7

Rev.4/08/02

SP3249E Intelligent +3.0V to +5.5V RS-232 Transceivers

© Copyright 2002 Sipex Corporation

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 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 (V

range. This is important to maintain compliant

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 (Figure 13). A descrip-

tion of each phase follows.

— V

the clock cycle, the positive side of capacitors

C

C

l

then switched to GND and the charge in C

1

transferred to C

2

2

V

now 2 times V

Figure 7. Loopback Test Circuit for RS-232 Driver Data

Transmission Rates

— V

connects the negative terminal of C

storage capacitor and the positive terminal of C

2

to GND. This transfers a negative generated

voltage to C

regulated to a minimum voltage of -5.5V.

Simultaneous with the transfer of the voltage to

C

to V

GND.

Figure 8. Loopback Test Circuit Result at 120kbps

(All Drivers Fully Loaded)

Figure 9. Loopback Test Circuit result at 250kbps

(All Drivers Fully Loaded)

SP3249E

TxIN

TxOUT

C1+

C1-

C2+

C2-

V+

V-

0.1

µF

0.1

µF

0.1

µF

+

C2

C5

C1

+

+

C3

C4

+

+

0.1

µF

0.1

µF

LOGIC

INPUTS

5k

Ω

RxIN

RxOUT

LOGIC

OUTPUTS

GND

1000pF