1µA Supply-Current, True +3V to +5.5V

RS-232 Transceivers with AutoShutdown

_______________________________________________________________________________________

9

Power-Supply Decoupling

In most circumstances a 0.1µF bypass capacitor is

adequate. In applications that are sensitive to power-

of the same value as the charge-pump capacitor C1.

Connect bypass capacitors as close to the IC as possible.

Transmitter Outputs

when Exiting Shutdown

Figure 6 shows two transmitter outputs when exiting

shutdown mode. As they become active, the two trans-

mitter outputs are shown going to opposite RS-232

levels. Each transmitter is loaded with 3k

Ω in parallel

with 2500pF. The transmitter outputs display no ringing

or undesirable transients as they come out of shut-

down. Note that the transmitters are enabled only when

the magnitude of V- exceeds approximately 3V. Figure

7a shows the MAX3243’s transmitter output voltage

vs. current per transmitter. Figure 7b is a mouse driver

test circuit.

Mouse Driveability (MAX3243)

The MAX3243 has been specifically designed to power

serial mice while operating from low-voltage power sup-

plies. 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 MAX3243 successfully drove all serial

mice and met their respective current and voltage

requirements. Figure 7a shows the transmitter outputs

under increasing load current. The MAX3243’s switching

regulator ensures the transmitters will supply at least ±5V

during worst-case conditions. The AutoShutdown feature

does not work with a mouse, so FORCEOFF and

High Data Rates

The MAX3221/MAX3223/MAX3243 maintain the RS-232

±5.0V minimum transmitter output voltage even at high

data rates. Figure 8 shows a transmitter loopback test

circuit. Figure 9 shows a loopback test result at

120kbps, and Figure 10 shows the same test at

235kbps. For Figure 9, all three transmitters were driv-

en simultaneously at 120kbps into RS-232 loads in par-

allel with 1000pF. For Figure 10, a single transmitter

was driven at 235kbps, but all three transmitters were

loaded with an RS-232 receiver in parallel with 1000pF.

Interconnection with 3V and 5V Logic

The MAX3221/MAX3223/MAX3243 can directly inter-

face with various 5V logic families, including ACT and

HCT CMOS. See Table 4 for more information on possi-

ble combinations of interconnections.

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

01

23

45

67

LOAD CURRENT PER TRANSMITTER (mA)

Figure 7a. MAX3243 Transmitter Output Voltage vs. Load

Current per Transmitter

5V/div

C1–C4 = 0.1

µF

2V/div

T2

50

µs/div

T1

Figure 6. Transmitter Outputs when Exiting Shutdown or

Powering Up