XR32220 Datasheet, PDF (9/13 Pages) Exar Corporation – 2-Driver/2-Receiver Intelligent RS-232 Transceiver

XR32220
Phase 2
V- transfer — Phase two of the clock connects the negative
terminal of C2 to the VSS storage capacitor and the posi-
tive terminal of C2 to GND. This transfers a negative gener-
ated voltage to C3. This generated voltage is regulated to a
minimum voltage of -5.5V. Simultaneous with the transfer of
the voltage to C3, the positive side of capacitor C1 is
switched to V_CC and the negative side is connected to
GND.
VCC
C1
C2
-5.5V
C4
V+ Storage Capacitor
V- Storage Capacitor
C3
Figure 8: Charge Pump - Phase 2
Phase 3
V+ charge storage — The third phase of the clock is identi-
cal to the first phase — the charge transferred in C1 pro-
duces –V_CC in the negative terminal of C1, which is applied
to the negative side of capacitor C2. Since C2+ is at V_CC,
the voltage potential across C2 is 2 times V_CC.
VCC
C1
-VCC
+VCC
C2
-VCC
C4
V+ Storage Capacitor
V- Storage Capacitor
C3
fer of the voltage to C4, the positive side of capacitor C1 is
switched to V_CC and the negative side is connected to
GND, allowing the charge pump cycle to begin again. The
charge pump cycle will continue as long as the operational
conditions for the internal oscillator are present.
VCC
+5.5V
C1
C2
C4
V+ Storage Capacitor
V- Storage Capacitor
C3
Figure 10: Charge Pump - Phase 4
Since both V+ and V– are separately generated from V_CC,
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 greater than
250kHz. The external capacitors can be as low as 0.1μF
with a 16V breakdown voltage rating.
The Exar-patented charge pumps are designed to operate
reliably with a range of low cost capacitors. Either polarized
or non polarized capacitors may be used. If polarized
capacitors are used they should be oriented as shown in
the Typical Applications Circuit. The V+ capacitor may be
connected to either ground or Vcc (polarity reversed.)
The charge pump operates with 0.1μF capacitors for 3.3V
operation. For other supply voltages, see the table for
required capacitor values. Do not use values smaller than
those listed. Increasing the capacitor values (e.g., by dou-
bling in value) reduces ripple on the transmitter outputs and
may slightly reduce power consumption. C2, C3, and C4
can be increased without changing C1’s value.
Figure 9: Charge Pump - Phase 3
Phase 4
V+ transfer — The fourth phase of the clock connects the
negative terminal of C2 to GND, and transfers this positive
generated voltage across C2 to C4, the V+ storage capaci-
tor. This voltage is regulated to +5.5V. At this voltage, the
internal oscillator is disabled. Simultaneous with the trans-
© 2014 Exar Corporation
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exar.com/XR32220
Rev 1A