SP333 Datasheet, PDF (8/13 Pages) Sipex Corporation – +5V Only RS-232/AppleTalk Programmable Transceiver

Phase 1
-Vss charge storage- During this phase of the
clock cycle, the positive side of capacitors C1
and C2 are initially charged to +5V. C1+ is then
switched to ground and charge in C1- is trans-
ferred to C2-. Since C2+ is connected to +5V,
the voltage potential across capacitor C2 is
now 10V.
Phase 2
-Vss transfer- Phase two of the clock connects
the negative terminal of C2 to the Vss storage
capacitor and the positive terminal of C2 to
ground, and transfers the generated -10V to C3.
Simultaneously, the positive side of capacitor
C1 is switched to +5V and the negative side is
connected to ground.
Phase 3
-Vdd charge storage- The third phase of the
clock is identical to the first phase- the trans-
ferred charge in C1 produces -5V in the negative
terminal of C1, which is applied to the negative
side of capacitor C2. Since C2+ is at +5V, the
voltage potential across C2 is 10V.
Phase 4
-Vdd transfer- The fourth phase of the clock
connects the negative terminal of C2 to ground
and transfers the generated 10V across C2 to C4,
the Vdd storage capacitor. Simultaneously with
this, the positive side of capacitor C1 is switched
to +5V and the negative side is connected to
ground, and the cycle begins again.
Since both V+ and V- are separately generated
from Vcc 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 15kHz. The external capacitors should
be 0.1µF with a 16V breakdown rating.
External Power Supplies
For applications that do not require +5V only,
external supplies can be applied at the V+ and
V- pins. The value of the external supply volt-
ages must be no greater than ±10V. The current
drain from the ±10V supplies is used for the RS-
232 drivers. For the RS-232 driver, the current
requirement is 3.5mA per driver. The external
power supplies should provide a power supply
sequence of either: +10V, –10V, and then +5V;
or –10V, +10V, and then +5V. It is critical that
the
±10V
supplies
are
on
before
V.
CC
+10V
a) C₂₊
GND
GND
b) C_2–
–10V
Figure 11. Charge Pump Waveforms
TGoddard/SP333/9617R0
SP333 RS-232/AppleTalk Programmable Transceiver
8
© Copyright 2000 Sipex Corporation