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Multichannel RS-232 Drivers/Receivers

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MAX220–MAX249
+5V-Powered, Multichannel
RS-232 Drivers/Receivers
Detailed Description
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.
Dual Charge-Pump Voltage Converter
The MAX220–MAX249 have two internal charge-pumps
that convert +5V to ±10V (unloaded) for RS-232 driver
operation. The first converter uses capacitor C1 to dou-
ble the +5V input to +10V on C3 at the V+ output. The
second converter uses capacitor C2 to invert +10V to
-10V on C4 at the V- output.
A small amount of power may be drawn from the +10V
(V+) and -10V (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 using the shutdown feature in the MAX222,
MAX225, MAX230, MAX235, MAX236, MAX240,
MAX241, 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. For applica-
tions where a +10V external supply is applied to the V+
pin (instead of using the internal charge pump to gen-
erate +10V), the C1 capacitor must not be installed and
the SHDN pin must be connected to VCC. This is
because V+ is internally connected to VCC in shutdown
mode.
RS-232 Drivers
The typical driver output voltage swing is ±8V when
loaded with a nominal 5kΩ RS-232 receiver and VCC =
+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.
These include a minimum 3kΩ load, VCC = +4.5V, and
maximum operating temperature. Unloaded driver out-
put voltage ranges from (V+ -1.3V) to (V- +0.5V).
Input thresholds are both TTL and CMOS compatible.
The inputs of unused drivers can be left unconnected
since 400kΩ input pullup resistors to VCC are built in
(except for the MAX220). The pullup resistors force the
outputs of unused drivers low because all drivers invert.
The internal input pullup resistors typically source 12µA,
except in shutdown mode where the pullups 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
www.maximintegrated.com
mode, in three-state mode, or when device power is
removed. Outputs can be driven to ±15V. The power-
supply current typically drops to 8µA in shutdown mode.
The MAX220 does not have pullup resistors to force the
outputs of the unused drivers low. Connect unused
inputs to GND or VCC.
The MAX239 has a receiver three-state control line, and
the MAX223, MAX225, MAX235, MAX236, MAX240,
and MAX241 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–
MAX241), and are also high-impedance whenever the
shutdown control line is high.
When in low-power shutdown mode, the driver outputs
are turned off and their leakage current is less than 1µA
with the driver output pulled to ground. The driver output
leakage remains less than 1µA, even if the transmitter
output is backdriven between 0V and (VCC + 6V). Below
-0.5V, the transmitter is diode clamped to ground with
1kΩ series impedance. The transmitter is also zener
clamped to approximately VCC + 6V, with a series
impedance of 1kΩ.
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
10V/µs loaded with 3Ω and 2500pF.
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.
Table 2. Three-State Control of
Receivers
PART SHDN SHDN EN EN(R) RECEIVERS
MAX223
Low X
__ High Low
High High
High Impedance
__ Active
High Impedance
MAX225 __
__
__
Low
High
High Impedance
Active
MAX235 Low
MAX236 Low
MAX240 High
__
Low High Impedance
__ High Active
X High Impedance
Maxim Integrated | 14
Maxim
Maxim


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