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ADM242AN Datasheet(PDF) 7 Page - Analog Devices |
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ADM242AN Datasheet(HTML) 7 Page - Analog Devices |
7 / 9 page ADM222/ADM232A/ADM242 REV. 0 –7– GENERAL INFORMATION The ADM222/ADM232A/ADM242 are high speed RS-232 drivers/receivers requiring a single digital +5 V supply. The RS- 232 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 parts achieve this by integrating step up voltage converters and level shifting trans- mitters and receivers onto the same chip. CMOS technology is used to keep the power dissipation to an absolute minimum. All devices contains an internal charge pump voltage doubler and a voltage inverter that generates ±10 V from the +5 V input. Four external 0.1 µF capacitors are required for the internal charge pump voltage converter. The ADM222/ADM232A/ADM242 is a modification, enhance- ment and improvement to the AD230-AD241 family and derivatives thereof. It is essentially plug-in compatible and does not have materially different applications. CIRCUIT DESCRIPTION The internal circuitry consists of four main sections. These are: A Charge Pump Voltage Converter TTL/CMOS to RS-232 Transmitters RS-232 to TTL/CMOS Receivers Enable and Shutdown Functions. Charge Pump DC-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 input 5 V level. This is done in two stages using a switched capacitor technique. The 5 V input supply is doubled to 10 V using capacitor C1 as the charge storage element. The –10 V level is also generated from the input 5 V supply using C1 and C2 as the storage elements. 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. Please refer to the typical performance characteristics which shows the V+, V– output voltage vs. current. In the shutdown mode the charge pump is disabled and V+ decays to V CC while V– decays to 0 V. Transmitter (Driver) Section The Drivers convert TTL/CMOS input levels into RS-232 output levels. With V CC = +5 V and driving a typical RS-232 load, the output voltage swing is ±9 V. Even under worst case conditions the drivers are guaranteed to meet the ±5 V RS-232 minimum requirement. The input threshold levels are both TTL and CMOS compat- ible with the switching threshold set at V CC/4. With a nominal V CC = 5 V the switching threshold is 1.25 V typical. 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. As required by the RS-232 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. Receiver Section The receivers are inverting level shifters which accept RS-232 input levels ( ±3 V to ±15 V) and translate them into 5 V TTL/ CMOS 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 guaranteed switching thresholds are 0.8 V mini- mum and 2.4 V maximum which are well within the ±3 V RS- 232 requirement. The low level threshold is deliberately positive as it ensures that an unconnected input will be interpreted as a low level. The receivers have Schmitt trigger input with a hysteresis level of 0.5 V. This ensures error free-reception for both noisy inputs and for inputs with slow transition times Enable and Shutdown Functions On the ADM222, both receivers are fully disabled during shutdown. On the ADM242, both receivers continue to operate normally. This function is useful for monitoring activity so that when it occurs, the device can be taken out of the shutdown mode. The ADM242 also contains a receiver enable function (EN) which can be used to fully disable the receivers, independent of SHDN . APPLICATIONS INFORMATION A selection of typical operating circuits is shown in Figures 13 to 19. Figure 19. Transmitter Output Disable Timing 10 90 100 0% 5V A1 5µS 2.0V 5V |
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