6
LTC1911-1.5/LTC1911-1.8
1911f
APPLICATIO S I FOR ATIO
WUUU
General Operation
The LTC1911 uses a switch capacitor-based DC/DC con-
version to provide the efficiency advantages associated
with inductor-based circuits as well as the cost and
simplicity advantages of a linear regulator. The LTC1911’s
unique constant frequency architecture provides a low
noise regulated output as well as lower input noise than
conventional switch-capacitor charge pump regulators.
The LTC1911 uses an internal switch network and frac-
tional conversion ratios to achieve high efficiency over
widely varying V
IN
and output load conditions. Internal
control circuitry selects the appropriate step-down con-
version ratio based on V
IN
and load conditions to optimize
efficiency. The part has three possible step-down modes:
2-to-1, 3-to-2 or 1-to-1 step-down mode. Only two exter-
nal flying caps are needed to operate in all three modes.
2-to-1 mode is chosen when V
IN
is greater than two times
the desired V
OUT
. 3-to-2 mode is chosen when V
IN
is
greater than 1.5 times V
OUT
but less than 2 times V
OUT
. 1-
to-1 mode is chosen when V
IN
falls below 1.5 times V
OUT
.
An internal load current sense circuit controls the switch
point of the step-down ratio as needed to maintain output
regulation over all load conditions.
Regulation is achieved by sensing the output voltage and
regulating the amount of charge transferred per cycle.
This method of regulation provides much lower input and
output ripple than that of conventional switched capacitor
charge pumps.
The constant frequency charge transfer
also makes additional output or input filtering much less
demanding than conventional switched capacitor charge
pumps.
The LTC1911 also has a Burst Mode function that delivers
a minimum amount of charge for one cycle then goes into
a low current state until the output drops enough to require
another burst of charge. Burst Mode operation allows the
LTC1911 to achieve high efficiency even at light loads. The
part has shutdown capability as well as user-controlled
inrush current limiting. In addition, the part has short-
circuit and overtemperature protection.
Step-Down Charge Transfer Operation
Figure 1a shows the switch configuration that is used for
2-to-1 step down mode. In this mode, a 2-phase clock
generates the switch control signals. On phase one of the
clock, the top plate of C1 is connected to V
IN
through R
A
and S4, the bottom plate is connected to V
OUT
through S3.
The amount of charge transferred to C1 (and V
OUT
) is set
by the value of R
A
.
On phase two, flying capacitor C1 is connected to V
OUT
through S1 and to GND through S2. The charge that was
transferred onto C1 from the previous cycle is now trans-
ferred to the output. Thus, in 2-to-1 mode, charge is
transferred to V
OUT
on both phases of the clock. Since
charge current is sourced from GND on the second phase
of the clock, current multiplication is realized with respect
to V
IN
, i.e., I
OUT
equals approximately 2 • I
IN
. This results
in significant efficiency improvement relative to a linear
regulator. The value of R
A
is set by the control loop of the
regulator.
V
IN
V
OUT
C1
R
A
C1
+
C1
–
1911 F01a
S4
φ1
S1
φ2
S3
φ1
S2
φ2
Figure 1a. Step-Down Charge Transfer in 2-to-1 Mode
The 3-to-2 conversion mode also uses a nonoverlapping
clock for switch control but requires two flying capacitors
and a total of seven switches (see Figure 1b). On phase one
of the clock, the two capacitors are connected in parallel to
V
IN
through R
A
by switches S5 and S7, and to V
OUT
through S4 and S6. The amount of charge transferred to
C1||C2 (and V
OUT
) is set by the regulator control loop
which determines the value of R
A
. On phase two, C1 and
C2 are connected in series from V
OUT
to GND through
switches S1, S2 and S3. On phase two, half of the charge
