sp385e-1 Exar Corporation, sp385e-1 Datasheet - Page 6

• Manufacturer Part Number: sp385e-1
• Description: 232 Line Driver/receiver
• Manufacturer: Exar Corporation

CHARGE PUMP
The charge pump is a Sipex–patented design
(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 symmetrical 10V
power supplies. There is a free–running
oscillator that controls the four phases of the
voltage shifting. A description of each phase
follows.
Phase 1
— V
the clock cycle, the positive side of capacitors
C
then switched to ground and the charge in C
is transferred to C
+5V, the voltage potential across capacitor C
is now 10V.
Phase 2
— V
connects the negative terminal of C
V
terminal of C
generated –l0V to C
positive side of capacitor C
+5V and the negative side is connected to
ground.
Phase 3
— V
the clock is identical to the first phase — the
charge transferred in C
negative terminal of C
1
SS
Figure 1. Charge Pump — Phase 1
Rev. 07/26/02
and C
storage capacitor and the positive
SS
SS
DD
C
charge storage —During this phase of
transfer — Phase two of the clock
charge storage — The third phase of
1
2
–5V
+
–
V
are initially charged to +5V. C
CC
= +5V
2
to ground, and transfers the
2
C
–
2
. Since C
+5V
–5V
+
–
3
. Simultaneously, the
1
1
, which is applied to
produces –5V in the
1
+
–
2
C
C
is switched to
+
4
3
+
–
is connected to
V
V
DD
SS
SP385E-1 True +3V to +5V RS-232 Line Driver/Receiver
Storage Capacitor
Storage Capacitor
2
to the
l
+
is
1
–
2
6
the negative side of capacitor C
at +5V, the voltage potential across C
Phase 4
— V
clock connects the negative terminal of C
ground,
and transfers the generated l0V across C
C
neously
with this, the positive side of capacitor C
switched to +5V and the negative side is
connected to ground, and the cycle begins
again.
Since both V
from V
will
proaches that generate V
a decrease in the magnitude of V
to V
design.
The clock rate for the charge pump typically
operates at 15kHz. The external capacitors
can be as low as 0.1µF with a 16V breakdown
voltage rating.
be symmetrical. Older charge pump ap-
C2-
4
C2+
Figure 2. Charge Pump — Phase 2
Figure 3. Charge Pump Waveforms
, the V
GND
GND
-10V
10V
+
DD
due to the inherent inefficiencies in the
CC
transfer — The fourth phase of the
C
DD
; in a no–load condition V
1
+
–
V
CC
storage capacitor. Again, simulta-
+
= +5V
and V
C
2
–10V
–
+
–
are separately generated
© Copyright 2002 Sipex Corporation
–
from V
+
–
C
C
4
+
3
–
2
V
V
. Since C
–
DD
SS
+
Storage Capacitor
Storage Capacitor
compared
will show
+
and V
2
is l0V.
2
1
2
2
to
+
is
to
–
is