AN654 データシートの表示（PDF） - Maxim Integrated

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AN654

New RS-232 ICs Feature 1µA Supply Current, ±15kV ESD Protection, and 3V Operation

Maxim Integrated 

Also simplifying applications is an internal, digitally controlled switch that transforms a Maxim RS-232

transceiver from a DTE port (Data Terminal Equipment) to a DCE port (Data Communications

Equipment).

The Move to 3V Operation

The standard supply voltage for notebook computers and other portable equipment is rapidly changing to

3V. To meet the needs of this market, many 5V RS-232 devices have been recharacterized for 3V

operation. While these parts do not generate the ±5V output swings required by RS-232 communications,

they do meet the EIA/TIA-562 requirements of ±3.7V output swings. EIA/TIA-562 is interoperable with

RS-232, although its output voltage is not sufficient to power a mouse, whose microcontroller typically

requires 5mA at 5V.

To overcome the limitations of these recharacterized devices, Maxim has developed the MAX3241 family

of 3V transceivers, which feature a low quiescent current, the capability to drive a mouse, a low-power

standby mode in which some (or all) receivers are active, a flow-through pinout, and operation to 230kbaud

(to support high-speed modems).

Unique Output Stage Uses 50% Less Power

Maxim's key innovation in developing 3V parts is a driver output structure with very low voltage drop from

input to output. Low voltage drop is important because the ideal DC/DC converter for 3.3V RS-232

transceivers is a capacitive voltage doubler. A perfect doubler would produce 6V for 3V minimum inputs,

leaving a drop of just 1V for losses in the driver output stage and the DC/DC converter itself.

Moreover, the output swing for an ideal RS-232 transceiver would be ±5V with a tolerance of zero. A

minimum of ±5V is needed to comply with the RS-232 specification, but any swing above 5V or below -

5V simply wastes power. Regardless of input voltage, therefore, members of the MAX3241 family regulate

their internal, voltage-doubling DC/DC converter to 5.4V-just enough to provide a safety margin after

covering the 200mV drop in the driver output stage. The result is minimal power consumption at the

nominal 3.3V supply rail.

An ideal (lossless) capacitive voltage doubler, unregulated, produces 6.6V with a 3.3V input and 10V with

a 5V input. Thus, an RS-232 transceiver with internal 5V doubler wastes the 5V difference between its

output (10V) and the desired ±5V as specified by the RS-232 standard. An internal 3.3V doubler, which

wastes only 1.6V, is therefore much more efficient.

Similarly, an ideal 3.3V capacitive tripler generates 9.9V. The desired output is 5V, so the overall

efficiency is only 5/9.9 (51%). Another way to compare the 3.3V doubler with the 3.3V tripler is to note

that, for every 1mA drawn by the RS-232 load, the doubler draws 2mA (from the 3.3V supply) while the

tripler must draw 3mA. Thus, the power saved by a 3.3V doubler is even greater when driving the

capacitive load of a long RS-232 cable at high speed (Figure 1).

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