sp207et-tr Exar Corporation, sp207et-tr Datasheet - Page 6

• Manufacturer Part Number: sp207et-tr
• Description: Sp207 -5v, 5-driver/3-receiver Rs-232, Uses Small 0.1?f Capacitors
• Manufacturer: Exar Corporation

Date:05/26/2005
Figure 1. Charge Pump — Phase 1
attain symmetrical ±10V power supplies.
Figure 3a shows the waveform found on the
positive side of capcitor C
shows the negative side of capcitor C
a free–running oscillator that controls the four
phases of the voltage shifting. A description of
each phase follows.
Phase 1
— V
clock cycle, the positive side of capacitors C
and C
switched to ground and the charge in C
transferred to C
+5V, the voltage potential across capacitor C
now 10V.
Phase 2
— V
nects the negative terminal of C
storage capacitor and the positive terminal of C
to ground, and transfers the generated –l0V to
C
tor C
is connected to ground.
Phase 3
— V
clock is identical to the first phase — the charge
Figure 2. Charge Pump — Phase 2
3
. Simultaneously, the positive side of capaci-
SS
DD
SS
1
2
charge storage —During this phase of the
is switched to +5V and the negative side
transfer — Phase two of the clock con-
are initially charged to +5V. C
charge storage — The third phase of the
2
–
. Since C
C
2
1
2
+
, and Figure 3b
+
–
C
is connected to
V
1
CC
–5V
+
–
V
2
= +5V
CC
to the V
2
= +5V
. There is
l
+
+5V High-Speed RS232 Transceivers
is then
C
1
2
–
–10V
2
+
–
C
is
is
SS
2
1
2
+5V
–5V
+
–
6
transferred in C
terminal of C
side of capacitor C
voltage potential across C
Phase 4
— V
connects the negative terminal of C
and transfers the generated l0V across C
the V
with this, the positive side of capacitor C
switched to +5V and the negative side is con-
nected to ground, and the cycle begins again.
Since both V
from V
be symmetrical. Older charge pump approaches
that generate V
the magnitude of V
inherent inefficiencies in the 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.
The SP200 Series devices are made up of three
basic circuit blocks — 1) transmitter/driver, 2)
receiver and 3) the Sipex proprietary charge
+
–
C
C
DD
4
DD
+
3
–
+
–
C
C
CC
transfer — The fourth phase of the clock
storage capacitor. Again, simultaneously
4
3
V
V
+
–
; in a no–load condition V
DD
SS
V
V
Storage Capacitor
Storage Capacitor
DD
SS
1
+
, which is applied to the negative
Storage Capacitor
Storage Capacitor
and V
–
1
from V
produces –5V in the negative
–
2
. Since C
–
compared to V
are separately generated
© Copyright 2005 Sipex Corporation
+
will show a decrease in
2
is l0V.
2
+
is at +5V, the
+
2
+
and V
to ground,
due to the
2
to C
–
will
1
is
4
,