MAX244EQH-D中文资料(14)

+5V-Powered, Multichannel RS-232Drivers/ReceiversMAX220–MAX249

_______________Detailed Description

The MAX220–MAX249 contain four sections: dualcharge-pump DC-DC voltage converters, RS-232 dri-vers, RS-232 receivers, and receiver and transmitterenable control inputs.

mode, in three-state mode, or when device power isremoved. 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 theoutputs of the unused drivers low. Connect unusedinputs to GND or VCC.

The MAX239 has a receiver three-state control line, andthe MAX223, MAX225, MAX235, MAX236, MAX240,and MAX241 have both a receiver three-state controlline and a low-power shutdown control. Table 2 showsthe 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 enableline is high (for the MAX225/MAX235/MAX236/MAX239–MAX241), and are also high-impedance whenever theshutdown control line is high.

When in low-power shutdown mode, the driver outputsare turned off and their leakage current is less than 1µAwith the driver output pulled to ground. The driver outputleakage remains less than 1µA, even if the transmitteroutput is backdriven between 0V and (VCC+ 6V). Below-0.5V, the transmitter is diode clamped to ground with1kΩseries impedance. The transmitter is also zenerclamped to approximately VCC+ 6V, with a seriesimpedance of 1kΩ.

The driver output slew rate is limited to less than 30V/µsas required by the EIA/TIA-232E and V.28 specifica-tions. Typical slew rates are 24V/µs unloaded and10V/µs loaded with 3Ωand 2500pF.

Dual Charge-Pump Voltage Converter

The MAX220–MAX249 have two internal charge-pumpsthat convert +5V to ±10V (unloaded) for RS-232 driveroperation. The first converter uses capacitor C1 to dou-ble the +5V input to +10V on C3 at the V+ output. Thesecond 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 Characteristicssection),except on the MAX225 and MAX245–MAX247, wherethese 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 whensourcing 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 shutdown, 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 andthe SHDNpin must be tied to VCC. This is because V+is internally connected to VCCin shutdown mode.

RS-232 Receivers

EIA/TIA-232E and V.28 specifications define a voltagelevel greater than 3V as a logic 0, so all receivers invert.Input thresholds are set at 0.8V and 2.4V, so receiversrespond to TTL level inputs as well as EIA/TIA-232E andV.28 levels.

The receiver inputs withstand an input overvoltage upto ±25V and provide input terminating resistors with

RS-232 Drivers

The typical driver output voltage swing is ±8V whenloaded with a nominal 5kΩRS-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 3kΩload, VCC= +4.5V, andmaximum 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 unconnectedsince 400kΩinput pullup resistors to VCCare built in(except for the MAX220). The pullup resistors force theoutputs 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 typicallymicroamperes (maximum 25µA)—when in shutdown

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Table 2. Three-State Control of Receivers