LTC1046 Datasheet, PDF (5/12 Pages) Linear Technology – “Inductorless” 5V to + 5V Converter

LTC1046
APPLICATI S I FOR ATIO
Rewriting in terms of voltage and impedance equivalence,
( ) I = V1– V2 = V1– V2.
1/ fC1 ^REQUIV
A new variable, R_EQUIV, has been defined such that
R_EQUIV = 1/fC1. Thus, the equivalent circuit for the switched
capacitor network is as shown in Figure 3.
R_EQUIV
V1
V2
R_EQUIV
=
1
fC1
C2
R_L
1046 F03
Figure 3. Switched Capacitor Equivalent Circuit
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.
BOOST
3x
(1)
OSC
OSC
(7)
V⁺
(8)
φ
+2
φ
SW1
SW2
CAP+
(2)
+
C1
CAP–
(4)
V_OUT
(5)
C2
LV
(6)
CLOSED WHEN
V⁺ > 3.0V
GND
(3)
1046 F04
Figure 4. LTC1046 Switched Capacitor
Voltage Converter Block Diagram
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.
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)
The internal logic of the LTC1046 runs between V⁺ and LV
(Pin 6). For V⁺ greater than or equal to 3V, an internal
switch shorts LV to GND (Pin 3). For V⁺ less than 3V, the
LV pin should be tied to ground. For V⁺ greater than or
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.
V⁺
BOOST
(1)
2I
I
OSC SCHMITT
(7) TRIGGER
∼14pF
2I
I
LV
1046 F05
(6)
Figure 5. Oscillator
5