8

TGoddard/SP333/9617R0

SP333 RS-232/AppleTalk Programmable Transceiver

© Copyright 2000 Sipex Corporation

Phase 1

-Vss charge storage- During this phase of the

clock cycle, the positive side of capacitors C1

and C2 are initially charged to +5V. C1+ is then

switched to ground and charge in C1- is trans-

ferred to C2-. Since C2+ is connected to +5V,

the voltage potential across capacitor C2 is

now 10V.

Phase 2

-Vss transfer- Phase two of the clock connects

the negative terminal of C2 to the Vss storage

capacitor and the positive terminal of C2 to

ground, and transfers the generated -10V to C3.

Simultaneously, the positive side of capacitor

C1 is switched to +5V and the negative side is

connected to ground.

Phase 3

-Vdd charge storage- The third phase of the

clock is identical to the first phase- the trans-

ferred charge in C1 produces -5V in the negative

terminal of C1, which is applied to the negative

side of capacitor C2. Since C2+ is at +5V, the

voltage potential across C2 is 10V.

Phase 4

-Vdd transfer- The fourth phase of the clock

connects the negative terminal of C2 to ground

and transfers the generated 10V across C2 to C4,

the Vdd storage capacitor. Simultaneously with

this, the positive side of capacitor C1 is switched

to +5V and the negative side is connected to

ground, and the cycle begins again.

Since both V+ and V- are separately generated

from Vcc in a no load condition, V+ and V- will

be symmetrical. Older charge pump approaches

that generate V- from V+ will show a decrease

in the magnitude of V- compared to V+ due to

the inherent inefficiencies in the design.

The clock rate for the charge pump typically

operates at 15kHz. The external capacitors should

be 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

V- pins. The value of the external supply volt-

ages must be no greater than

±10V. The current

drain from the

±10V supplies is used for the RS-

232 drivers. For the RS-232 driver, the current

requirement is 3.5mA per driver. The external

power supplies should provide a power supply

sequence of either: +10V, –10V, and then +5V;

or –10V, +10V, and then +5V. It is critical that

the

±10V supplies are on before V

Figure 11. Charge Pump Waveforms

–10V

b) C

2

–

GND

GND

a) C

2

+

+10V