Datasheet

LTC3112
13
3112fc
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are pulse width modulated to produce the required duty
cycle to support the output regulation voltage.
As the input voltage nears the output voltage, switches
A and D are on for a greater portion of the switching
period, providing a direct current path from V
IN
to V
OUT
.
Switches B and C are turned on only enough to ensure
proper regulation and/or provide charging of the BST1
and BST2 capacitors. The internal control circuitry will
determine the proper duty cycle in all modes of operation,
which will vary with load current.
As the input voltage drops well below the output voltage,
the converter operates solely in boost mode. Switch A turns
on at maximum duty cycle and switch B turns on just long
enough to refresh the voltage on the BST1 capacitor used
to drive A. Switches C and D are pulse width modulated to
produce the required duty cycle to regulate the output voltage.
This switching algorithm provides a seamless transition
between operating modes and eliminates discontinuities
in average inductor current, inductor current ripple, and
loop transfer function throughout the operational modes.
These advantages result in increased efficiency and stab-
ility in comparison to the traditional 4-switch buck-boost
converter.
Powering V
CC
from an External Source
The LTC3112’s V
CC
regulator can be powered or back-fed
from an external source up to 5.5V. Advantages of back-
feeding V
CC
from a voltage above 4.2V include higher
efficiency and improved maximum duty cycle at lower
input voltages. These advantages are shown in the Typical
Performance Characteristics curves “MOSFET Resistance
vs V
CC
” and “Minimum SW1 Low Times.” For 5V
OUT
ap-
plications, V
CC
can be easily powered from V
OUT
using an
external low current Schottky diode as shown in several
applications circuits in the Typical Applications section.
Back-feeding V
CC
also improves a light load PWM mode
output voltage ripple that occurs when the inductor passes
through zero current. Back-feeding V
CC
reduces the switch
pin anti-cross conduction times, minimizing the V
OUT
ripple during this light-load condition. One disadvantage
of powering V
CC
from V
OUT
is that no-load quiescent
current increases at low V
IN
in Burst Mode operation as
operaTion
shown in the Typical Performance Characteristics curves
(compared to V
CC
powered from V
IN
).
Considerations for Boost Applications
In boost mode, the maximum output current that can be
supported at higher V
OUT
/V
IN
ratios is reduced. This ef-
fect is illustrated in the Maximum Output Current PW
M
Mode curves in the Typical Performance Characteristics
section. For example at 12V
OUT
, the LTC3112 needs V
IN
>
4V to support 1A. As described previously, powering V
CC
from a 5V source (if available) can improve output current
capabilities at low input voltages.
At even lower input voltages (below 3.6V for 12V
OUT
), the
LTC3112 can run into duty cycle limitations. This occurs
since SW1 and SW2 maximum duty cycles are multiplied,
giving an approximate 70% maximum duty cycle at the
nominal 750kHz switching frequency. Reducing the switch
-
ing frequency
with the PWM/SYNC pin will increase the
maximum duty cycle, allowing a higher boost ratio to be
achieved. Do not attempt operating the LTC3112 beyond
the duty cycle limitations described as this may result in
unstable operation.
Burst Mode OPERATION
When the PWM/SYNC pin is held low, the buck-boost
converter operates utilizing a variable frequency switch
-
ing algorithm
designed to improve efficiency at light load
and reduce the standby current at zero load. In Burst
Mode operation, the inductor is charged with fixed peak
amplitude current pulses and as a result only a
fraction
of the maximum output current can be delivered when in
Burst Mode operation.
These current pulses are repeated as often as necessary
to maintain the output regulation voltage. The maximum
output current, I
MAX
, which can be supplied in Burst Mode
operation is dependent upon the input and output voltage
as approximated by the following formula:
I
MAX
=
0.5 • V
IN
V
IN
+ V
OUT
(A)
If the buck-boost load exceeds the maximum Burst Mode
current capability, the output rail will lose regulation. In