SP3508CF datasheet(25/33 Pages) SIPEX

25 / 33 page

Date: 06/14/04

SP3508 Enhanced WAN Multi–Mode Serial Transceiver

Phase 2

-VSS1 transfer -Phase two of the clock connects the negative terminal of C2 to the VSS1 storage capacitor

and the positive terminal of C2 to ground, and transfers the negative generated voltage to CVSS1. This

generated voltage is regulated to -5.5V. Simultaneously, the positive side of the capacitor C1 is switched

to VCC and the negative side is connected to ground.

FEATURES

Phase 3

-VDD charge storage -The third phase of the clock is identical to the first phase-the charge transferred in

C1 produces -VCC in the negative terminal of C1 which is applied to the negative side of the capacitor C2.

Since C2+ is at VCC, the voltage potential across C2 is 2xVCC.

VCC = +3V

–6V

VSS Storage Capacitor

VDD Storage Capacitor

–

CVDD

CVSS1

Figure 46. Charge Pump - Phase 2.

VCC = +3V

–3V

+3V

–3V

VSS1 Storage Capacitor

VDD Storage Capacitor

–

CVSS1

CVDD

Figure 47.Charge Pump - Phase 3.

Phase 4

-VDD transfer -The fourth phase of the clock connects the negative terminal of C2 to ground, and transfers

the generated 5.5V across C2 to CVDD, the VDD storage capacitor. This voltage is regulated to +5.5V. At

the regulated voltage, the internal oscillator is disabled and simultaneously with this, the positive side of

capacitor C1 is switched to VCC and the negative side is connected to ground, and the cycle begins again.

The charge pump cycle will continue as long as the operational conditions for the internal oscillator are

present. 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 250kHz. The external capacitors can be as low as 1

µF with a 16V breakdown voltage

rating.

SP3508CF 数据表(PDF) 25 Page - Sipex Corporation

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