LTC1144
TEST CIRCUITS
1
2
C1
10碌F
8
7
LTC1144
6
5
EXTERNAL
OSCILLATOR R
L
V
+
15V
I
S
+
3
4
I
L
V
OUT
Figure 1.
APPLICATI
S I FOR ATIO
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, capacitor
C1 will charge to voltage V1. The total charge on C1 will be
q1 = C1V1. The switch then moves to the right, discharg-
ing 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:
鈭唓
= q1 鈥?q2 = C1(V1 鈥?V2)
V1
f
R
L
C1
C2
1144 F02
V1
C2
R
EQUIV
=
1
f
脳
C1
R
L
Figure 3. Switched-Capacitor Equivalent Circuit
V2
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
theory will explain how the LTC1144 behaves. The loss,
V
+
(8)
SW1
SW2
Figure 2. Switched-Capacitor Building Block
If the switch is cycled f times per second, the charge
transfer per unit time (i.e., current) is:
I = f
脳 鈭唓
= f
脳
C1(V1 鈥?V2)
Rewriting in terms of voltage and impedance equivalence,
BOOST
10X
(1)
OSC
OSC
(7)
梅
2
I
=
V1
鈭?/div>
V2 V1
鈭?/div>
V2
=
铮?/div>
1
铮?/div>
R
EQUIV
铮?/div>
f
脳
C1
铮?/div>
铮?/div>
铮?/div>
SHDN
(6)
A new variable R
EQUIV
has been defined such that R
EQUIV
= 1/(f
脳
C1). Thus, the equivalent circuit for the switched-
capacitor network is as shown in Figure 3.
Figure 4. LTC1144 Switched-Capacitor
Voltage Converter Block Diagram
+
C
OSC
C2
10碌F
1144 F01
U
R
EQUIV
V2
1144 F03
W
U
UO
CAP
+
(2)
蠁
蠁
+
C1
CAP
鈥?/div>
(4)
V
OUT
(5)
C2
+
GND
(3)
1144 F04
5
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