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SP232AEP-L Datasheet(PDF) 4 Page - Exar Corporation |
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SP232AEP-L Datasheet(HTML) 4 Page - Exar Corporation |
4 / 11 page Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-7000 • Fax (50)668-707 • www.exar.com SP232A_00_05308 4 DESCRiPTion DETAiLED DESCRiPTion The SP232A transceiver is a two driver and two receiver device that meets the EIA/TIA- 232 and V.28 serial communication protocol. The device is pin-to-pin compatible with popular industry standard pinouts. The SP232A offers 20kbps data rate, 0V/ μs slew rate and a regulated charge pump that operates from a single 5V supply. The proprietary on-board charge pump generates a regulated output of +/-6.5V for RS- 232 compliant voltage levels. Theory of operation The SP232A is made up of three basic circuit blocks: . Driver, 2. Receiver, and 3. charge pump. Drivers The drivers are inverting level transmitters that convert TTL or CMOS logic levels to EIA/TIA- 232 levels with an inverted sense relative to the input logic levels. Typically, the RS-232 output voltage swing is +/-6.5V with no load and +/- 5.0V minimum when fully loaded. The driver out- puts are protected against infinite short-circuits to ground without degradation in reliability. The drivers can guarantee output data rates of 20Kbps under worst case loading of 3k ohms and 2500pF. The driver output Slew Rate is internally limited to 30V/ μs in order to meet the EIA standards (EIA-232F). Additionally, the driver output LOW to HIGH transition meet the monotonicity output requirements of the standard. Receivers The receivers convert EIA/TIA-232 signal levels to TTL or CMOS logic output levels. Since the input is usually from a transmission line, where long cable length and system interference can degrade the signal, the receiver inputs have a typical hysteresis margin of 500mV. This en- sures that the receiver is virtually immune to noisy transmission lines. Should a input be left unconnected, an internal 5Kohm pull-down re- sistor to ground will commit the output of the re- ceiver to a logic HIGH state. The input voltage range for the SP232A Receiver is +/-5V. Charge pump The charge pump is a patented design and uses a unique approach compared to older less efficient designs. The charge pump requires 4 external capacitors and uses a four phase volt- age shifting technique. The internal power sup- ply consists of a regulated dual charge pump that provides output voltages of +/-6.5V. This is important to maintain compliant RS-232 lev- els regardless of power supply fluctuations. The charge pump operates in a discontinous mode using an internal oscillator. If the voltages are less than a magnitude of 6.5V, the charge pump is enabled. If the output voltage exceed a magnitude of 6.5V then the charge pump is disabled. The internal oscillator controls the four phases of the voltage shifting. A description of each phase follows: Phase 1 Vss charge store and double: The positive ter- minals of capacitors C and C2 are charged from Vcc with their negative terminals initially connected to ground. C+ is then connected to ground and the stored charge from C- is superimposed onto C2-. Since C2+ is still con- nected to Vcc the voltage potential across C2 is now 2 x Vcc. Phase 2 Vss transfer and invert: Phase two connects the negative terminal of C2 to the Vss storage capacitor and the positive terminal of C2 to ground. This transfers the doubled and inverted (V-) voltage onto C4. Meanwhile, capacitor C is charged from Vcc in order to prepare it for its next phase. Phase 3 Vdd charge store and double: Phase three is identical to the first phase. The positive termi- nals of C and C2 are charged from Vcc with their negative terminals initially connected to ground. C+ is then connected to ground and the stored charge from C- is superimposed onto C2-. Since C2+ is still connected to Vcc the voltage potential across capacitor C2 is now 2 x Vcc. |
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