MAX3380E/MAX3381E
+2.35V to +5.5V, 1µA, 2Tx/2Rx RS-232 Transceivers
with ±15kV ESD-Protected I/O and Logic Pins
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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.
Under a decaying V
CC
, the charge pump will generate
an output voltage of ±5.5V with a V
CC
input range
between +3.1V and +5.5V. When V
CC
drops below the
switchover point of +3.1V, the charge pump switches
into RS-232-compatible mode generating ±4V.
When V
CC
is rising, the charge pump will generate an
output voltage of ±4.0V, while V
CC
is between +2.5V
and +3.5V. When V
CC
rises above the switchover volt-
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
V
CC
falls below approximately +3.1V. The reduced lev-
els are RS-232-compatible and reduce supply current
requirements that help preserve the battery. Built-in
hysteresis of approximately 400mV for V
CC
ensures
that the RS-232 output levels do not change if V
CC
is
noisy or has a sudden current draw causing the supply
voltage to drop slightly. The outputs will return to RS-
232-compliant levels (±5.5V) when V
CC
rises above
approximately +3.5V.
The MAX3380E/MAX3381E transmitters guarantee a data
rate of 460kbps/250kbps, respectively, with worst-case
loads of 3kΩ 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 400kΩ active positive
feedback resistor. They will retain a valid logic level if
the driving signal is removed or goes high impedance.
Connect unused transmitter inputs to V
CC
or ground.
RS-232 Receivers
The receivers convert RS-232 signals to logic levels
referred to V
L
. Both receivers are active in shutdown
(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).
