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Exar Corporation 48720 Kato Road, Fremont CA, 94538 510-668-7017 www.exar.com
SP3238E_100_020111
9
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
— V
SS charge storage — During this phase of the
clock cycle, the positive side of capacitors C
1 and
C
2 are initially charged to VCC. Cl
+
is then switched
to GND and the charge in C
1
–
is transferred to C
2
–
.
SinceC
2
+
isconnectedtoV
CC, the voltage potential
across capacitor C
2 is now 2 times VCC.
Phase 2
— V
SS transfer — Phase two of the clock
connects the negative terminal of C
2 to the VSS
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.
SimultaneouswiththetransferofthevoltagetoC
3,
the positive side of capacitor C
1 is switched to VCC
and the negative side is connected to GND.
Phase 3
— V
DD charge storage — The third phase of the
clock is identical to the first phase — the charge
transferred in C
1 produces –VCC in the negative
terminal of C
1, which is applied to the negative
side of capacitor C
2. Since C2
+
is at V
CC, the volt-
age potential across C
2 is 2 times VCC.
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 C4, the VDD storage capacitor. This
voltage is regulated to +5.5V. At this voltage,
the internal oscillator is disabled. Simultane-
ous with the transfer of the voltage to C
4, the
positive side of capacitor C
1 is switched to VCC
and the negative side is connected to GND, al-
lowing 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 generated
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 inefficiencies in the design.
The clock rate for the charge pump typically
operates at 500kHz. The external capacitors
can be as low as 0.1F with a 16V breakdown
voltage rating.
Figure 10. Charge Pump Waveform