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ADM211ARS-REEL Arkusz danych(PDF) 11 Page - Analog Devices |
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ADM211ARS-REEL Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 16 page ADM206–ADM211/ADM213 Rev. C | Page 11 of 16 GENERAL INFORMATION The ADM206–ADM211/ADM213 family of RS-232 drivers/ receivers is designed to solve interface problems by meeting the EIA-232-E specifications while using a single digital 5 V supply. The EIA-232-E standard requires transmitters that will deliver ±5 V minimum on the transmission channel and receivers that can accept signal levels down to ±3 V. The ADM206–ADM211/ ADM213 meet these requirements by integrating step-up voltage converters and level shifting transmitters and receivers onto the same chip. CMOS technology is used to keep the power dissi- pation to an absolute minimum. A comprehensive range of transmitter/receiver combinations is available to cover most communication needs. The ADM206–ADM211/ADM213 are modifications, enhancements, and improvements to the AD230– AD241 family and derivatives thereof. They are essentially plug- in compatible and do not have materially different applications. The ADM206, ADM211, and ADM213 are particularly useful in battery-powered systems because they feature a low power shut- down mode that reduces power dissipation to less than 5 µW. The ADM209 includes only a negative charge pump converter and is intended for applications where a +12 V is available. To facilitate sharing a common line or for connection to a microprocessor data bus, the ADM206, the ADM209, the ADM211, and the ADM213 feature an enable (EN) function. When the receivers are disabled, their outputs are placed in a high impedance state. CIRCUIT DESCRIPTION The internal circuitry in the ADM206–ADM211/ADM213 consists of three main sections: (a) a charge pump voltage converter; (b) RS-232-to-TTL/CMOS receivers; and (c) TTL/CMOS-to-RS-232 transmitters. Charge Pump DC-to-DC Voltage Converter The charge pump voltage converter consists of an oscillator and a switching matrix. The converter generates a ±10 V supply from the 5 V input. This is done in two stages using a switched capacitor technique, as illustrated in Figure 22 and Figure 23. First, the 5 V input supply is doubled to 10 V using capacitor C1 as the charge storage element. The 10 V level is then inverted to generate –10 V using C2 as the storage element. + + VCC GND S1 S2 C1 S3 S4 C3 VCC V+ = 2VCC INTERNAL OSCILLATOR Figure 22. Charge Pump Voltage Doubler + + V+ GND S1 S2 C2 S3 S4 C4 GND V– = –(V+) INTERNAL OSCILLATOR FROM VOLTAGE DOUBLER Figure 23. Charge Pump Voltage Inverter Capacitors C3 and C4 are used to reduce the output ripple. Their values are not critical and can be reduced if higher levels of ripple are acceptable. The charge pump capacitors C1 and C2 may also be reduced at the expense of higher output impedance on the V+ and V– supplies. The V+ and V– supplies may also be used to power external circuitry if the current requirements are small. Transmitters (Drivers) The drivers convert TTL/CMOS input levels into EIA-232-E output levels. With VCC = +5 V and driving a typical EIA-232-E load, the output voltage swing is ±9 V. Even under worst-case conditions, the drivers are guaranteed to meet the ±5 V EIA- 232-E minimum requirement. The input threshold levels are both TTL- and CMOS-compatible with the switching threshold set at VCC/4. With a nominal VCC = 5 V, the switching threshold is 1.25 V typical. Unused inputs may be left unconnected, because an internal 400 kΩ pull-up resistor pulls them high, forcing the outputs into a low state. As required by the EIA-232-E standard, the slew rate is limited to less than 30 V/µs, without the need for an external slew limiting capacitor, and the output impedance in the power-off state is greater than 300 Ω. Receivers The receivers are inverting level shifters that accept EIA-232-E input levels (±5 V to ±15 V) and translate them into 5 V TTL/ CMOS levels. The inputs have internal 5 kΩ pull-down resistors to ground and are also protected against overvoltages of up to ±30 V. The guaranteed switching thresholds are 0.8 V minimum and 2.4 V maximum, well within the ±3 V EIA-232-E require- ment. The low level threshold is deliberately positive, since it ensures that an unconnected input will be interpreted as a low level. The receivers have Schmitt-trigger inputs with a hysteresis level of 0.65 V. This ensures error-free reception for both noisy inputs and inputs with slow transition times. Shutdown (SD) The ADM206–ADM211/ADM213 feature a control input that may be used to disable the part and reduce the power consump- ion to less than 5 ΩW. This is very useful in battery-operated systems. During shutdown, the charge pump is turned off, the transmitters are disabled, and all receivers except R4 and R5 on the ADM213 are put into a high impedance disabled state. |
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