3V to 5.5V, up to 250kbps True RS-232 Transceiver

with 4µA AutoShutdown Plus and Power-On Reset

_______________________________________________________________________________________

9

When using the minimum required capacitor values,

make sure the capacitor value does not degrade

excessively with temperatures outside the 0°C to +70°C

range. If in doubt, use capacitors with a larger nominal

value. The capacitor's equivalent series resistance

(ESR), which usually rises at low temperatures, influ-

ences the amount of ripple on V+ and V-.

Transmitter Outputs

when Exiting Shutdown

Figure 5 shows two transmitter outputs when exiting

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

mitter outputs go to opposite RS-232 levels (one trans-

mitter input is high; the other is low). Each transmitter is

loaded with 3k

Ω in parallel with 2500pF. The transmitter

outputs display no ringing or undesirable transients as

they exit shutdown. Note that the transmitters are

enabled only when the magnitude of V- exceeds

approximately 3V.

High Data Rates

The MAX3320 maintains the RS-232 ±5V minimum

transmitter output voltage even at high data rates.

Figure 6 shows a transmitter loop-back test circuit.

Figure 7 shows a loop-back test result at 120kbps, and

Figure 8 shows the same test at 250kbps. For Figure 7,

both transmitters were driven simultaneously at

120kbps into RS-232 loads in parallel with 1000pF. For

Figure 8, a single transmitter was driven at 250kbps,

and both transmitters were loaded with an RS-232

receiver in parallel with 1000pF.

Interconnection with 3V and 5V Logic

The MAX3320 can directly interface with various 5V

logic families, including ACT and HCT CMOS. See

Table 4 for more information on possible combinations

of interconnections.

C1 - C4 = 0.1

µF

5

µs/div

2V/div

2V/div

T2OUT

T1OUT

FORCEON =

FORCEOFF

Figure 5. Transmitter Outputs when Exiting Shutdown or

Powering Up

MAX3320

5k

R_ IN

R_ OUT

C2-

C2+

C1-

C1+

V-

V+

C4

C3*

C1

C2

FORCEOFF

T_ OUT

T_ IN

GND

*C3 CAN BE RETURNED TO

FORCEON

1000pF

Figure 6. Loop-Back Test Circuit

MAX3320

GND

100k

RESET

0.1

0.47

3 to 5.5

0.047

0.33

4.5 to 5.5

C1

(µF)

0.1

(µF)

0.1

(V)

3 to 3.6

Table 3. Required Minimum

Capacitance Values