standards [EIA RS-232–D 2.1.7, Paragraph (5)]. The
transition of the loaded output from V
OL
to V
OH
clearly
meets the monotonicity requirements of the standard
[EIA RS-232–D 2.1.7, Paragraphs (1) & (2)].
Receivers
The receivers convert RS-232 input signals to in-
verted TTL signals. Since the input is usually from a
transmission line, where long cable lengths and sys-
tem 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.
The input thresholds are 0.8V minimum and 2.4V
maximum, again well within the
±3V
RS-232 re-
quirements. The receiver inputs are also protected
against voltages up to
±30V.
Should an input be left
unconnected, a 5kΩ pulldown resistor to ground will
commit the output of the receiver to a high state.
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. This occurs,
for example, when a PC user attempts to print, only to
realize the printer wasn’t turned on. 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 series
devices are fully protected. Again, to facilitate use in
“real-world” applications, the receiver outputs can be
tri–stated by bringing the ENABLE (EN) pin high,
with the driver remaining full active.
Charge Pump
The charge pump section of the
SP230A
series allows
the circuit to operate from a single +5V,
±10%
power
10.5
10.0
9.5
9.0
8.5
V+ (Abs.)
S1
S3
V+ = 2V
CC
+
+
S4
V
CC
S2
C1
C3
GND
V
CC
INTERNAL
OSCILLATOR
Figure 1. Charge Pump Voltage Doubler
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.
As shown in
Figure 1,
an internal oscillator
triggers the charge accumulation and voltage
inversion. The voltage doubler momentarily
stores a charge on capacitor C
1
equal to V
CC
,
reference to ground. During the next transition
of the oscillator this charge is boot–strapped to
transfer charge to capacitor C
3
. The voltage
across C
3
is now from V
CC
to V
+
.
In the inverter section (Figure
2),
the voltage
across C
3
is transferred to C
2
forcing a range of
0V to V
+
across C
2
. Boot–strapping of C
2
will
then transfer charge to C
4
to generate V
-
.
The values of the capacitors are somewhat
non-critical and can be varied, however the
performance will be affected. As C
3
and C
4
are
reduced, higher levels of ripple will appear.
Lower values of C
1
and C
2
will increase the
10.5
10.0
9.5
9.0
8.5
V- (Abs.)
8.0
7.5
7.0
6.5
6.0
5.5
5.0
0
5
10
15
20
25
V =5.5V
CC
8.0
7.5
7.0
6.5
V =5V
CC
V
CC
=4.5V
V =5.5V
CC
V =5V
CC
V
CC
=4.5V
6.0
5.5
5.0
30
35
40
0
5
10
15
20
25
30
35
40
a)
V+ I
OUT
(mA)
b)
V- I
OUT
(mA)
Charge Pump Output Loading versus VCC; a) V
+
; b) V
–
Date: 8/3/04
+ 5V Powered Multi-Channel RS-232 Drivers/Receivers
© Copyright 2004 Sipex Corporation
7
SIPEX [ SIPEX CORPORATION ]
