5

LTC1046

S

APPLICATI

I FOR ATIO

Rewriting in terms of voltage and impedance equivalence,

I

VV

fC

VV

12

11

12

–

/

–

.

capacitor network is as shown in Figure 3.

As frequency is decreased, the output impedance will

eventually be dominated by the 1/fC1 term and power

efficiency will drop. The typical curves for power effi-

ciency versus frequency show this effect for various capaci-

tor values.

Note also that power efficiency decreases as frequency

goes up. 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 switching frequency, becomes a current

loss. At high frequency this loss becomes significant and

the power efficiency starts to decrease.

LV (Pin 6)

equal to 3V, the LV pin can be tied to ground or left floating.

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.

Figure 3. Switched Capacitor Equivalent Circuit

Figure 4. LTC1046 Switched Capacitor

Voltage Converter Block Diagram

Figure 5. Oscillator

Examination of Figure 4 shows that the LTC1046 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 typical curve), this simple

theory will explain how the LTC1046 behaves. The loss,

and hence the efficiency, is set by the output impedance.

C2

1046 F03

V2

V1

1

fC1

1046 F04

CAP+

(2)

CAP–

(4)

GND

(3)

(5)

(8)

LV

(6)

3x

(1)

OSC

(7)

OSC

+2

CLOSED WHEN

C1

C2

BOOST

SW1

SW2

φ

φ

+

OSC

(7)

1046 F05

LV

(6)

BOOST

(1)

∼14pF

I

2I

I

2I

SCHMITT

TRIGGER