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ADM2209EARUZ 数据表(PDF) 8 Page - Analog Devices |
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ADM2209EARUZ 数据表(HTML) 8 Page - Analog Devices |
8 / 12 page REV. 0 ADM2209E –8– GENERAL DESCRIPTION The ADM2209E is a rugged dual-port RS-232 line driver/re- ceiver that operates from a single, +12 V supply, thus removing the need for a –12 V power supply. It contains ten receivers and six drivers, and provides a one-chip solution for both serial ports in desktop or portable personal computers. Features include low power consumption, high transmission rates and compatibility with the EU directive on electromagnetic compatibility. EM compatibility includes protection against radiated and conducted interference including high levels of electrostatic discharge. All RS-232 inputs and outputs contain protection against electrostatic discharges up to ±15 kV and electrical fast tran- sients up to ±2 kV. This ensures compliance to IE1000-4-2 and IEC1000-4-4 requirements. This device is ideally suited for operation in electrically harsh environments or where RS-232 cables are frequently being plugged/unplugged. They are also immune to high RF field strengths without special shielding precautions. Emissions are also controlled to within very strict limits. A novel feature of this device is that one receiver (R5) in each port can be kept active by a low-current, +3 V/+5 V power supply, while the rest of the system is powered down. This al- lows the system to be awakened when peripheral devices begin to communicate with it. CIRCUIT DESCRIPTION The internal circuitry consists of four main sections. These are: 1. A charge pump dc-to-dc converter 2. Logic (3 V/5 V)-to-EIA-232 transmitters 3. EIA-232-to-logic receivers 4. Transient protection circuit on all I-O lines Charge Pump DC-DC Converter The dc-dc converter generates a negative supply voltage from the +12 V supply, thus removing the need for a separate –12 V rail. It consists of an on-chip 200 kHz oscillator, switching ma- trix and two external capacitors, as shown in Figure 20. + + INTERNAL OSCILLATOR GND C2 C1 S1 S2 S3 S4 VDD +12V GND C2– Figure 20. Charge Pump DC-DC Converter When S1 and S2 are closed, S3 and S4 are open, and C1 charges to +12 V. S1 and S2 are then opened, while S3 and S4 are closed to connect C1 across C2, dumping charge into C2. Since the positive terminal of C2 is at ground, a negative voltage will be built up on its negative terminal with each cycle of the oscil- lator. This voltage depends on the current drawn from C2. If the current is small, the voltage will be close to –12 V, but will fall as the current drawn increases. Standby Operation The ADM2209E automatically enters a standby or shutdown mode when the VDD power supply is removed. An on-chip com- parator circuit generates an internal shutdown signal. This sig- nal disables the internal oscillator and hence the charge pump. The inverted output V– goes to GND. All transmitter outputs are disabled and receivers R1 through R4 on each port are three- stated. The remaining receiver on each port (R5) remains fully active. The standby current ISTBY remains less than 200 µA in this mode. VDD (12V) VSTBY (5V) INTERNAL SHUTDOWN SIGNAL Figure 21. Standby Detection Circuit Charge Pump Capacitors And Supply Decoupling For proper operation of the charge pump, the capacitors should have an equivalent series resistance (ESR) less than 1 Ω. As the charge pump draws current pulses from VDD, the VDD decou- pling capacitor should also have low ESR. The V– decoupling capacitor and reservoir capacitor should also have low ESR because they determine how effectively ESD pulses are clamped to VDD or V– by the on-chip clamp diodes. Tantalum or mono- lithic ceramic capacitors are suitable for these components. If using tantalum capacitors, do not forget to observe polarity. Transmitter (Driver) Section The drivers convert 5 V logic input levels into EIA-232 output levels. With VDD = +12 V and driving an EIA-232 load, the output voltage swing is typically ±9 V. Unused inputs may be left unconnected, as an internal 400 k Ω pull-up resistor pulls them high, forcing the outputs into a low state. The input pull-up resistors typically source 10 µA when grounded, so unused inputs should either be connected to VSTBY or left unconnected in order to minimize power consumption. Receiver Section The receivers are inverting level shifters that accept EIA-232 input levels and translate them into 5 V logic output levels. The inputs have internal 5 k Ω pull-down resistors to ground and are also protected against overvoltages of up to ±30 V. The guaran- teed switching thresholds are 0.4 V minimum and 2.4 V maxi- mum. Unconnected inputs are pulled to 0 V by the internal 5k Ω pull-down resistor. This, therefore, results in a Logic 1 output level for unconnected inputs or for inputs connected to GND. The receivers have Schmitt trigger input with a hysteresis level of 0.25 V. This ensures error-free reception for both noisy in- puts and for inputs with slow transition times. HIGH BAUD RATE The ADM2209E features high slew rates permitting data trans- mission at rates well in excess of the EIA-232-E specifications. RS-232 levels are maintained at data rates up to 920 kb/s. This allows for high speed data links between two terminals and, indeed, is suitable for the new generation modem standards. ESD/EFT TRANSIENT PROTECTION SCHEME The ADM2209E uses protective clamping structures on all in- puts and outputs, which clamps the voltage to a safe level and dissipates the energy present in ESD (Electrostatic) and EFT |
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类似说明 - ADM2209EARUZ |
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