SP211EHE Datasheet, PDF (7/19 Pages) Exar Corporation – Low Power, High ESD 5V RS-232 Transceivers

V_CC = +5V
+7V
+
C_{1 –}
+
C_{2 –}
C₄
+ – _{VDD Storage Capacitor}
– + _{VSS Storage Capacitor}
C₃
Figure 4. Charge Pump — Phase 4
The Exar charge-pump generates V+ and V-
icnodnedpiteionndeVn+tlyafnrodmVV-_CCw. Hillebnecesiynmamnoe–trliocaadl.
Older charge pump approaches generate
V+ and then use part of that stored charge
to generate V-. Because of inherent losses,
the magnitude of V- will be smaller than V+
on these older designs.
Under lightly loaded conditions the intelligent
pump oscillator maximizes efficiency by
running only as needed to maintain V+ and
V-. Since interface transceivers often spend
much of their time at idle, this power-efficient
innovation can greatly reduce total power
consumption. This improvement is made
possible by the independent phase sequence
of the Exar charge-pump design.
The clock rate for the charge pump typically
operates at greater than 15kHz, allowing
the pump to run efficiently with small 0.1µF
capacitors.Efficient operation depends on
rapidly charging and discharging C₁ and
C₂, therefore capacitors should be mounted
close to the IC and have low ESR (equivalent
series resistance).Low cost surface mount
ceramic capacitors (such as are widely used
for power-supply decoupling) are ideal for
use on the charge pump.
However the charge pumps are designed
to be able to function properly with a wide
range of capacitor styles and values. If
polarized capacitors are used, the positive
and negative terminals should be connected
as shown.
Voltage potential across any of the capaci-
tors will never exceed 2 x V_CC. Therefore
capacitors with working voltages as low as
10V rating may be used with a nominal V_CC
supply. C₁ will never see a potential greater
than V_CC , so a working voltage of 6.3V is
adequate. The reference terminal of the
V_DD capacitor may be connected either to
V_CC or ground, but if connected to ground a
minimum 16V working voltage is required.
Higher working voltages and/or capacitance
values may be advised if operating at higher
V_CC or to provide greater stability as the
capacitors age.
+7V
a) C₂₊
GND
GND
b) C_2–
–7V
Figure 5. Typical waveforms seen on ca-
pacitor C2 when all drivers are at maximum
load.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP207E_101_101512