LTC3115-2_15 Datasheet, PDF(13/42 Page) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

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LTC3115-2_15 Datasheet, PDF (13/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

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LTC3115-2

Operation

INTRODUCTION

The LTC3115-2 is a monolithic buck-boost converter that

can operate with input and output voltages from as low

as 2.7V to as high as 40V. Four internal low resistance N-

channel DMOS switches minimize the size of the application

circuit and reduce power losses to maximize efficiency.

Internal high side gate drivers, which require only the

addition of two small external capacitors, further simplify

the design process. A proprietary switch control algorithm

allows the buck-boost converter to maintain output volt-

age regulation with input voltages that are above, below

or equal to the output voltage. Transitions between these

operating modes are seamless and free of transients and

subharmonic switching. The LTC3115-2 can be configured

to operate over a wide range of switching frequencies, from

100kHz to 2MHz, allowing applications to be optimized for

board area and efficiency. With its configurability and wide

operating voltage range, the LTC3115-2 is ideally suited to

a wide range of power systems especially those requiring

compatibility with a variety of input power sources such

as lead-acid batteries, USB ports, and industrial supply

rails as well as from power sources which have wide or

poorly controlled voltage ranges such as FireWire and

unregulated wall adapters.

The LTC3115-2 has an internal fixed-frequency oscillator

with a switching frequency that is easily set by a single

external resistor. In noise sensitive applications, the con-

verter can also be synchronized to an external clock via

the PWM/SYNC pin. The LTC3115-2 has been optimized to

reduce input current in shutdown and standby for applica-

tions which are sensitive to quiescent current draw, such

as battery-powered devices. In Burst Mode operation, the

no-load standby current is only 50ÂµA (typical) and in shut-

down the total supply current is reduced to 3ÂµA (typical).

provides seamless transitions between operating modes

and eliminates discontinuities in the average inductor cur-

rent, inductor current ripple, and loop transfer function

throughout all regions of operation. These advantages

result in increased efficiency, improved loop stability, and

lower output voltage ripple in comparison to the traditional

4-switch buck-boost converter.

Figure 1 shows the topology of the LTC3115-2 power stage

which is comprised of four N-channel DMOS switches and

their associated gate drivers. In PWM mode operation both

switch pins transition on every cycle independent of the input

and output voltage. In response to the error amplifier output,

an internal pulse width modulator generates the appropriate

switch duty cycles to maintain regulation of the output voltage.

When stepping down from a high input voltage to a lower

output voltage, the converter operates in buck mode and

switch D remains on for the entire switching cycle except

for the minimum switch low duration (typically 100ns).

During the switch low duration switch C is turned on which

forces SW2 low and charges the flying capacitor, CBST2, to

ensure that the voltage of the switch D gate driver supply

rail is maintained. The duty cycle of switches A and B are

adjusted to provide the appropriate buck mode duty cycle.

If the input voltage is lower than the output voltage, the

converter operates in boost mode. Switch A remains on

for the entire switching cycle except for the minimum

switch low duration (typically 100ns) while switches C

and D are modulated to maintain the required boost mode

duty cycle. The minimum switch low duration ensures that

flying capacitor CBST1 is charged sufficiently to maintain

the voltage on the BST1 rail.

CBST1

CBST2

PWM MODE OPERATION

PVCC

BST1

PVIN SW1

SW2 PVOUT

BST2

PVCC

With the PWM/SYNC pin forced high or driven by an ex-

ternal clock, the LTC3115-2 operates in a fixed-frequency

pulse width modulation (PWM) mode using a voltage mode

control loop. This mode of operation maximizes the output

current that can be delivered by the converter, reduces

output voltage ripple, and yields a low noise fixed-frequency

switching spectrum. A proprietary switching algorithm

PVCC

PGND

LTC3115-2

PVCC

PGND

Figure 1. Power Stage Schematic

31152 F01

31152fa

For more information www.linear.com/LTC3115-2

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