DUAL RS-232 TRANSMITTER/
RECEIVER AND POWER SUPPLY
TC232
DETAILED DESCRIPTION
The TC232 contains a +5V to
±10V
dual charge pump
voltage converter, a dual transmitter and a dual receiver.
OUTPUT VOLTAGE (|V|)
V
+
, V
–
Output Voltages vs. Load Current
10
9
8
7
6
5
CONDITIONS:
4 A) TA = +25°C
B) TRANSMITTER OUTPUTS
OPEN CIRCUIT
3
0
1 2 3 4 5 6 7 8
|I
| (mA)
LOAD
V–
(VCC = 4.5V)
V+
(VCC = 4.5V)
+5V to
±
10V Dual Charge Pump
Voltage Converter
The TC232 power supply consists of two charge pumps.
One uses external capacitor C1 to double the +5V input to
+10V, with output impedance of about 200Ω. The other
uses C2 to invert +10V to – 10V, with overall output
impedance of 450Ω (including effects of +5V to +10V
doubler impedance).
The clock in the doubler circuit will start at
≈4.2V
in the
typical part, but external loads may make this point rise to as
high as 4.5V with a load of 2 kΩ on each of the two output
voltages.
Because of this, use of the doubler and inverter to run
additional external circuits should be limited. The maximum
current should be no more than 2.5 mA from the +10V and
- 10V. in order to guarantee start-up of the doubler clock.
The test circuit employs 22
µF
capacitors for C1 to C4,
but the value is not critical. These capacitors usually are low-
cost aluminum or tantalum electrolytic capacitors.
Increasing C1 and C2 to 47
µF
lowers the output
impedance of the +10V doubler and the - 10V inverter by
the change in the ESR of the capacitors.
Increasing C3 and C4 lowers ripple on the
±10V
out-
puts and 16 kHz ripple on the RS-232 outputs. Where size
is critical, the value of C1 to C4 can be lowered to 1
µF.
The
use of a low ESR capacitor will help lower the output ripple
and keep the output impedance of the
±10V
as low as
possible.
V+
(VCC = 5V)
V–
(VCC = 5V)
9
10
The outputs are protected and can be short-circuited to
ground indefinitely.
Dual Receiver
TC232 receivers meet RS-232 input specifications.
Input impedance is between 3 kΩ and 7 kΩ. Switching
thresholds are within the
±3V
limits, and the receivers
withstand up to
±30V
inputs. RS-232 and TTL/CMOS input
compatible, the receivers have 0.8V V
IL
and 2.4V V
IH
with
0.5V hysteresis to reject noise.
The TTL/CMOS compatible receiver output is LOW
when an RS-232 input is greater than 2.4V. It is HIGH
when an input is floating or between +0.8V and – 30V.
TEST CIRCUIT
TC232
C1 +
22
µF
22
µF
+
C3
3
C2
22
µF
+
4
5
6
7
3 kΩ
8
RS-232
INPUT
±30V
R2 IN
1
2
+
C1
V+
–
C1
+
C2
–
C2
V–
T2 OUT
+4.5V TO +5.5V
INPUT
V CC 16
GND 15
3 kΩ
T1OUT 14
R1IN 13
R1OUT 12
T1 IN 11
T2 IN 10
R2OUT
9
RS-232
OUTPUT
RS-232
INPUT
±30V
TTL/CMOS OUTPUT
TTL/CMOS INPUT
TTL/CMOS
INPUT
TTL/CMOS OUTPUT
Dual Transmitter
TC232 transmitters are CMOS inverters driven by
±10V
internally-generated voltages. The input is TTL/CMOS com-
patible, with a logic threshold of about 26% of V
CC
(1.3V for
5V V
CC
). The input of an unused transmitter can be left
unconnected, since an internal 400 kΩ pull-up resistor
connected between the transmitter input and V
CC
pulls the
input HIGH and forces the unused transmitter output to the
LOW state.
With V
CC
at 5V, the outputs will go from (V+ – 0.6V) to
–
with no load and will swing
±9V
when loaded with 3 kΩ.
V
The minimum output voltage swing, with V
CC
at 4.5V and at
maximum ambient temperature, is
±5V.
This conforms to
RS-232 specifications for "worst-case" conditions.
EIA/TIA RS-232E specs limit the slew rate at output
to less than 30V/µs.
The powered-down output impedance (V
CC
= 0V) is
a minimum of 300Ω with
±2V
applied to outputs.
© 2001 Microchip Technology Inc.
DS21396A
+
RS-232
OUTPUT
C4
22
µF
3
TC232-6 10/21/96
MICROCHIP [ MICROCHIP TECHNOLOGY ]
