LTC660 Linear Technology, LTC660 Datasheet - Page 8

• Manufacturer Part Number: LTC660
• Description: 100mA CMOS Voltage Converter
• Manufacturer: Linear Technology

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
floated, since LV is internally switched to ground (Pin 3)
for V
LTC660
TYPICAL APPLICATIONS
The output voltage (Pin 5) characteristics of the circuit are
those of a nearly ideal voltage source in series with a 6.5
resistor. The 6.5 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 = 150 F, the
first term is:
Notice that the equation for R
reactance equation (X
2 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 = 150 F, the output
impedance goes from 6.5 at f
f
to the switch on-resistance term, the output resistance is
determined by 1/fC only.
8
OSC
R
5 10
= 100Hz. As the 1/fC term becomes large compared
+
EQUIV
•
+
3V.
3
C1
150 F
=
•
150 10
1
f
OSC
Figure 7. Voltage Inverter
1
2
3
4
•
1
/2
BOOST
CAP
GND
CAP
C1
C
+
–
–
LTC660
6
= 1/ C) and does not contain a
V
OSC
OUT
1 3
V
LV
.
+
EQUIV
8
7
6
5
OSC
U
+
= 10kHz to 110 at
C2
150 F
is not a capacitive
+
N
V
1.5V TO 5.5V
V
IN
OUT
3V, it may be
= –V
LTC660 • F07
IN
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
load. The diode has no effect on the output voltage.
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.
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).
TO 5.5V
2.5V
V
T
IN
MIN
* SCHOTTKY DIODE IS FOR START-UP ONLY
V
2
+
I
T
L
A
+
0.002%
150 F
100nA
Figure 9. Ultraprecision Voltage Divider
T
MAX
C1
C1
150 F
+
Figure 8. Voltage Doubler
+
C2
150 F
1
2
3
4
1
2
3
4
1N5817*
BOOST
CAP
GND
CAP
+
–
LTC660
LTC660
V
OSC
OUT
LV
V
+
8
7
6
5
8
7
6
5
+
V
3V TO 11V
+
IN
C2
150 F
without a
LTC660 • F09
V
OUT
LTC660 • F08
= 2V
IN