SP231ADS/01

SP231A Enhanced RS-232 Line Drivers/Receivers

© Copyright 2000 Sipex Corporation

5

98

R IN

R OUT

2

12

13

R IN

R OUT

1

10

7

T IN

2

T OUT

2

11

14

T IN

1

T OUT

1

15

GND

400k

Ω

400k

Ω

3

1

C +

C -

1

1

6

16

V

CC

V+

+

+

0.1 F

6.3V

µ

+5V to +10V

Voltage Doubler

+5V INPUT

2

V-

2

1

5k

Ω

0.1 F

16V

µ

5k

Ω

5

4

C +

C -

2

2

+

0.1 F

16V

µ

+10V to -10V

Voltage Inverter

0.1 F 6.3V

µ

10 F 6.3V

µ

SP232A

+

+

*

*The negative terminal of the V+ storage capacitor can be tied

is recommended.

Figure 3. Typical Circuits using the SP231A and 232A.

65

R IN

R OUT

2

910

R IN

R OUT

1

7

4

T IN

2

T OUT

2

8

11

T IN

1

T OUT

1

12

GND

400k

Ω

400k

Ω

2

1

C +

C -

1

1

3

13

V

CC

V+

+

+

0.1 F

6.3V

µ

+5V INPUT

14

V-

2

1

5k

Ω

0.1 F

16V

µ

5k

Ω

+12V to -12V

Voltage Inverter

SP231A

V+ (+8.5V to +13.2V)

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

and system 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 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.

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 of these enhanced devices

are fully protected.

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.

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 C

ground. During the next transition of the oscillator

this charge is boot-strapped to transfer charge to

capacitor C

V

In the inverter section (Figure 2), the voltage

across C

transfer charge to C

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 0.1

µF. Because

the charge pump runs at a much higher frequency,

the 0.1

µF capacitors are sufficient to transfer and

sustain charges to the two transmitters.

APPLICATION HINTS

Protection From Shorts to

±15V

The driver outputs are protected against shorts

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

voltages exceeding

±15V,twoback-to-backzener

diodes connected from each output to ground will

clamp the outputs to an acceptable voltage level.