SP331CT datasheet(9/13 Pages) SIPEX

9 / 13 page

Date: 03-04-05

Programmable Dual RS-232/RS-485 Transceiver

Figure 11. Charge Pump Phase 1.

Figure 12. Charge Pump Phase 2.

Figure 13. Charge Pump Phase 3.

Figure 14. Charge Pump Phase 4.

simultaneously with this, the positive side of

capacitor C

1 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 V

CC 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 must

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

±l0V. The current

drain for the

±10V supplies is used for RS-232.

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.

VCC = +5V

–5V

+5V

VSS Storage Capacitor

VDD Storage Capacitor

–

VCC = +5V

–10V

VSS Storage Capacitor

VDD Storage Capacitor

–

VCC = +5V

–5V

+5V

–5V

VSS Storage Capacitor

VDD Storage Capacitor

–

VCC = +5V

+10V

VSS Storage Capacitor

VDD Storage Capacitor

–

Figure 15. Charge Pump Waveforms

+10V

a) C

GND

-10V

b) C

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