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SP3220UET Datasheet(PDF) 11 Page - Sipex Corporation |
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SP3220UET Datasheet(HTML) 11 Page - Sipex Corporation |
11 / 18 page 11 Rev. 7/8/03 SP3220B/U +3.0 to +5.0V RS-232 Transceivers © Copyright 2003 Sipex Corporation Table 2. Truth Table Logic for Shutdown and Enable Control N D H SN ET U O x TT U O x R 00 e t a t s - i r Te v i t c A 01 e t a t s - i r Te t a t s - i r T 10 e v i t c Ae v i t c A 11 e v i t c Ae t a t s - i r T Receivers The receiver converts EIA/TIA-232 levels to TTL or CMOS logic output levels. The receiver has an inverting high-impedance output. This receiver output (RxOUT) is at high-impedance when the enable control EN = HIGH. In the shutdown mode, the receiver can be active or inactive. EN has no effect on TxOUT. The truth table logic of the SP3220B/U driver and receiver outputs can be found in Table 2. Since receiver input is usually from a transmission line where long cable lengths and system interference can degrade the signal, the inputs have a typical hysteresis margin of 300mV. This ensures that the receiver is virtually immune to noisy transmission lines. Should an input be left unconnected, a 5k Ω pulldown resistor to ground will commit the output of the receiver to a HIGH state. Charge Pump The charge pump is a Sipex–patented design (U.S. 5,306,954) and uses a unique approach compared to older less–efficient designs. The charge pump still requires four external capacitors, but uses a four–phase voltage shifting technique to attain symmetrical 5.5V power supplies. The internal power supply consists of a regulated dual charge pump that provides output voltages 5.5V regardless of the input voltage (V CC) over the +3.0V to +5.5V range. In most circumstances, decoupling the power supply can be achieved adequately using a 0.1 µF bypass capacitor at C5 (refer to Figures 11). In applications that are sensitive to power- supply noise, decouple V CC to ground with a capacitor of the same value as charge-pump capacitor C1. Physically connect bypass capacitors as close to the IC as possible. The charge pumps operate in a discontinuous mode using an internal oscillator. If the output voltages are less than a magnitude of 5.5V, the charge pumps are enabled. If the output voltage exceed a magnitude of 5.5V, the charge pumps are disabled. This oscillator controls the four phases of the voltage shifting. A description of each phase follows. Phase 1 — V SS charge storage — During this phase of the clock cycle, the positive side of capacitors C 1 and C2 are initially charged to VCC. Cl + is then switched to GND and the charge in C 1 – is transferred to C 2 –. Since C 2 + is connected to V CC, the voltage potential across capacitor C 2 is now 2 times V CC. Phase 2 — V SS transfer — Phase two of the clock connects the negative terminal of C 2 to the VSS storage capacitor and the positive terminal of C 2 to GND. This transfers a negative generated voltage to C 3. This generated voltage is regulated to a minimum voltage of -5.5V. Simultaneous with the transfer of the voltage to C 3, the positive side of capacitor C1 is switched to V CC and the negative side is connected to GND. Phase 3 — V DD charge storage — The third phase of the clock is identical to the first phase — the charge transferred in C 1 produces –VCC in the negative terminal of C 1, which is applied to the negative side of capacitor C 2. Since C 2 + is at V CC, the voltage potential across C 2 is 2 times VCC. |
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