Date: 10/17/ 05

SP232A Series Enhanced RS232 Transceivers

© Copyright 2005 Sipex Corporation

6

Figure 4. Charge Pump — Phase 1

Figure 5. Charge Pump — Phase 2

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 is a Sipex–patented design

(5,306,954) and uses a unique approach com-

pared to older less–efficient designs. The charge

pump still requires four external capacitors, but

uses a four–phase voltage shifting technique to

attain symmetrical power supplies. There is a

free–running oscillator that controls the four

phases of the voltage shifting. A description of

each phase follows.

Phase 1

— V

the clock cycle, the positive side of capacitors

C

then switched to ground and the charge in C

1

transferred to C

2

2

+5V, the voltage potential across capacitor C

now 10V.

Phase 2

— V

nects the negative terminal of C

storage capacitor and the positive terminal of C

2

to ground, and transfers the generated –l0V to

C

tor C

is connected to ground.

Phase 3

— V

clock is identical to the first phase — the charge

transferred in C

terminal of C

side of capacitor C

voltage potential across C

Phase 4

— V

connects the negative terminal of C

and transfers the generated l0V across C

the V

with this, the positive side of capacitor C

switched to +5V and the negative side is con-

nected to ground, and the cycle begins again.

from V

be symmetrical. Older charge pump approaches

–Vcc

+Vcc

+

+

++

–

–

–

–

–Vcc

Vss

+

+

++

–

–

–

–