CONVERTER

X3

X5

EN

LN

LP

V-

GND

+5V

LOGIC

SIGNALS

200k

V+

Figure 2. Connecting to +5V Logic