SP385ECT-L Datasheet, PDF (8/15 Pages) Exar Corporation – Enhanced +3V or +5V RS-232 Line Driver/Receiver

Charge Pump
The charge pump is a Exar–patented design
(5,306,954) and uses a unique approach
compared to older less–efficient designs. The
charge pump still requires four external capaci-
tors, but uses a four–phase voltage shifting
technique to attain symmetrical 10V power
supplies. There is a free–running oscillator
that controls the four phases of the voltage
shifting. A description of each phase follows.
Phase 1
— V_SS charge storage —During this phase of
the clock cycle, the positive side of capacitors
C1 and C2 are initially charged to +5V. Cl+
is then switched to ground and the charge in
C1– is transferred to C2–. Since C2+ is con-
nected to +5V, the voltage potential across
capacitor C2 is now 10V.
Phase 2
— V_SS transfer — Phase two of the clock con-
nects the negative terminal of C2 to the V_SS
storage capacitor and the positive terminal of
C2 to ground, and transfers the generated
–l0V to C3. Simultaneously, the positive side of
capacitor C 1 is switched to +5V and the nega-
tive side is connected to ground.
Phase 3
— V_DD charge storage — The third phase of
the clock is identical to the first phase — the
charge transferred 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 l0V.
Phase 4
— V_DD transfer — The fourth phase of the
clock connects the negative terminal of C2
to ground, and transfers the generated l0V
across C2 to C4, the V_DD storage capacitor.
Again, 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 gener-
ated 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– com-
pared to V+ due to the 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.
+10V
a) C2+
GND
GND
b) C2–
–10V
Figure 1. Charge Pump Waveform
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

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