6

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:

∆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

VV

fC

VV

=

−

=

−

12

11

12

/

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.

Figure 2. Switched-Capacitor Building Block

Figure 3. Switched-Capacitor Equivalent Circuit

LTC660 • F04

(2)

(4)

GND

(3)

(5)

(8)

LV

(6)

4.5

×

(1)

OSC

(7)

OSC

+2

CLOSED WHEN

C1

C2

BOOST

SW1

SW2

φ

φ

+

Figure 4. LTC660 Switched-Capacitor Voltage Converter

Block Diagram

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

µF

capacitors the effective output impedance is determined

by the internal switch resistances and the capacitor ESRs.

LV (Pin 6)

≥3V,aninternalswitchshortsLVtoground

≥3V,theLVpincanbetiedtogroundorleftfloating.

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.

C1

C2

V2

660 F02

V1

C2

V2

660 F03

V1

1

fC1