SP238A Datasheet, PDF (5/12 Pages) Exar Corporation – +5V Powered Multi-Channel RS-232 Drivers/Receivers

DESCRIPTION
Phase 1
Vss charge store and double: The positive ter-
minals of capacitors C1 and C2 are charged
from Vcc with their negative terminals initially
connected to ground. C1+ is then connected
to ground and the stored charge from C1- is
superimposed onto C2-. Since C2+ is still con-
nected to Vcc the voltage potential across C2 is
now 2 x Vcc.
Phase 2
Vss transfer and invert: Phase two connects
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. Meanwhile, capacitor C1
is charged from Vcc in order to prepare it for its
next phase.
Phase 3
Vdd charge store and double: Phase three is
identical to the first phase. The positive termi-
nals of C1 and C2 are charged from Vcc with
their negative terminals 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 posi-
tive terminal of C2 to the Vdd storage capacitor .
This transfers the doubled (V+) voltage onto C3.
Meanwhile, capacitor C1 is charged from Vcc to
prepare it for its next phase.
Under lightly loaded conditions, the intelligent
pump oscillator maximizes efficiency by running
only as needed to maintain V+ and V- voltage
levels. Since interface transceivers spend most
of their time at idle, this power-efficient innova-
tion can greatly reduce total power consump-
tion. This improvement is made possible by
the independent phase sequences of the Exar
charge pump design.
The clock rate of the charge pump typically op-
erates greater than 70kHz allowing the pump to
operate efficiently using small 0.1uF capacitors.
Efficient operation depends on the rapid charg-
ing and discharging of C1 and C2. Therefore,
the capacitors should be mounted as close as
possible to the IC and have a low ESR (equiva-
lent series resistance). Inexpensive, surface
mount ceramic capacitors are ideal for use on
charge pump. If polarized capacitors are used
the positive and negative terminals should be
connected as shown on the typical operating
circuit. A diagram of the 4 individual phases is
shown in Figure 1.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com

SP238A_100_071410