SP6660DS/11 SP6660 200mA Charge Pump Inverter or Doubler © Copyright 2000 Sipex Corporation
15
Optimizing Capacitor Selection
Refer to Figure 24 for the total output resistance
for various capacitance values and oscillator
frequencies. The reservoir and charge pump
capacitor values are equal. The capacitance
values required to maintain comparable ripple
and output resistance typically diminish
proportionately as the pump frequency of the
SP6660 increases.
The test conditions for the curves of Figure 24
are the same as for Figures 2 to 20 for the circuits
in Figures 1 and 2; additional conditions are as
follows:
C1 = C2 = 0.2Ω ESR capacitors
R
OUT
= 4.2Ω
The flat portion of the curves shown at a 5.2Ω
effective output resistance is a result of the
SP6660's 5.25Ω output resistance where
5.2Ω = R
OUT(SP6660)
+ (4 x ESR
C1
) + ESR
C2
.
Instead of the typical 5.2Ω, R
OUT
= 4.2Ω is used
because the typical specification includes the
effect of the ESRs of the capacitors used in the
test circuit in Figures 1 and 2.
Refer to Figures 17, 18, 19 and 20 for the output
currents using 0.33µF to 220µF capacitors.
Output currents are plotted for 3.0V and 4.5V
inputs taking into consideration a 10% to 20%
loss in the input voltage. The SP6660 5.2Ω
series resistance limits increases in output current
vs. capacitance for values much higher than
47µF. Larger values may still be useful to reduce
ripple.
Designing a Multiple of the SP6660
Negative Inverted Output Voltage
The SP6660 can be cascaded to allow a designer
to provide a multiple of the negative inverted
output voltage of a single SP6660 device. The
approximate total output resistance, R
TOT
,of the
cascaded SP6660 devices is equal to the sum of
the individual SP6660 output resistance values,
R
OUT
. The output voltage, V
TOT
, is a multiple of
the number of cascaded SP6660 devices and the
output voltage of an individual SP6660 device,
V
OUT
. Refer to Figure 25 for the circuit cascading
SP6660 devices. Note that the capacitance value
of C1 for the charge pump and C2 at V
OUT
is
multiplied respectively to the number of cascaded
SP6660 devices.
Connecting the SP6660 in Parallel
SP6660 devices can be connected in parallel
to reduce the total output resistance. The
approximate total output resistance, R
TOT
, of the
multiple devices connected in parallel is equal
to the output resistance of an individual SP6660
device divided by the total number of devices
connected. Refer to Figure 26 for the circuit
connecting multiple SP6660 devices in parallel.
Note that only the charge pump capacitor value
of C1 is multiplied respectively by the number
of SP6660 connected in parallel. A single
capacitor C2 at the output voltage V
OUT
of the
"nth" device connected in parallel serves all
devices connected.
