SP3221E Datasheet, PDF (12/20 Pages) Sipex Corporation – Intelligent +3.0V to +5.5V RS-232 Transceiver

Charge Pump Capacitor selection
The Exar charge pump is designed to
operate reliably with a range of low cost
capacitors.Either polarized or non polar-
ized capacitors may be used. If polarized
capacitors are used they should be oriented
as shown in the Typical Operating Circuit.
The V+ capacitor may be connected to either
ground or Vcc (polarity reversed.)
The charge pump operates with 0.1µF ca-
pacitors 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 doubling 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
For best charge pump efficiency locate the
charge pump and bypass capacitors as
close as possible to the IC. Surface mount
capacitors are best for this purpose. Using
capacitors with lower equivalent series re-
sistance (ESR) and self-inductance, along
with minimizing parasitic PCB trace induc-
tance will optimize charge pump operation.
Designers are also advised to consider that
capacitor values may shift over time and
operating temperature.
Minimum recommended charge pump capacitor value
Input Voltage V_CC
Charge pump capacitor value
3.0V to 3.6V
C1 - C4 = 0.1µF
4.5V to 5.5V
C1 = 0.047µF, C2-C4 = 0.33µF
3.0V to 5.5V
C1 - C4 = 0.22µF
Table 4. Capacitor selection table
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com
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