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-ecient
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 gures 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
output voltages exceed a magnitude of 5.5V,
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
SS
charge storage — During this phase
of the clock cycle, the positive side of capaci-
tors C
1
and C
2
are initially charged to V
CC
.
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
CC
, the voltage potential across
capacitor C
2
is now 2 times V
CC
.
Phase 2
— V
SS
transfer — Phase two of the clock
connects the negative terminal of C
2
to the V
SS
storage capacitor and the positive terminal of
C
2
to GND. This transfers a negative gener-
ated voltage to C
3
. This generated voltage is
regulated to a minimum voltage of -5.5V.
Simultaneous with the transfer of the volt-
age to C
3
, the positive side of capacitor C
1
is switched to V
CC
and the negative side is
connected to GND.
Phase 3
— V
DD
charge storage — The third phase of
the clock is identical to the rst phase — the
charge transferred in C
1
produces –V
CC
in
the negative terminal of C
1
, which is applied
to the negative side of capacitor C
2
. Since
C
2
+
is at V
CC
, the voltage potential across C
2
is 2 times V
CC
.
Phase 4
— V
DD
transfer — The fourth phase of
the clock connects the negative terminal
of C
2
to GND, and transfers this positive
generated voltage across C
2
to C
4
, the
V
DD
storage capacitor. 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
4
, the
positive side of capacitor C
1
is switched
to V
CC
and 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.
Since both V
+
and V
–
are separately gener-
ated from V
CC
, 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
ineciencies in the design.
DESCRIPTION
