460kbps, 1µA Supply Current,

RS-232-Compatible Transceivers in µMAX

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Detailed Description

Single Charge-Pump Voltage Converter

The MAX3311/MAX3313 internal power supply has a

single inverting charge pump that provides a negative

voltage from a single +5V supply. The charge pump

operates in a discontinuous mode and requires a flying

capacitor (C1) and a reservoir capacitor (C2) to gener-

ate the V- supply.

RS-232-Compatible Driver

The transmitter is an inverting level translator that con-

verts CMOS-logic levels to EIA/TIA-232-compatible lev-

els. It guarantees data rates up to 460kbps with

worst-case loads of 3k

Ω in parallel with 1000pF. When

SHDN is driven low, the transmitter is disabled and put

into three state. The transmitter input does not have an

internal pullup resistor.

RS-232 Receiver

The MAX3311/MAX3313 receiver converts RS-232 sig-

nals to CMOS-logic output levels. The MAX3311 receiv-

er will remain active during shutdown mode. The

MAX3313 INVALID indicates when an RS-232 signal is

present at the receiver input, and therefore when the

port is in use.

The MAX3313 INVALID output is pulled low when no

valid RS-232 signal level is detected on the receiver

input.

MAX3311 Shutdown Mode

In shutdown mode, the charge pump is turned off, V- is

pulled to ground, and the transmitter output is disabled

(Table 1). This reduces supply current typically to 1µA.

The time required to exit shutdown is typically less than

100µs.

Applications Information

Capacitor Selection

The capacitor type used for C1 and C2 is not critical for

proper operation; either polarized or nonpolarized

capacitors are acceptable. If polarized capacitors are

used, connect polarity as shown in the Typical

Operating Circuit. The charge pump requires 0.1µF

capacitors. Increasing the capacitor values (e.g., by a

factor of 2) reduces power consumption. C2 can be

increased without changing C1’s value. However, do

not increase C1’s value without also increasing the

(C1 to the other capacitors).

When using the minimum 0.1µF capacitors, make sure

the capacitance does not degrade excessively with

temperature. If in doubt, use capacitors with a larger

nominal value. The capacitor’s equivalent series resis-

tance (ESR) usually rises at low temperatures and influ-

ences the amount of ripple on V-.

To reduce the output impedance at V-, use larger

capacitors (up to 10µF).

tions sensitive to power-supply noise generated by the

tor the same size as (or larger than) charge-pump

capacitors C1 and C2.

Transmitter Output when Exiting

Shutdown

Figure 1 shows the transmitter output when exiting

shutdown mode. The transmitter is loaded with 3k

Ω in

parallel with 1000pF. The transmitter output displays no

ringing or undesirable transients as the MAX3311

comes out of shutdown. Note that the transmitter is

enabled only when the magnitude of V- exceeds

approximately -3V.

Table 1. MAX3311 Shutdown Logic Truth

Table

SHDN

TRANSMITTER

OUTPUT

RECEIVER

OUTPUT

CHARGE

PUMP

L

High Z

Active

Inactive

H

Active

Active

Active

Figure 1. Transmitter Output when Exiting Shutdown or

Powering Up

10

µs/div

SHDN

TOUT

5V/div

0

1.5V/div

0

TIN = GND