AN654 データシートの表示（PDF） - Maxim Integrated
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AN654
Maxim Integrated
AN654 Datasheet PDF : 8 Pages
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Figure 1. The MAX3241 (with voltage doubler) consumes only half as much as power as does the
competitive device based on a voltage tripler. Note also, the MAX3241 maintains valid RS-232
output levels at quadruple the data rate.
RS-232 drivers must also supply output current for driving the input resistance (3k to 7k ) associated
with the RS-232 receiver at the far end of the line, and for charging and discharging the load capacitance
(up to 2.5nF, as specified by the RS-232 standard). This charge/discharge current increases with frequency,
and exceeds the resistive current at a data rate of 80k bits/sec (40kHz). Thus, a voltage doubler at high data
rates saves even more power.
Auto-Shutdown—The Ideal RS-232 IC
The RS-232 port in most portable systems transmits and receives for only a small percentage of the time
that power is applied; for the rest of the time it may waste power needlessly. An ideal RS-232 transceiver,
therefore, should shut itself down when not transmitting or receiving.
Early RS-232 ICs for portable systems provided a shutdown pin, but the result was complete shutdown
(deep sleep) in which the chip had no way to detect incoming data. So, the next step was to provide
receivers that remained active during shutdown.
The operating system can theoretically shut down an RS-232 port if, after a suitable delay, it sees no
incoming data transitions or status-line changes. But the choice of delay period presents a problem-you can
miss data if you happen to power down just as a data burst begins, and you'll probably miss some of the
data that wakes up the system and initiates power-up. For these reasons, designers seldom go to the trouble
of introducing a monitoring delay by rewriting the BIOS/operating system.
Maxim engineers had the following goals in designing a new RS-232 transceiver:
1) Use power only when transmitting and receiving data.
2) Meet goal #1 with no compromise in performance.
3) Meet goal #1 with no increase in cost.
An obvious approach is to include a timer that shuts down the IC after a desired time interval. But this
thwarts goal #3 by increasing the die area. The better solution is to monitor all incoming data lines for valid
levels of RS-232 signal voltage. All receiver inputs will be near ground, for example, if the RS-232 port is
not connected or if the far-end transceiver is turned off. Either way, the absence of valid signal levels
causes the chip to enter its shutdown mode automatically, reducing the typical no-load supply current to
1µA.
Maxim has recently introduced four devices with automatic shutdown (Table 2). Most include an output
(valid RS-232) that indicates to the system processor whether an active RS-232 port is connected at the
other end of the cable. The MAX3212 goes one step further; it includes a transition-detect circuit whose
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