SP334DS/10

Programmable RS-232/RS-485 Transceiver

© Copyright 2000 Sipex Corporation

8

THEORY OF OPERATION

The SP334 is made up of four separate circuit

blocks — the charge pump, drivers, receivers,

and decoder. Each of these circuit blocks is

described in more detail below.

Charge–Pump

The charge pump is a Sipex–patented design

(U.S. 5,306,954) and uses a unique approach

compared to older less–efficient designs. The

charge pump still requires four external

capacitors, but uses a four–phase voltage

shifting technique to attain symmetrical 10V

power supplies. Figure 17(a) shows the

waveform found on the positive side of capcitor

C2, and figure 17(b) shows the negative side of

capcitor C2. 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 charge on C

1

transferred to C

2

2

+5V, the voltage potential across capacitor C

2

is now 10V.

Phase 2

— V

connects 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

clock connects the negative terminal of C

ground and transfers the generated l0V across

C

simultaneously with this, the positive side of

Figure 13. Charge Pump Phase 1.

Figure 14a. Charge Pump Phase 2.

capacitor C

side is connected to ground, and the cycle begins

again.

from V

be symmetrical. Older charge pump approaches

the inherent inefficiencies in the design.

The clock rate for the charge pump typically

operates at 15kHz. The external capacitors

must be a minimum of 0.1

µF with a 16V

breakdown rating.

External Power Supplies

For applications that do not require +5V only,

external supplies can be applied at the V+ and

ages must be no greater than

±l0V. The current

drain for the

±10V supplies is used for RS232.

For the RS-232 driver the current requirement

will be 3.5mA per driver. The external power

supplies should provide a power supply se-

quence of :+l0V, then +5V, followed by –l0V.

Drivers

The SP334 has three independent RS-232 single-

ended drivers and two differential RS-485

drivers. Control for the mode selection is done

by the RS-232/RS-485 select pin. The drivers

are pre-arranged such that for each mode of

–5V

–5V

+5V

+

+

++

–

–

–

–

–10V

+

+

++

–

–

–

–