+5V-Powered, Multichannel RS-232
Drivers/Receivers
mode, in three-state mode, or when device power is
_______________Detailed Description
removed. Outputs can be driven to ±±5V. The power-
supply current typically drops to 8µA in shutdown mode.
The MAX220 does not have pull-up resistors to force the
ouputs of the unused drivers low. Connect unused inputs
The MAX220–MAX249 contain four sections: dual
charge-pump DC-DC voltage converters, RS-232 dri-
vers, RS-232 receivers, and receiver and transmitter
enable control inputs.
to GND or V
.
CC
Dual Charge-Pump Voltage Converter
The MAX220–MAX249 have two internal charge-pumps
that convert +5V to ±±0V (unloaded) for RS-232 driver
operation. The first converter uses capacitor C± to dou-
ble the +5V input to +±0V on C3 at the V+ output. The
second converter uses capacitor C2 to invert +±0V to
-±0V on C4 at the V- output.
The MAX239 has a receiver three-state control line, and
the MAX223, MAX225, MAX235, MAX236, MAX240,
and MAX24± have both a receiver three-state control
line and a low-power shutdown control. Table 2 shows
the effects of the shutdown control and receiver three-
state control on the receiver outputs.
The receiver TTL/CMOS outputs are in a high-imped-
ance, three-state mode whenever the three-state enable
line is high (for the MAX225/MAX235/MAX236/MAX239–
MAX24±), and are also high-impedance whenever the
shutdown control line is high.
A small amount of power may be drawn from the +±0V
(V+) and -±0V (V-) outputs to power external circuitry
(see the Typical Operating Characteristics section),
except on the MAX225 and MAX245–MAX247, where
these pins are not available. V+ and V- are not regulated,
so the output voltage drops with increasing load current.
Do not load V+ and V- to a point that violates the mini-
mum ±5V EIA/TIA-232E driver output voltage when
sourcing current from V+ and V- to external circuitry.
When in low-power shutdown mode, the driver outputs
are turned off and their leakage current is less than ±µA
with the driver output pulled to ground. The driver output
leakage remains less than ±µA, even if the transmitter
output is backdriven between 0V and (V
+ 6V). Below
CC
When using the shutdown feature in the MAX222,
MAX225, MAX230, MAX235, MAX236, MAX240,
MAX24±, and MAX245–MAX249, avoid using V+ and V-
to power external circuitry. When these parts are shut
down, V- falls to 0V, and V+ falls to +5V. ꢁor applica-
tions where a +±0V external supply is applied to the V+
pin (instead of using the internal charge pump to gen-
erate +±0V), the C± capacitor must not be installed and
-0.5V, the transmitter is diode clamped to ground with
±kΩ series impedance. The transmitter is also zener
clamped to approximately V
impedance of ±kΩ.
+ 6V, with a series
CC
The driver output slew rate is limited to less than 30V/µs
as required by the EIA/TIA-232E and V.28 specifica-
tions. Typical slew rates are 24V/µs unloaded and
±0V/µs loaded with 3Ω and 2500pꢁ.
the SHDN pin must be tied to V . This is because V+
CC
is internally connected to V
in shutdown mode.
CC
RS-232 Receivers
EIA/TIA-232E and V.28 specifications define a voltage
level greater than 3V as a logic 0, so all receivers invert.
Input thresholds are set at 0.8V and 2.4V, so receivers
respond to TTL level inputs as well as EIA/TIA-232E and
V.28 levels.
RS-232 Drivers
The typical driver output voltage swing is ±8V when
loaded with a nominal 5kΩ RS-232 receiver and V
=
CC
+5V. Output swing is guaranteed to meet the EIA/TIA-
232E and V.28 specification, which calls for ±5V mini-
mum driver output levels under worst-case conditions.
The receiver inputs withstand an input overvoltage up
to ±25V and provide input terminating resistors with
These include a minimum 3kΩ load, V
= +4.5V, and
CC
maximum operating temperature. Unloaded driver out-
put voltage ranges from (V+ -±.3V) to (V- +0.5V).
Table 2. Three-State Control of Receivers
Input thresholds are both TTL and CMOS compatible.
The inputs of unused drivers can be left unconnected
PART
SHDN
EN
EN(R)
RECEIVERS
SHDN
Low
X
High Impedance
Active
High Impedance
since 400kΩ input pull-up resistors to V
are built in
CC
MAX223
__
High Low
High High
__
(except for the MAX220). The pull-up resistors force the
outputs of unused drivers low because all drivers invert.
The internal input pull-up resistors typically source ±2µA,
except in shutdown mode where the pull-ups are dis-
abled. Driver outputs turn off and enter a high-imped-
ance state—where leakage current is typically
microamperes (maximum 25µA)—when in shutdown
Low
High
High Impedance
Active
MAX225
__
__
__
__
__
MAX235 Low
MAX236 Low
MAX240 High
Low
High
X
High Impedance
Active
High Impedance
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