SP3224E Datasheet, PDF (11/23 Pages) Sipex Corporation – 3.0V to 5.5V RS-232 Transceivers with Auto On-Line® Plus

Description
The SP3224E and SP3225E are 2-driver/
2-receiver devices. The SP3226E and
SP3227E are 1-driver/1-receiver devices.
All are ideal for serial ports in embedded,
consumer, portable, or handheld applica-
tions. The transceivers meet the EIA/TIA-
232 and ITU-T V.28/V.24 communication
protocols for reliable serial communication.
The devices feature Exar’s proprietary and
patented (U.S. 5,306,954) on-board charge
pump circuitry that generates ±5.5V RS-
232 voltage levels from a single +3.0V to
+5.5V power supply.
THEORY OF OPERATION
rates fully loaded with 3KΩ in parallel with
1000pF (SP3224E/SP3226E), or 3KΩ in
parallel with 250pF (SP3225E/SP3227E)
ensuring compatibility with PC-to-PC com-
munication software. The slew rate of the
driver output on the SP3224E/SP3226E is
internally limited to a maximum of 30V/μs
in order to meet the EIA standards (EIA
RS-232D 2.1.7, Paragraph 5). The Slew
Rate of SP3225E/SP3227E is not limited
to enable higher speed data transfers. The
transition of the loaded output from HIGH
to LOW also meets the monotonic signal
transitionrequirements of the standard.
The SP3224E-SP3227E are ideal choic-
es for power sensitive designs. With the
Auto On-line^® Plus enabled, the SP3224E-
SP3227E reduce supply current to 1µA
whenever the transceivers are in idle. In
shutdown, the internal charge pump and
the drivers will shut down. This feature al-
lows design engineers to address power
saving concerns without major design
changes.
Theory Of Operation
The SP3224E-SP3227E are made up of
four basic circuit blocks: 1. Drivers, 2. Re-
ceivers, 3. the Exar proprietary charge
pump, and 4. Auto On-line^® Plus circuitry.
Drivers
The drivers are inverting level transmit-
ters that convert TTL or CMOS logic lev-
els to 5.0V EIA/TIA-232 levels with an
inverted sense relative to the input logic
levels. Typically, the RS-232 output volt-
age 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 reliabili-
ty. These drivers comply with the EIA-TIA-
232-F and all previous RS-232 versions.
Unused driver inputs should be connected
to GND or VCC.
The drivers can guarantee output data
Receivers
The receivers convert EIA/TIA-232 signal
levels to TTL or CMOS logic output lev-
els.Receivers remain active during device
shutdown. Since receiver input is usually
from a transmission line where long cable
lengths and system interference can de-
grade 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 5KΩ pulldown re-
sistor to ground will commit the output of
the receiver to a HIGH state.
Charge Pump
The charge pump is a Exar–patented de-
sign (US Patent #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 5.5V regardless of the in-
put voltage (Vcc) over the +3.0V to +5.5V
range. This is important to maintain com-
pliant RS-232 levels regardless of power
supply fluctuations.
The charge pump operates in a discontinu-
ous mode using an internal oscillator. If the
output voltages are less than a magnitude
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
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