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SP21111ECT-L Datasheet(PDF) 6 Page - Exar Corporation |
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SP21111ECT-L Datasheet(HTML) 6 Page - Exar Corporation |
6 / 19 page Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com SP207E_00_072309 6 Receiver input thresholds are between .2 to .7 volts typical. This allows the receiver todetectstandardTTLorCMOSlogic-level signals as well as RS-232 signals. If a re- ceiver input is left unconnected or un-driven, a5kΩpulldownresistortogroundwillcommit the receiver to a logic- output state. Highly Efficient Charge–Pump The onboard dual-output charge pump is usedtogeneratepositiveandnegativesignal voltagesfortheRS-232drivers. Thisenables fullycompliantRS-232andV.28signalsfrom a single power supply device. The charge pumps use four external capaci- tors to hold and transfer electrical charge. The Exar–patented design (US Patent #5,306,954) uses a unique approach com- paredtoolder,less–efficientdesigns.The pumps use a four–phase voltage shifting technique to attain symmetrical V+ and V- power supplies. An intelligent control oscil- lator regulates the operation of the charge pump to maintain the proper voltages at maximumefficiency. Phase 1 V SS charge store and double — The positive terminalsofcapacitorsC and C2 are charged from V CC with their negative terminals initially connected to ground. C l+ is then connected to ground and the stored charge from C – is superimposed onto C 2 –. Since C 2+ is still connectedtoV CC the voltage potential across capacitor C 2 is now 2 x VCC. VCC = +5V –5V –5V +5V VSS Storage Capacitor VDD Storage Capacitor C1 C2 C3 C4 + + + + – – – – Phase 2 — V SS transfer and invert — Phase two con- nects the negative terminal of C 2 to the VSS storage capacitor and the positive terminal of C 2 to ground. This transfers the doubled andinverted(V-)voltageontoC 3. Meanwhile, capacitor C charged from VCC to prepare it for its next phase. Phase 3 V DD charge store and double —Phase three isidenticaltothefirstphase.Thepositive terminalsofcapacitorsC and C2 are charged from V CC with their negative terminals initially connected to ground. C l+ is then connected to ground and the stored charge from C – is superimposed onto C 2–. Since C2+ is still connectedtoV CC the voltage potential across capacitor C 2 is now 2 x VCC. Phase 4 V DD transfer — The fourth phase connects the negative terminal of C 2 to ground and the positive terminal of C 2 to the VDD stor- age capacitor. This transfers the doubled (V+) voltage onto C 4. Meanwhile, capacitor C is charged from VCC to prepare it for its next phase. Figure . Charge Pump — Phase VCC = +5V –10V VSS Storage Capacitor VDD Storage Capacitor C1 C2 C3 C4 + + + + – – – – Figure 2. Charge Pump — Phase 2 VCC = +5V –5V –5V +5V VSS Storage Capacitor VDD Storage Capacitor C1 C2 C3 C4 + + + + – – – – Figure 3. Charge Pump — Phase 3 |
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