SP3224E Datasheet, PDF (12/23 Pages) Sipex Corporation – 3.0V to 5.5V RS-232 Transceivers with Auto On-Line® Plus

of 5.5V, the charge pump is enabled. If the
output voltages exceed a magnitude of
5.5V, the charge pump is disabled. This os-
cillator controls the four phases of the volt-
age shifting. A description of each phase
follows.
Highly Efficient Charge Pump
The charge pump is used to generate posi-
tive and negative signal voltages for the
RS-232 drivers. This enables fully compli-
ant RS-232 and V.28 signals from a single
3.0 or 5.5V power supply.
The charge pumps use four external capac-
itors to hold and transfer electrical charge.
The Exar patented design (US Patent
#5,306,954) uses a unique approach com-
pared to older less efficient designs. 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
maximum efficiency.
Phase 1
Vss charge store and double: The posi-
tive terminals of capacitors C1 and C2 are
charged from Vcc with their negative termi-
nals initially connected to ground. C1+ is
then connected to ground and the stored
charge from C1– is superimposed onto
C2– . Since C2+ is still connected to Vcc
the voltage potential across capacitor C2 is
now 2 x Vcc.
Phase 2
Vss transfer and invert: Phase two con-
nects the negative terminal of C2 to the Vss
storage capacitor and the positive terminal
of C2 to ground. This transfers the doubled
and inverted (V–) voltage onto C4. Mean-
while, capacitor C1 charged from Vcc to
prepare it for its next phase.
Phase 3
Vdd charge store and double: Phase three
THEORY OF OPERATION
is identical to the first phase. The posi-
tive terminals of capacitors C1 and C2 are
charged from Vcc with their negative termi-
nals initially connected to ground. C1+ is
then connected to ground and the stored
charge from C1– is superimposed onto
C2–. Since C2+ is still connected to Vcc
the voltage potential across capacitor C2 is
now 2 x Vcc.
Phase 4
Vdd transfer: The fourth phase connects
the negative terminal of C2 to ground and
the positive terminal of C2 to the Vdd stor-
age capacitor. This transfers the doubled
(V+) voltage onto C3. Meanwhile, capaci-
tor C1 charged from Vcc to prepare it for
its next phase.
The Exar charge pump generates V+ and
V– independently from Vcc. Hence in a no-
load condition V+ and V- will be symmetri-
cal. Older charge pump approaches gen-
erate V+ and then use part of that stored
charge to generate V-.Because of inherent
losses the magnitude of V- will be smaller
than V+ on these older designs.
Under lightly loaded conditions the intelli-
gent pump oscillator maximizes efficiency
by running only as needed to maintain V+
and V–. Since interface transceivers often
spend much of their time at idle this power-
efficient innovation can greatly reduce total
power consumption. This improvement is
made possible by the independent phase
sequence of the Exar charge pump
design.
The clock rate for the charge pump typi-
cally operates at greater than 70kHz allow-
ing the pump to run efficiently with small
0.1μF capacitors. Efficient operation de-
pends on rapidly charging and discharging
C1 and C2, therefore capacitors should be
mounted close to the IC and have low ESR
(equivalent series resistance). Inexpensive
surface mount ceramic capacitors (such as
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
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