RS232 Transceiver with Automatic Power Shutdown Control
Design Note 30
Sean Gold
The LTВ®1180/81 RS232 transceivers with on-chip
charge pumps offer some unique features that greatly
enhance serial interface performance. Like the LT1080
and LT1130 series transceivers, the LT1180 is fully
compliant with all RS232 specifications. The LT1180 is
unique since it utilizes a charge pump which oscillates
at 150kHz to 200kHz – about twice the frequency of the
standard transceivers. In addition to providing excellent
current delivery capability, the high speed charge pump
can operate with storage capacitors as small as 0.1ОјF.
Reducing storage capacitor size to 0.1ОјF shrinks board
space, thereby lowering production costs. Small capacitors also shorten the transceiver turn-on time to
less than 200Ојs, which makes the LT1180 ideal for applications which must address the RS232 transceiver quickly. The interface described here takes advantage
of fast turn-on to reduce power dissipation.
The circuit shown in Figure 1 automatically shuts down
when there is no data flow through the interface. A data
stream on either the RS232 or logic inputs activates
the transceiver. The data must begin with a logic 1
preamble, and the data stream must contain a sufficient number of 1’s to keep the transceiver active. The
preamble may be as short as 50Ојs. Alternatively, the …