7
LTC1046
1046fb
U
SA
O
PP
L
IC
AT
ITY
P
I
CA
L
Figure 7. Negative Voltage Converter
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 GND (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 an 27Ω
resistor. The 27Ω 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 30kHz and C1 = 10µF, the first
term is:
R=
1
f/2
EQUIV
OSC
()
=
=
•
••
.
•
–
C1
1
15 10 10 10
67
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
Figure 8. Voltage Doubler
Figure 9. Ultraprecision Voltage Divider
the typical curves of output impedance and power effi-
ciency versus frequency. For C1 = C2 = 10µF, the output
impedance goes from 27Ω at f
OSC
= 30kHz to 225Ω at
f
OSC
= 1kHz. As the 1/fC term becomes large compared to
switch ON resistance term, the output resistance is deter-
mined by 1/fC only.
Voltage Doubling
Figure 8 shows a two diode, capacitive voltage doubler.
With a 5V input, the output is 9.1V with no load and 8.2V
with a 10mA load.
1
2
3
4
8
7
6
5
V
+
OSC
LV
V
OUT
BOOST
CAP
+
GND
CAP
–
LTC1046
10µF
10µF
1046 F07
V
+
1.5V TO 6V
V
OUT
= –V
+
REQUIRED FOR V
+
< 3V
T
MIN
≤ T
A
≤ T
MAX
+
+
1
2
3
4
8
7
6
5
V
+
OSC
LV
V
OUT
BOOST
CAP
+
GND
CAP
–
LTC1046
10µF10µF
V
D
V
D
+
+
1046 F08
V
+
1.5V TO 6V
V
OUT
= 2
(V
IN
–1)
REQUIRED
FOR
V
+
< 3V
+ +
Ultraprecision Voltage Divider
An ultraprecision voltage divider is shown in Figure 9. To
achieve the 0.0002% 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
V
+
OSC
LV
V
OUT
BOOST
CAP
+
GND
CAP
–
LTC1046
C1
10µF
C2
10µF
T
MIN
≤ T
A
≤ T
MAX
I
L
≤ 100nA
REQUIRED FOR V
+
< 6V
1046 F09
V
+
3V TO 12V
+
+
±0.002%
V
+
2
