Datasheet

Detailed Description
The MAX220–MAX249 contain four sections: dual charge-
pump DC-DC voltage converters, RS-232 drivers, 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 double
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 minimum ±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 Vto
power external circuitry. When these parts are shut down,
V- falls to 0V, and V+ falls to +5V. For applications where
a +10V external supply is applied to the V+ pin (instead of
using the internal charge pump to generate +10V), the C1
capacitor must not be installed and the SHDN pin must be
connected to V
CC
. This is because V+ is internally con-
nected to V
CC
in shutdown mode.
RS-232 Drivers
The typical driver output voltage swing is ±8V when load-
ed 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 minimum driver
output levels under worst-case conditions. These include
a minimum 3kΩ load, V
CC
= +4.5V, and maximum oper-
ating temperature. Unloaded driver output 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 V
CC
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 disabled. Driver
outputs turn off and enter a high-impedance state—where
leakage current is typically microamperes (maximum
25μA)—when in shutdown mode, in three-state mode, or
when device power is removed. Outputs can be driven
to ±15V. The powersupply 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 V
CC
.
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 (V
CC
+ 6V). Below
-0.5V, the transmitter is diode clamped to ground with
1kΩ series impedance. The transmitter is also zener
clamped to approximately V
CC
+ 6V, with a series imped-
ance 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
High
High
X
Low
High
—
High Impedance
Active
High Impedance
MAX225 — — —
Low
High
High Impedance
Active
MAX235
MAX236
MAX240
Low
Low
High
— —
Low
High
X
High Impedance
Active
High Impedance
MAX220–MAX249 +5V-Powered, Multichannel
RS-232 Drivers/Receivers
www.maximintegrated.com
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