LTC114 4
6
1144fa
For more information www.linear.com/LTC1144
TesT circuiT
applicaTions inForMaTion
1
2
3
4
8
7
6
5
+
+
C1
C2
10µF
I
S
V
V
+
15V
I
L
R
L
EXTERNAL
OSCILLATOR
C
OSC
LTC1144
Figure 1.
Figure 2. Switched-Capacitor Building Block
Figure 3. Switched-Capacitor Equivalent Circuit
Figure 4. LTC1144 Switched-Capacitor
Voltage Converter Block Diagram
Theory of Operation
To understand the theory of operation of the LTC1144,
a review of a basic switched-capacitor building block is
helpful.
In Figure 2, when the switch is in the left position, capaci
-
tor C
1 will charge to voltage V1. The total charge on C1
will
be q1 = C1V1. The switch then moves to the right,
discharging C1 to voltage V2. After this discharge time,
the charge on C1 is q2 = C1V2. Note that charge has been
transferred from the source V1 to the output V2. The
amount of charge transferred is:
∆q = q1 – q2 = C1(V1 – V2)
If the switch is cycled f times per second, the charge
transfer per unit time (i.e., current) is:
I = f × ∆q = f × C1(V1 – V2)
Rewriting in terms of voltage and impedance equivalence,
I=
1
f×C1
=
R
EQUIV
A new variable R
EQUIV
has been defined such that
R
L
C2
C1
f
V2
R
L
EQUIV
C2
V1
EQUIV
=
1
R
EQUIV
= 1/(f × C1). Thus, the equivalent circuit for the
switched-capacitor network is as shown in Figure 3.
Examination of Figure 4 shows that the LTC1144 has the
same switching action as the basic switched-capacitor
building block. With the addition of finite switch on-
resistance and output voltage ripple, the simple theory,
although not exact, provides an intuitive feel for how the
device works.
For example, if you examine power conversion efficiency
as a function of frequency (see Figure 5), this simple
SHDN
(6)
OSC
(7)
10X
(1)
BOOST
1144 F04
OSC
÷
2
V
+
(8)
SW1 SW2
CAP
+
(2)
CAP
–
(4)
GND
(3)
V
OUT
(5)
C1
+
+
φ
φ
