SP3223B Datasheet, PDF (17/24 Pages) Sipex Corporation – Intelligent +3.0V to +5.5V RS-232 Transceivers

AUTO ON-LINE^® Circuitry
The SP3223B and SP3243B devices have a
patent pending AUTO ON-LINE^® circuitry on
board that saves power in applications such as
laptop computers, palmtop (PDA) computers,
and other portable systems.
The SP3223B and SP3243B devices incorporate
an AUTO ON-LINE^® circuit that automatically
enables itself when the external transmitters are
enabled and the cable is connected. Conversely,
the AUTO ON-LINE^® circuit also disables most
of the internal circuitry when the device is not
being used and goes into a standby mode where
the device typically draws 1µA. This function
can also be externally controlled by the ONLINE
pin. When this pin is tied to a logic LOW, the
AUTO ON-LINE^® function is active. Once
active, the device is enabled until there is no
activity on the receiver inputs. The receiver
input typically sees at least ±3V, which are
generated from the transmitters at the other end
of the cable with a ±5V minimum. When the
external transmitters are disabled or the cable is
disconnected, the receiver inputs will be pulled
down by their internal 5kΩ resistors to ground.
When this occurs over a period of time, the
internal transmitters will be disabled and the
device goes into a shutdown or standby mode.
When ONLINE is HIGH, the AUTO ON-LINE^®
mode is disabled.
The AUTO ON-LINE^® circuit has two stages:
1) Inactive Detection
2) Accumulated Delay
The first stage, shown in Figure 28, detects an
inactive input. A logic HIGH is asserted on
R_XINACT if the cable is disconnected or the
external transmitters are disabled. Otherwise,
R_XINACT will be at a logic LOW. This circuit is
duplicated for each of the other receivers.
The second stage of the AUTO ON-LINE^® cir-
cuitry, shown in Figure 29, processes all the
receiver's R_XINACT signals with an accumu-
lated delay that disables the device to a 1µA
supply current.
The STATUS pin goes to a logic LOW when the
cable is disconnected, the external transmitters
are disabled, or the SHUTDOWN pin is
invoked. The typical accumulated delay is
around 20µs.
When the SP3223B and SP3243B drivers or
internal charge pump are disabled, the supply
current is reduced to 1µA. This can commonly
occur in hand-held or portable applications where
the RS-232 cable is disconnected or the RS-232
drivers of the connected peripheral are turned off.
The AUTO ON-LINE^® mode can be disabled by
the SHUTDOWN pin. If this pin is a logic LOW,
the AUTO ON-LINE^® function will not operate
regardless of the logic state of the ONLINE pin.
Table 3 summarizes the logic of the AUTO ON-
LINE^® operating modes. The truth table logic of
the SP3223B and SP3243B driver and receiver
outputs can be found in Table 2.
The STATUS pin outputs a logic LOW signal
if the device is shutdown. This pin goes to a
logic HIGH when the external transmitters are
enabled and the cable is connected.
When the SP3223B and SP3243B devices
are shut down, the charge pumps are turned off.
V+ charge pump output decays to V_CC, the
V- output decays to GND. The decay time will
depend on the size of capacitors used for the
charge pump. Once in shutdown, the time
required to exit the shut down state and have
valid V+ and V- levels is typically 200µs.
For easy programming, the STATUS can be
used to indicate DTR or a Ring Indicator signal.
Tying ONLINE and SHUTDOWN together
will bypass the AUTO ON-LINE^® circuitry so
this connection acts like a shutdown input pin.
Rev. 6/30/03
SP3223B/3243B +3.0V to +5.5V RS-232 Transceivers
17
© Copyright 2003 Sipex Corporation