SP3281EB Datasheet, PDF (7/15 Pages) Sipex Corporation – Intelligent +2.35V to +5.5V RS-232 Transceivers

VCC
+
C5 0.1µF
+
C1 0.1µF
+
C2 0.1µF
VCC
C1+
V+
C1-
C2+ SP3281EB _V-
C2-
LOGIC
INPUTS
TxIN
TxOUT
LOGIC
OUTPUTS
VCC
RxOUT
ONLINE
SHUTDOWN
RxIN
5kΩ
GND
+
C3 0.1µF
C4 0.1µF
+
1000pF
Figure 6. Loopback Test Circuit for RS-232 Driver Data
Transmission Rates
Since receiver input is usually from a transmission
line where long cable lengths and system
interference can degrade the signal, the inputs
have a typical hysteresis margin of 300mV. This
ensures that the receiver is virtually immune to
noisy transmission lines. Should an input be left
unconnected, an internal 5kΩ pulldown resistor to
ground will commit the output of the receiver to a
HIGH state.
Charge Pump
The charge pump is a Sipex–patented design
(U.S. #5,306,954) and uses a unique approach
compared to older less–efficient designs. The
charge pump uses a four–phase voltage
shifting technique to attain symmetrical ±5.5V
power supplies. The internal power supply con-
sists of a regulated dual charge pump that provides
output voltages ±5.5V regardless of the input
voltage (V_CC) over the +3.3V to +5.5V range.
This is important to maintain compliant RS-232
levels regardless of power supply
fluctuations. The charge pump will provide out-
put voltage levels of ±4.0V when the input voltage
(V_CC) is from +3.1V to +2.35V.
The charge pump operates in a discontinuous
mode using an internal oscillator. If the output
voltages are less than a magnitude of 5.5V ( V_CC
> 3.3V ) and 4.0V (V_CC < 3.1V), the charge pump
is enabled. If the output voltages exceed a
magnitude of 5.5V (V_CC > 3.3V) and 4.0V (V_CC <
3.1V), the charge pump is disabled. This oscilla-
tor controls the four phases of the voltage shifting
(Figure 10).
A description of each phase follows.
Phase 1 (Figure 11)
Figure 7. Loopback Test Circuit Result at 120kbps
(All Drivers Fully Loaded)
a) C2+
1
2
2
[
T
T
]
+6V
0V
0V
b) C2-
T
-6V
Ch1 2.00V Ch2 2.00V M 1.00µs Ch1 1.96V
Figure 8. Loopback Test Circuit result at 250kbps
(All Drivers Fully Loaded)
Figure 10. Charge Pump Waveforms
Rev. 6/19/03
SP3281EB Intelligent +2.35V to +5.5V RS-232 Transceivers
7
© Copyright 2003 Sipex Corporation