Datasheet

LTC3101
18
3101fb
OPERATION
BUCK-BOOST CONVERTER OPERATION
The buck-boost converter is a synchronous 5-switch
DC/DC converter with the capability to operate effi ciently
with input voltages that are above, below or equal to
the output regulation voltage. A proprietary switching
algorithm provides a smooth transition between opera-
tional modes while maintaining high effi ciency and low
noise performance. Referring to the Block Diagram, the
buck-boost converter has two P-channel input power
switches, A and A´. This provides the capability for the
buck-boost converter to operate directly from either input
power source, USB or battery. The buck-boost converter
automatically and seamlessly transitions to the higher
voltage input supply.
PWM Mode Operation
When the PWM pin is held high, the LTC3101 buck-boost
converter operates in a fi xed frequency pulse width modu-
lation mode using voltage mode control. A proprietary
switching algorithm allows the converter to transition
between buck, buck-boost, and boost modes without
discontinuity in inductor current or loop characteristics.
The switch topology for the buck-boost converter is shown
in Figure 2.
When the input voltage is signifi cantly greater than the
output voltage, the buck-boost converter operates in
buck mode. Switch D turns on continuously and switch
C remains off. Switches A (or A´) and B are pulse width
modulated to produce the required duty cycle to support the
output regulation voltage. As the input voltage decreases,
switch A remains on for a larger portion of the switching
cycle. When the duty cycle reaches approximately 85%,
the switch pair AC begins turning on for a small fraction
of the switching period. As the input voltage decreases
further, the AC switch pair remains on for longer durations
and the duration of the BD phase decreases proportionally.
As the input voltage drops below the output voltage, the
AC phase will eventually increase to the point that there is
no longer any BD phase. At this point, switch A remains on
continuously while switch pair CD is pulse width modu-
lated to obtain the desired output voltage. At this point,
the converter is operating solely in boost mode.
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 all three operational
modes. These advantages result in increased effi ciency
and stability in comparison to the traditional 4-switch
buck-boost converter.
Error Amplifi er and Internal Compensation
The buck-boost converter utilizes a voltage mode error
amplifi er with an internal compensation network as shown
in Figure 3.
Notice that resistor R2 of the external resistor divider
network plays an integral role in determining the frequency
response of the compensation network. The ratio of R2 to
R1 is set to program the desired output voltage but this
still allows the value of R2 to be adjusted to optimize the
A´ D
L
A
BC
SW3ABAT2USB2 SW3B
LTC3101
V
OUT3
3101 F02
Figure 2. Buck-Boost Switch Topology
+
–
0.599V
GND (EXPOSED PAD)
LTC3101
V
OUT3
V
OUT3
R2
FB3
R1
Figure 3. Buck-Boost Error Amplifi er and Compensation