LTC3255 datasheet(10/20 Pages) AD

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LTC3246

3246fa

For more information www.linear.com/LTC3246

APPLICATIONS INFORMATION

General Operation

The LTC3246 uses switched capacitor based DC/DC

conversion to provide the efficiency advantages associ-

ated with inductor based circuits as well as the cost and

simplicity advantages of a linear regulator. The LTC3246

uses an internal switch network and fractional conversion

ratios to achieve high efficiency and regulation over widely

varying VIN and output load conditions.

Internalcontrolcircuitryselectstheappropriateconversion

ratio based on VIN and load conditions. The device has

threepossibleconversionmodes:2:1step-downmode,1:1

step-down mode and 1:2 step-up mode. Only one external

flying capacitor is needed to operate in all three modes.

2:1 mode is chosen when VIN is greater than two times the

desired VOUT. 1:1 mode is chosen when VIN falls between

two times VOUT and VOUT. 1:2 mode is chosen when VIN

falls below the desired VOUT. The internal mode control

logic maintains output regulation over all load conditions.

Regulation is achieved by sensing the output voltage

and enabling charge transfer when the output falls below

regulation. When the charge pump is enabled, it controls

the current into the flying capacitor to limit the output

ripple beyond that of conventional switched capacitor

charge pumps. The part has two SEL pins that select the

output regulation (fixed 5V, fixed 3.3V or adjustable) as

well as shutdown.

Thechargepumpoperatesatanominalfrequencyofabout

450kHz, though actual output ripple frequency will vary

with output load, operating mode and output capacitance.

The LTC3246 is designed for applications requiring high

systemreliability.Thepartincludesoutputsupplymonitor-

ing and watchdog timing circuitry as well as overvoltage,

short-circuit and overtemperature protection.

VOUT Regulation and Mode Selection

Regulation is achieved by sensing the output voltage and

enabling charge transfer when the output falls below the

programmed regulation voltage. The amount of charge

transferred per cycle is controlled over the full input range

to minimize output ripple. The regulation voltage (fixed

5V, fixed 3.3V or adjustable) is selected through the SEL1

and SEL2 pins per Table 1 in the Pin Function section.

The optimal conversion ratio is chosen based on VIN, VOUT

and output conditions. Two internal comparators are used

to select the default conversion ratio. The conversion ratio

switchpointisoptimizedtoprovidepeakefficiencyoverall

supply and load conditions while maintaining regulation.

Each comparator also has built-in hysteresis to reduce the

tendency of oscillating between modes when a transition

point is reached.

The LTC3246 will attempt to regulate its output over the

full operating range (2.7V to 38V), but like any regulator

the output will drop out of regulation if inadequate supply

voltage exists to the operating load. As the input voltage

drops, the LTC3246 will eventually end up in the 1:2 step

up mode. As the input voltage drops further, the output

will eventually drop out of regulation. At this point, the 1:2

step-up charge pump impedance can be calculated as:

ROUT =

2 • VIN – VOUT

IOUT

This equation can be rewritten to determine the output

current at which the output will drop out for a given input

voltage as:

IOUT =

2 • VIN – VOUT

ROUT

IOUT 500mA

For a typical 1:2 step-up charge pump impedance of 4

with 5V output voltage and 3V input voltage, the output

current at dropout will be about:

IOUT =

2 • 3 – 4.8

mA = 300mA

Thus, typically the part should be able to output 300mA

without dropping out. To be conservative, the max 1:2

step-up charge pump impedance of 8 should be used

whichgivesamoreconservativeoutputcurrentof150mA.

Any supply impedance in series with the LTC3246 must

be doubled and added to the 1:2 step-up charge pump

impedance. It is also important to have the specified COUT

andCFLYcapacitancetoachievethespecifiedoutputimped-

ance. Observing dropout will allow the user to calculate

the output impedance for their specific application.

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