As shown in Figure 1, an internal oscillator trig-
gers the charge accumulation and voltage inver-
sion. The voltage doubler momentarily stores a
charge on capacitor C1 equal to Vcc, referenced to
ground. During the next transition of the oscillator
this charge is boot-strapped to transfer charge to
capacitor C3. The voltage across C3 is now from
Vcc to V+.
Receivers
The receivers convert RS-232 input signals to
inverted TTL signals. Since the input is usually
from a transmission line, where long cable lengths
andsysteminterferencecandegradethesignal,the
inputs have a typical hysteresis margin of 500mV.
This ensures that the receiver is virtually immune
to noisy transmission lines.
In the inverter section (Figure 2), the voltage
across C3 is transferred to C2 forcing a range of 0V
to V+ across C2. Boot-strapping of C2 will then
transfer charge to C4 to genrate V-.
The input thresholds are 0.8V minimum and
2.4V maximum, again well within the ±3V
RS-232 requirements. The receiver inputs are also
protected against voltages up to ±30V. Should an
input be left unconnected, a 5kOhm pulldown
resistor to ground will commit the output of the
receiver to a high state.
One of the significant enhancements over
previous products of this type is that the values of
the capacitors are no longer critical and have been
decreased in size considerably to 0.1µF. Because
the charge pump runs at a much higher frequency,
the 0.1µF capacitors are sufficient to transfer and
sustain charges to the two transmitters.
In actual system applications, it is quite possible
for signals to be applied to the receiver inputs
before power is applied to the receiver circuitry.
Thisoccurs, forexample, whenaPCuserattempts
toprint,onlytorealizetheprinterwasn’tturnedon.
In this case an RS-232 signal from the PC will
appear on the receiver input at the printer. When
the printer power is turned on, the receiver will
operate normally. All of these enhanced devices
are fully protected.
APPLICATION HINTS
Protection From Shorts to ±15V
The driver outputs are protected against shorts
to ground, other driver outputs, and V+ or V-.
If the possibility exists that the outputs could be
inadvertently connected to voltages higher than
±15V, then it is recommended that external
protection be provided. For protection against
voltagesexceeding±15V,twoback-to-backzener
diodes connected from each output to ground will
clamp the outputs to an acceptable voltage level.
Charge Pump
The charge pump section of the these devices
allows the circuit to operate from a single +5V
±10% power supply by generating the required
operating voltages internal to the devices. The
charge pump consists of two sections — 1) a
voltage doubler and 2) a voltage inverter.
+5V INPUT
10
+
µF 6.3V
+5V INPUT
V+ (+8.5V to +13.2V)
+
16
1
0.1µF 6.3V
V
C
C
C
C
CC
13
+
+
1
2
6
+
0.1
6.3V
µF
1
2
V+
V-
+5V to +10V
Voltage Doubler
*
V
C
C
+
-
3
4
CC
-
+
µF
1
1
14
0.1
6.3V
V+
+12V to -12V
Voltage Inverter
1
+
-
0.1
16V
µ
F
2
+
0.1
16V
µF
0.1
16V
µ
F
+10V to -10V
Voltage Inverter
5
2
3
+
V-
400k
Ω
Ω
11
14
400k
Ω
Ω
T
T
IN
T
OUT
T
1
1
1
8
11
400k
T
T
IN
T
OUT
T
1
1
1
10
12
7
400k
IN
T
OUT
IN
T
2
2
1
2
7
9
4
IN
T
OUT
IN
T
2
2
1
2
13
R
R
OUT
OUT
R
R
1
2
1
10
5kΩ
5kΩ
R
R
OUT
OUT
R
R
1
2
1
9
8
5kΩ
5kΩ
R
IN
R
2
2
6
5
R
IN
R
2
SP232A
2
SP231A
GND 15
GND 12
*The negative terminal of the V+ storage capacitor can be tied
to either VCC or GND. Connecting the capacitor to VCC (+5V)
is recommended.
Figure 3. Typical Circuits using the SP231A and 232A.
SP231ADS/01
SP231A Enhanced RS-232 Line Drivers/Receivers
© Copyright 2000 Sipex Corporation
5
SIPEX [ SIPEX CORPORATION ]
