8
LTC660
TYPICAL APPLICATIONS N
U
Voltage Doubling
Figure 8 shows the LTC660 operating in the voltage
doubling mode. The external Schottky (1N5817) diode is
for start-up only. The output voltage is 2 • V
IN
without a
load. The diode has no effect on the output voltage.
1
2
3
4
8
7
6
5
BOOST
CAP
+
GND
CAP
–
LTC660 • F08
C1
150µF
V
OUT
= 2V
IN
V
IN
2.5V
TO 5.5V
LTC660
1N5817*
V
+
OSC
LV
V
OUT
C2
150µF
* SCHOTTKY DIODE IS FOR START-UP ONLY
+
+
Figure 8. Voltage Doubler
Negative Voltage Converter
Figure 7 shows a typical connection which will provide a
negative supply from an available positive supply. This
circuit operates over full temperature and power supply
ranges without the need of any external diodes. The LV pin
(Pin 6) is shown grounded, but for V
+
≥ 3V, it may be
floated, since LV is internally switched to ground (Pin 3)
for V
+
≥ 3V.
The output voltage (Pin 5) characteristics of the circuit are
those of a nearly ideal voltage source in series with a 6.5Ω
resistor. The 6.5Ω output impedance is composed of two
terms: 1) the equivalent switched-capacitor resistance
(see Theory of Operation), and 2) a term related to the on-
resistance of the MOS switches.
At an oscillator frequency of 10kHz and C1 = 150µF, the
first term is:
R=
1
f/2
EQUIV
OSC
()
=
=
C1
1
5 10 150 10
13
36
•• •
.
–
Ω.
Notice that the equation for R
EQUIV
is not a capacitive
reactance equation (X
C
= 1/ωC) and does not contain a
2π term.
The exact expression for output impedance is complex,
but the dominant effect of the capacitor is clearly shown on
the typical curves of output impedance and power effi-
ciency versus frequency. For C1 = C2 = 150µF, the output
impedance goes from 6.5Ω at f
OSC
= 10kHz to 110Ω at
f
OSC
= 100Hz. As the 1/fC term becomes large compared
to the switch on-resistance term, the output resistance is
determined by 1/fC only.
Ultraprecision Voltage Divider
An ultraprecision voltage divider is shown in Figure 9. To
achieve the 0.002% accuracy indicated, the load current
should be kept below 100nA. However, with a slight loss
in accuracy, the load current can be increased.
1
2
3
4
8
7
6
5
BOOST
CAP
+
GND
CAP
–
C2
150µF
LTC660 • F07
C1
150µF
V
OUT
= –V
IN
V
IN
1.5V TO 5.5V
LTC660
V
+
OSC
LV
V
OUT
+
+
Figure 7. Voltage Inverter
Battery Splitter
A common need in many systems is to obtain positive and
negative supplies from a single battery or single power
supply system. Where current requirements are small, the
circuit shown in Figure 10 is a simple solution. It provides
symmetrical positive or negative output voltages, both
equal to one-half the input voltage. The output voltages are
both referenced to Pin 3 (Output Common).
Figure 9. Ultraprecision Voltage Divider
8
7
6
54
3
2
1
C1
150µF
± 0.002%
V
+
3V TO 11V
LTC660 • F09
T
MIN
≤ T
A
≤ T
MAX
I
L
≤ 100nA
C2
150µF
2
V
+
LTC660
+
+
