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SP385EEA-L-TR Datasheet, PDF (8/16 Pages) Exar Corporation – Enhanced +3V or +5V RS-232 Line Driver/Receiver | |||
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Charge Pump
The charge pump is a Exarâpatented design
(5,306,954) and uses a unique approach
compared to older lessâefficient designs. The
charge pump still requires four external capaci-
tors, but uses a fourâphase voltage shifting
technique to attain symmetrical 10V power
supplies. There is a freeârunning oscillator
that controls the four phases of the voltage
shifting. A description of each phase follows.
Phase 1
â VSS charge storage âDuring 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
C1â is transferred to C2â. Since C2+ is con-
nected to +5V, the voltage potential across
capacitor C2 is now 10V.
Phase 2
â VSS transfer â Phase two of the clock con-
nects the negative terminal of C2 to the VSS
storage capacitor and the positive terminal of
C2 to ground, and transfers the generated
âl0V 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
â VDD charge storage â The third phase of
the clock is identical to the first phase â the
charge transferred in C1 produces â5V 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
â VDD transfer â The fourth phase of the
clock connects the negative terminal of C2
to ground, and transfers the generated l0V
across C2 to C4, the VDD 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 Vâ are separately gener-
ated from VCC; in a noâload condition V+ and
Vâ will be symmetrical. Older charge pump
approaches that generate Vâ from V+ will
show a decrease in the magnitude of Vâ 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) C2â
â10V
Figure 1. Charge Pump Waveform
Exar Corporation 48720 Kato Road, Fremont CA, 94538 ⢠510-668-7017 ⢠www.exar.com
SP385E_100_030811
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