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Exar Corporation 48720 Kato Road, Fremont CA, 94538 (50)668-707 www.exar.com
SP3203E_00_2080
DESCRIPTION
The
SP3203Eisa3-driver/2-receiverdevice
ideal for portable or handheld applications.
The
SP3203E transceivers meet the EIA/
TIA-232andITU-TV.28/V.24communication
protocolsandcanbeimplementedinbattery-
powered, portable, or handheld applications
such as notebook or palmtop computers,
PDA'sandcellphones. The
SP3203Edevice
features
Exar's proprietary and patented
(U.S.-- 5,306,954) on-board charge pump
circuitry that generates ±5.5V RS-232 volt-
age levels from a single +3.0V to +5.5V
power supply. The
SP3203E can operate at
a minimum data rate of 250kbps.
THEORY OF OPERATION
The
SP3203E is made up of four basic
circuit blocks:
. Drivers, 2. Receivers, 3. The Exar propri-
etary charge pump, and 4. V
L circuitry.
Drivers
The drivers are inverting level transmitters
thatconvertTTLorCMOSlogiclevelsto5.0V
EIA/TIA-232 levels with an inverted sense
relativetotheinputlogiclevels. Typically,the
RS-232 output voltage swing is +5.4V with
no load and +5V minimum fully loaded. The
driver outputs are protected against infinite
short-circuits to ground without degradation
in reliability. These drivers comply with the
EIA-TIA-232F and all previous RS-232 ver-
sions. The driver output stages are turned
off (High Impedance) when the device is in
shutdown mode.
The drivers can guarantee output data rates
fully loaded with 3k in parallel with 1000pF,
ensuring compatibility with PC-to-PC com-
munication software.
The slew rate of the driver output is internally
limited to a maximum of 30V/s in order
to meet the EIA standards (EIA RS-232D
2..7, Paragraph 5). The transition of the
loaded output from HIGH to LOW also
meets the monotonicity requirements of
the standard.
Figure 2 shows a loopback test circuit used
to test the RS-232 Drivers. Figure 3 shows
the test results with all drivers active at
20kbpswithtypicalRS-232loadsinparallel
with a 000pF capacitors. Figure 4 shows
the test results where one driver was active
at 250kbps and all three drivers loaded with
an RS-232 receiver in parallel with a 000pF
capacitor.The transmitter inputs do not have
pull-up resistors. Connect unused inputs to
ground or V
L.
Receivers
The receivers convert ±5.0V EIA/TIA-232
levels to TTL or CMOS logic output levels.
Receivers are disabled when in shutdown.
The truth table logic of the SP3203E driver
and receiver outputs can be found in Table
.
Since receiver input is usually from a trans-
mission line where long cable lengths and
system interference can degrade the signal,
the inputs have a typical hysteresis margin
of 500mV. This ensures that the receiver
is virtually immune to noisy transmission
lines. Should an input be left unconnected,
an internal 5k pull-down resistor to ground
will commit the output of the receiver to a
HIGH state.
Charge Pump
The charge pump is a patented design (U.S.
5,306,954) and uses a unique approach
compared to older less–efficient designs.
The charge pump still requires four external
capacitors, but uses a four–phase voltage
shifting technique to attain symmetrical
5.5V power supplies. The internal power
supply consists of a regulated dual charge
pump that provides output voltages of
+/-5.5V regardless of input voltage (V
CC)
over the +3.0V to +5.5V range. This
is important to maintain compliant RS-
232 levels regardless of power supply
fluctuations.