Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com

SP207E_00_072309

6

Receiver input thresholds are between .2

to .7 volts typical. This allows the receiver

todetectstandardTTLorCMOSlogic-level

signals as well as RS-232 signals. If a re-

ceiver input is left unconnected or un-driven,

a5kΩpulldownresistortogroundwillcommit

the receiver to a logic- output state.

Highly Efficient Charge–Pump

The onboard dual-output charge pump is

usedtogeneratepositiveandnegativesignal

voltagesfortheRS-232drivers. Thisenables

fullycompliantRS-232andV.28signalsfrom

a single power supply device.

The charge pumps use four external capaci-

tors to hold and transfer electrical charge.

The

Exar–patented design (US Patent

#5,306,954) uses a unique approach com-

paredtoolder,less–efficientdesigns.The

pumps use a four–phase voltage shifting

technique to attain symmetrical V+ and V-

power supplies. An intelligent control oscil-

lator regulates the operation of the charge

pump to maintain the proper voltages at

maximumefficiency.

Phase 1

V

terminalsofcapacitorsC

from V

connected to ground. C

to ground and the stored charge from C



– is

superimposed onto C

2

–. Since C

connectedtoV

capacitor C

–5V

–5V

+5V

+

+

+

+

–

–

–

–

Phase 2

— V

nects the negative terminal of C

storage capacitor and the positive terminal

of C

andinverted(V-)voltageontoC

capacitor C

for its next phase.

Phase 3

V

isidenticaltothefirstphase.Thepositive

terminalsofcapacitorsC

from V

connected to ground. C

to ground and the stored charge from C

superimposed onto C

connectedtoV

capacitor C

Phase 4

V

the negative terminal of C

the positive terminal of C

age capacitor. This transfers the doubled

(V+) voltage onto C

C

next phase.

Figure . Charge Pump — Phase 

–10V

+

+

+

+

–

–

–

–

Figure 2. Charge Pump — Phase 2

–5V

–5V

+5V

+

+

+

+

–

–

–

–

Figure 3. Charge Pump — Phase 3