+2.35V to +5.5V, 1µA, 2Tx/2Rx RS-232 Transceivers

with ±15kV ESD-Protected I/O and Logic Pins

_______________________________________________________________________________________

7

Voltage Generation in the

Switchover Region

The MAX3380E/MAX3381E include a switchover circuit

between RS-232-compliant and RS-232-compatible

modes that has approximately 400mV of hysteresis

around the switchover point. The hysteresis is shown in

Figure 1. This large hysteresis helps to avoid mode

change under battery or power-supply bounce.

switchover point of +3.1V, the charge pump switches

into RS-232-compatible mode generating ±4V.

age of +3.5V, the charge pump switches to RS-232-

compliant mode to generate an output voltage of ±5.5V.

RS-232 Transmitters

The transmitters are inverting level translators that con-

vert CMOS-logic levels to RS-232-compatible levels.

The MAX3380E/MAX3381E will automatically reduce

the RS-232-compliant levels from ±5.5V to ±3.7V when

els are RS-232-compatible and reduce supply current

requirements that help preserve the battery. Built-in

noisy or has a sudden current draw causing the supply

voltage to drop slightly. The outputs will return to RS-

approximately +3.5V.

The MAX3380E/MAX3381E transmitters guarantee a data

rate of 460kbps/250kbps, respectively, with worst-case

loads of 3k

Ω in parallel with 1000pF. Transmitters can be

paralleled to drive multiple receivers.

When FORCEOFF is driven to ground, the transmitters

are disabled and the outputs go into high impedance;

receivers remain active. When the AutoShutdown Plus

circuitry senses that all receiver and transmitter inputs

are inactive for more than 30s, the transmitters are dis-

abled and the outputs go into a high-impedance state,

and the receivers remain active. When the power is off,

the MAX3380E/MAX3381E permit the outputs to be dri-

ven up to ±12V.

The transmitter inputs have a 400k

Ω active positive

feedback resistor. They will retain a valid logic level if

the driving signal is removed or goes high impedance.

RS-232 Receivers

The receivers convert RS-232 signals to logic levels

(Table 1).

AutoShutdown Plus Mode

The MAX3380E/MAX3381E achieve a 1µA supply current

with Maxim’s AutoShutdown Plus feature, which operates

when FORCEOFF is high and FORCEON is low. When

these devices do not sense a valid signal transition on

any receiver and transmitter input for 30s, the on-board

charge pumps are shut down, reducing supply current

to 1µA. This occurs if the RS-232 cable is disconnected

or if the connected peripheral transmitters are turned off,

and if the UART driving the transmitter inputs is inactive.

The system turns on again when a valid transition is

applied to any RS-232 receiver or transmitter input. As a

result, the system saves power without changes to the

existing BIOS or operating system.

Figures 2a and 2b show valid and invalid RS-232

receiver voltage levels. INVALID indicates the receiver

input’s condition, and is independent of the FORCEON

and FORCEOFF states. Figure 2 and Table 1 summa-

rize the MAX3380E/MAX3381E’s operating modes.

FORCEON and FORCEOFF override AutoShutdown

Plus circuitry. When neither control is asserted, the IC

selects between these states automatically based on

the last receiver or transmitter input edge received.

By connecting FORCEON to INVALID, the MAX3380E/

MAX3381E is shut down when no valid receiver level and

no receiver or transmitter edge is detected for 30s, and

wakes up when a receiver or transmitter edge is detect-

ed (Figure 2c).

20ms/div

2V/div

V+

2V/div

+4.5V

+2.5V

+5.8V

+4.4V

Figure 1. V+ Switchover for Changing Vcc