
Date: 02/24/05
SP3282EB Intelligent +2.35V to +5.5V RS-232 Transceivers
Copyright 2005 Sipex Corporation
8
— V
charge storage — During this phase of the
clock cycle, the positive side of capacitors C
and
C
are initially charged to V
CC
. C
to GND and the charge in C
1
. Since C
tial across capacitor C
2
is now 2 times V
CC
.
+
is then switched
–
is transferred to C
2
–
+
is connected to V
, the voltage poten-
Phase 2 (Figure 12)
— V
transfer — Phase two of the clock
connects the negative terminal of C
to the V
storage capacitor and the positive terminal of C
to
GND. This transfers a negative generated voltage
to
C
.
This
generated
regulated to a minimum voltage of -5.5V (V
CC
>
3.3V) and -4.0V (V
< 3.1V).
Simultaneous with the transfer of the voltage to C
3
,
the positive side of capacitor C
is switched to V
CC
and the negative side is connected to GND.
voltage
is
Phase 3 (Figure 13)
— V
charge storage — The third phase of the
clock is identical to the first phase — the charge
transferred in C
produces –V
in the negative
terminal of C
1
, which is applied to the negative side
of capacitor C
. Since C
potential across C
2
is 2 times V
CC
.
+
is at V
CC
, the voltage
Phase 4 (Figure 14)
— V
transfer — The fourth phase of the clock
connects the negative terminal of C
to GND, and
transfers this positive generated voltage across C
2
to C
, the V
storage capacitor. This voltage is
regulated to +5.5V (V
> 3.3V) and +4.0V
(V
<3.1V). At this voltage, the internal oscillator
is disabled. Simultaneous with the transfer of the
voltage to C
4
, the positive side of capacitor C
is
switched to V
and the negative side is connected
to GND, allowing the charge pump cycle to begin
long as the operational conditions for theinternal
oscillator are present.
Since both V
+
and V
–
are separately generated
Charge Pump Capacitor Selection
The charge pump capacitors C1-C4 and bypass
C5 can be of any type including ceramic. If
polarized capacitors are used, refer to figure 3
application diagram for proper orientation. The
following chart illustrates the minimum capaci-
tor valve for a given input voltage range.
from V
, 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 should
be 0.22
μ
F with a 16V working voltage rating for
a V
CC
input range of +2.35V to +5.5V.
Figure 11. Charge Pump — Phase 1
V
CC
V
DD
Storage Capacitor
V
SS
Storage Capacitor
C
1
C
2
+
-
-
+
C
4
C
3
+
-
+
-
-V
CC
-V
CC
+V
CC
V
CC
(V)
3.0 to 3.6
4.5 to 5.5
2.35 to 5.5
C1 and C5 (
μ
F)
0.1
0.047
0.22
C2,C3,C4 (
μ
F)
0.1
0.33
0.22