LTC660 Linear Technology, LTC660 Datasheet - Page 6

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

APPLICATIONS
LTC660
Theory of Operation
To understand the theory of operation for the LTC660, 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,
discharging C1 to voltage V2. After this discharging 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:
If the switch is cycled “f” times per second, the charge
transfer per unit time (i.e., current) is:
Rewriting in terms of voltage and impedance equivalence,
A new variable R
R
capacitor network is as shown in Figure 3.
Figure 4 shows that the LTC660 has the same switching
action as the basic switched-capacitor building block.
6
EQUIV
I = f • q = f • C1 (V1 – V2)
I
q = q1 – q2 = C1 (V1 – V2)
V
= 1/fC1. Thus, the equivalent circuit for the switched-
1
Figure 3. Switched-Capacitor Equivalent Circuit
1
/
Figure 2. Switched-Capacitor Building Block
fC
V
1
2
V1
V1
V
R
R
EQUIV
1
EQUIV
U
EQUIV
R
EQUIV
=
V
fC1
2
1
C1
INFORMATION
U
has been defined such that
C2
C2
R
R
W
L
L
660 F02
660 F03
V2
V2
U
This simplified circuit does not include finite on-resistance
of the switches and output voltage ripple, however, it does
give an intuitive feel for how the device works. For ex-
ample, if you examine power conversion efficiency as a
function of frequency this simple theory will explain how
the LTC660 behaves. The loss and hence the efficiency is
set by the output impedance. As frequency is decreased,
the output impedance will eventually be dominated by the
1/fC1 term and voltage losses will rise decreasing the
efficiency. As the frequency increases the quiescent cur-
rent increases. At high frequency this current loss be-
comes significant and the power efficiency starts to de-
crease.
The LTC660 oscillator frequency is designed to run where
the voltage loss is a minimum. With the external 150 F
capacitors the effective output impedance is determined
by the internal switch resistances and the capacitor ESRs.
LV (Pin 6)
The internal logic of the LTC660 runs between V
(Pin 6). For V
(Pin 3). For V
For V
OSC (Pin 7) and BOOST (Pin 1)
The switching frequency can be raised, lowered or driven
from an external source. Figure 5 shows a functional
diagram of the oscillator circuit.
BOOST
4.5
OSC
(1)
(7)
Figure 4. LTC660 Switched-Capacitor Voltage Converter
Block Diagram
+
3V, the LV pin can be tied to ground or left floating.
OSC
(6)
LV
+
+
< 3V, the LV pin should be tied to ground.
3V, an internal switch shorts LV to ground
(8)
V
+2
CLOSED WHEN
+
V
+
> 3.0V
SW1
CAP
+
CAP
GND
(2)
(4)
(3)
+
–
C1
SW2
+
and LV
V
LTC660 • F04
(5)
OUT
+
C2