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LTC3101

3101fb

OPERATION

BUCK-BOOST CONVERTER OPERATION

The buck-boost converter is a synchronous 5-switch

DC/DC converter with the capability to operate efﬁciently

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 efﬁciency 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 ﬁxed 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 signiﬁcantly 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 efﬁciency

and stability in comparison to the traditional 4-switch

buck-boost converter.

Error Ampliﬁer and Internal Compensation

The buck-boost converter utilizes a voltage mode error

ampliﬁer 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´

SW3A

BAT2

USB2

SW3B

LTC3101

VOUT3

3101 F02

Figure 2. Buck-Boost Switch Topology

–

0.599V

GND (EXPOSED PAD)

LTC3101

VOUT3

FB3

Figure 3. Buck-Boost Error Ampliﬁer and Compensation

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