LTC3111 datasheet(13/32 Pages) LINER

13 / 32 page

LTC3111

3111fa

For more information www.linear.com/LTC3111

OPERATION

PWM MODE OPERATION

When the PWM/SYNC pin is held high, the LTC3111 buck-

boost converter operates in a fixed-frequency pulse-width

modulation (PWM) mode using voltage mode control. Full

outputcurrentisonlyavailableinPWMmode.Aproprietary

switching algorithm allows the converter to transition

between buck, buck-boost, and boost modes without

discontinuity in inductor current. The switch topology for

the buck-boost converter is shown in Figure 2.

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 stabil-

ity in comparison to the traditional 4-switch buck-boost

converter.

OUTPUT VOLTAGE PROGRAMMING

The output voltage is set via the external resistor divider

comprised of resistors R1 and R2 as show in Figures 1.

Theresistordividervaluesdeterminetheoutputregulation

voltage according to:

VOUT = 0.8V • 1+









In addition to setting the output voltage, the value of R1 is

instrumentalincontrollingthedynamicsofthecompensa-

tion network. When changing the value of this resistor,

care must be taken to understand the impact this will have

on the compensation network.

Inaddition,theTheveninequivalentresistanceoftheresis-

tor divider controls the gain of the input current limit. To

maintain sufficient gain in this loop, it is recommended

that the Thevenin resistance be greater than 100kΩ.

RUN Comparator

In addition to serving as a logic-level input to enable the IC,

the RUN pin includes an accurate internal comparator that

allows it to be used to set custom rising and falling on/off

thresholds with the addition of an external resistor divider.

When RUN is driven above its logic threshold (0.8V typi-

cal), the LDO regulator is enabled, which provides power

to the internal control circuitry of the IC. If the voltage

on RUN is increased further so that it exceeds the RUN

comparator accurate analog threshold (1.2V typical), all

functions of the buck-boost converter will be enabled and

a start-up sequence will ensue.

IfRUNisbroughtbelowtheaccuratecomparatorthreshold,

the buck-boost converter will inhibit switching, but the

VIN

VOUT

3111 F02

Figure 2. Buck-Boost Switch Topology

When the input voltage is significantly greater than the

output voltage, the buck-boost converter operates in buck

mode. Switch D turns on at maximum duty cycle and

switch C turns on just long enough to refresh the voltage

on the BST2 capacitor used to drive switch D. Switches A

and B 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 pe-

riod, providing a direct current path from VIN to VOUT.

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 ca-

pacitor used to drive A. Switches C and D are pulse-width

modulated to produce the required duty cycle to regulate

the output voltage.

LTC3111 датащи(PDF) 13 Page - Linear Technology

LTC3111 датащи(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

датащи скачать

ссылки URL