SP3249E Ver la hoja de datos (PDF) - Exar Corporation

Número de pieza

componentes Descripción

Fabricante

SP3249E

Intelligent +3.0V to +5.5V RS-232 Transceiver

Exar Corporation 

+

C5 0.1µF

+

C1 0.1µF

+

C2 0.1µF

LOGIC

INPUTS

LOGIC

OUTPUTS

VCC

VCC

C1+

V+

C1-

C2+ SP3249E V-

C2-

TxIN

TxOUT

RxOUT

RxIN

5kΩ

+

C3 0.1µF

C4 0.1µF

+

1000pF

GND

Figure 7. Loopback Test Circuit for RS-232 Driver

Data Transmission Rates

Charge Pump

The charge pump is an Exar–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 5.5V

power supplies. The internal power supply

consists of a regulated dual charge pump that

provides output voltages 5.5V regardless of

the input voltage (VCC) over the +3.0V to +5.5V

range. This is important to maintain compli-

ant RS-232 levels regardless of power supply

fluctuations.

The charge pump operates in a discontinuous

mode using an internal oscillator. If the output

voltages are less than a magnitude of 5.5V, the

charge pump is enabled. If the output voltages

exceed a magnitude of 5.5V, the charge pump

is disabled. This oscillator controls the four

phases of the voltage shifting. A description of

each phase follows.

Phase 1

— VSS charge storage — During this phase of the

clock cycle, the positive side of capacitors C1 and

C2 are initially charged to VCC. Cl+ is then switched

to GND and the charge in C1– is transferred to C2–.

Since C2+ is connected to VCC, the voltage potential

across capacitor C2 is now 2 times VCC.

Figure 8. Loopback Test results at 120kbps

(All Drivers Fully Loaded)

Figure 9. Loopback Test results at 250Kbps

(All Drivers Fully Loaded)

Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com



SP3249E_100_013111

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