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ADM3251E 数据表(PDF) 11 Page - Analog Devices |
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ADM3251E 数据表(HTML) 11 Page - Analog Devices |
11 / 16 page ADM3251E Rev. 0 | Page 11 of 16 THEORY OF OPERATION The ADM3251E is a high speed, 2.5 kV fully isolated, single- channel RS-232 transceiver device that operates from a single power supply. The internal circuitry consists of the following main sections: • Isolation of power and data • A charge pump voltage converter • A 5.0 V logic to EIA/TIA-232E transmitter • A EIA/TIA-232E to 5.0 V logic receiver DECODE RECT REG V– C4 0.1µF 16V VOLTAGE DOUBLER C1+ C1– V+ VISO C2+ C2– R T VOLTAGE INVERTER VCC ROUT TIN GND GNDISO RIN* TOUT ADM3251E OSC ENCODE ENCODE DECODE *5kΩ PULL-DOWN RESISTOR ON THE RS-232 INPUT. 0.1µF C3 0.1µF 10V C2 0.1µF 16V 0.1µF C1 0.1µF 16V Figure 13. Functional Block Diagram ISOLATION OF POWER AND DATA The ADM3251E incorporates a dc-to-dc converter section, which works on principles that are common to most modern power supply designs. VCC power is supplied to an oscillating circuit that switches current into a chip-scale air core transformer. Power is transferred to the secondary side, where it is rectified to a high dc voltage. The power is then linearly regulated to about 5.0 V and supplied to the secondary side data section and to the VISO pin. The VISO pin should not be used to power external circuitry. Because the oscillator runs at a constant high frequency independent of the load, excess power is internally dissipated in the output voltage regulation process. Limited space for transformer coils and components also adds to internal power dissipation. This results in low power conversion efficiency. The ADM3251E can be operated with the dc-to-dc converter enabled or disabled. The internal dc-to-dc converter state of the ADM3251E is controlled by the input VCC voltage. In normal operating mode, VCC is set between 4.5 V and 5.5 V and the internal dc-to-dc converter is enabled. When/if it is desired to disable the dc-to-dc converter, lower VCC to a value between 3.0 V and 3.7 V. In this mode, the user must externally supply isolated power to the VISO pin. An isolated secondary side voltage of between 3.0 V and 5.5 V and a secondary side input current, IISO, of 12 mA (maximum) is required on the VISO pin. The signal channels of the ADM3251E then continue to operate normally. The TIN pin accepts TTL/CMOS input levels. The driver input signal that is applied to the TIN pin is referenced to logic ground (GND). It is coupled across the isolation barrier, inverted, and then appears at the transceiver section, referenced to isolated ground (GNDISO). Similarly, the receiver input (RIN) accepts RS-232 signal levels that are referenced to isolated ground. The RIN input is inverted and coupled across the isolation barrier to appear at the ROUT pin, referenced to logic ground. The digital signals are transmitted across the isolation barrier using iCoupler technology. Chip-scale transformer windings couple the digital signals magnetically from one side of the barrier to the other. Digital inputs are encoded into waveforms that are capable of exciting the primary transformer of the winding. At the secondary winding, the induced waveforms are decoded into the binary value that was originally transmitted. There is hysteresis in the VCC input voltage detect circuit. Once the dc-to-dc converter is active, the input voltage must be decreased below the turn-on threshold to disable the converter. This feature ensures that the converter does not go into oscillation due to noisy input power. |
类似零件编号 - ADM3251E_1 |
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类似说明 - ADM3251E_1 |
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