Datasheet

7
LT1054
APPLICATIONS INFORMATION
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Theory of Operation
To understand the theory of operation of the LT1054, a
review of a basic switched-capacitor building block is
helpful.
In Figure 3 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, 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)
To obtain an equivalent resistance for the switched-capaci-
tor network we can rewrite this equation in terms of voltage
and impedance equivalence:
I = =
V1 – V2
(1/fC1)
V1 – V2
R
EQUIV
f
C1
C2
R
L
V2
LT1054 • F03
V1
Figure 3. Switched-Capacitor Building Block
A new variable R
EQUIV
is defined such that R
EQUIV
= 1/fC1.
Thus the equivalent circuit for the switched-capacitor
network is as shown in Figure 4. The LT1054 has the same
switching action as the basic switched-capacitor building
block. Even though this simplification doesn’t include finite
switch on-resistance and output voltage ripple, it provides
an intuitive feel for how the device works.
These simplified circuits explain voltage loss as a function
of frequency (see Typical Performance Characteristics). As
frequency is decreased, the output impedance will eventu-
ally be dominated by the 1/fC1 term and voltage losses will
rise.
C2
R
L
R
EQUIV
R
EQUIV
=
V2
LT1054 • F04
V1
1
fC1
Figure 4. Switched-Capacitor Equivalent Circuit
Note that losses also rise as frequency increases. This is
caused by internal switching losses which occur due to
some finite charge being lost on each switching cycle. This
charge loss per-unit-cycle, when multiplied by the switch-
ing frequency, becomes a current loss. At high frequency
this loss becomes significant and voltage losses again rise.
The oscillator of the LT1054 is designed to run in the
frequency band where voltage losses are at a minimum.
Regulation
T he error amplifier of the LT1054 servos the drive to the
PNP switch to control the voltage across the input capaci-
tor (C
IN
) which in turn will determine the output voltage.
Using the reference and error amplifier of the LT1054, an
external resistive divider is all that is needed to set the
regulated output voltage. Figure 5 shows the basic regu-
lator configuration and the formula for calculating the
appropriate resistor values. R1 should be chosen to be
Figure 5
R4
RESTART SHUTDOWN
C1
R2
C
IN

10µF
TANTALUM
C
OUT

100µF
TANTALUM
V
OUT
LT1054 • F05
V
IN
+
+
R1
2.2µF
+
R3
R2
R1
= ≈+ 1
WHERE V
REF
= 2.5V NOMINAL
*CHOOSE THE CLOSEST 1% VALUE
FOR EXAMPLE: TO GET V
OUT
= –5V REFERRED TO THE GROUND 
PIN OF THE LT1054, CHOOSE R1 = 20k, THEN
| V
OUT
|
) )
V
REF

2
– 40mV
R2 = 20k = 102.6k*
+ 1
| –5V |
) )
2.5V
2
– 40mV
) )
+ 1
| V
OUT
|
1.21V
LT1054
FB/SHDN
CAP
+
GND
CAP
–
V
+
OSC
V
REF
V
OUT