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MAX232-ACWE Datasheet(PDF) 14 Page - Maxim Integrated Products |
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MAX232-ACWE Datasheet(HTML) 14 Page - Maxim Integrated Products |
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14 / 38 page  _______________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 5k Ω 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 3k Ω 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 400k Ω 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 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 1k Ω series impedance. The transmitter is also zener clamped to approximately VCC + 6V, with a series impedance of 1k Ω. 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 3 Ω 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. The receiver inputs withstand an input overvoltage up to ±25V and provide input terminating resistors with +5V-Powered, Multichannel RS-232 Drivers/Receivers PART SHDN EN EN(R) RECEIVERS MAX223 __ Low High High X Low High High Impedance Active High Impedance MAX225 __ __ High Impedance Active __ MAX235 MAX236 MAX240 Low Low High __ __ Low High X High Impedance Active High Impedance Table 2. Three-State Control of Receivers Low High SHDN __ 14 Maxim Integrated MAX220–MAX249 MAX220–MAX249 |
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