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

SP3220E_EB_EU_102_102016

11

Charge Pump

The charge pump is an Exar-patended

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 sym-

metrical 5.5V power supplies. The internal

power supply consists of a regulated dual

charge pump that provides output voltages

of +/-5.5V regardless of the input voltage

(Vcc) over the +3.0V to +5.5V range.

In most circumstances, decoupling the

power supply can be achieved adequately

using a 0.1µF bypass capacitor at C5 (refer

to figures 6 and 7). In applications that are

sensitive to power-supply noise, decouple

Vcc to ground with a capacitor of the same

value as charge-pump capacitor C1. Physi-

cally connect bypass capacitor as close to

the IC as possible.

The charge pump operates in a discontinu-

ous mode using an internal oscillator. If the

output voltages are less than a magnitude

of 5.5V, the charge pump is enabled. If the

outputvoltages exceedamagnitudeof5.5V,

the charge pump is disabled. This oscillator

controls the four phases of the voltage shift-

ing. A description of each phase follows.

Phase 1

— V

of the clock cycle, the positive side of capaci-

tors C

C

l

+

is then switched to GND and the charge

in C

1

–

is transferred to C

2

–

. Since C

2

+

is con-

nected to V

capacitor C

Phase 2

— V

connectsthenegativeterminalofC

storagecapacitorandthepositiveterminalof

C

ated voltage to C

regulated to a minimum voltage of -5.5V.

Simultaneous with the transfer of the volt-

age to C

is switched to V

connected to GND.

Phase 3

— V

the clock is identical to the first phase — the

charge transferred in C

the negative terminal of C

to the negative side of capacitor C

C

2

+

is at V

is 2 times V

Phase 4

— V

the clock connects the negative terminal

of C

generated voltage across C

V

This voltage is

regulated to +5.5V. At this voltage, the in-

ternal oscillator is disabled. Simultaneous

with the transfer of the voltage to C

positive side of capacitor C

to V

the

negative side is con-

nected to GND, allowing the charge

pump cycle to begin again. The charge

pump cycle will continue as long as the

operational conditions for the internal

oscillator are present.

ated from V

+

inefficiencies in the design.

DESCRIPTION