LTC3126 datasheet(21/30 Pages) LINER

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LTC3126

3126f

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VCC Regulators and Bootstrapping with EXTVCC

The VCC rail powers the internal control circuitry and

power device gate drivers of the LTC3126. Two internal

lowdropoutlinearregulatorsprovidetheabilitytogenerate

this rail from either VIN1 or VIN2. When the IC is disabled

(ENA low) the VCC rail is powered by the higher voltage

input, VIN1 or VIN2, regardless of the state of the VSET1,

VSET2 and DIODE pins. When the IC is enabled, the input

voltage comparators on the VSET1 and VSET2 pins become

active and the VCC rail will be powered by the active chan-

nel. In ideal diode mode (DIODE high) the active channel

is the valid input with the higher voltage. In priority mode

(DIODE low) the active channel is VIN1 if VIN1 is valid or

VIN2 otherwise (assuming it is valid). If both VIN1 and VIN2

are in UVLO then the higher voltage input will be utilized

to power the VCC rail.

A third linear regulator allows the VCC rail to be powered

via the EXTVCC pin which can be connected to the buck

converter output or an auxiliary rail with a voltage above

3.15V. When operating at high input voltages, the losses

in the VCC regulator powered from the input voltage can

become a significant factor in conversion efficiency and

canevenbecomeasubstantialsourceofpowerdissipation.

A significant performance improvement can be obtained

by connecting the EXTVCC input to the buck converter

output so that the gate drive current is provided through

the high efficiency buck converter rather than the less

efficient linear regulator. This is of particular benefit at

higher input voltages, lower output voltages and higher

switching frequencies. The EXTVCC pin is only utilized to

power the VCC rail when the buck converter is operating.

Thermal Considerations

The LTC3126 is designed to operate continuously up to

its full rated 2.5A output current. However, when operat-

ing at high current levels there will be significant heat

generated within the IC. In addition, in many applications

the VCC regulator is operated with large input-to-output

voltage differential resulting in significant levels of power

dissipation in its pass element which can add significantly

to the total power dissipated within the IC. To ensure full

output current capability and optimal efficiency, careful

consideration must be given to the thermal environment

of the IC. This is even more important in applications that

function over an extended ambient temperature range.

Specifically, the exposed die attach pad of both the QFN

and TSSOP packages must be soldered to the PC board

and the PC board should be designed to maximize the

conduction of heat out of the IC package. This can be

accomplished by utilizing multiple vias from the die attach

pad connection to other PCB layers containing a large area

of exposed copper.

If the die temperature exceeds approximately 170°C, the

IC will enter overtemperature shutdown and all switching

will be inhibited. The part will remain disabled until the

die cools by approximately 10°C. The soft-start circuit is

re-initialized in overtemperature shutdown to provide a

smooth recovery when the fault condition is removed.

PCB Layout Guidelines

The LTC3126 buck converter switches large currents at

high frequencies. Special attention must be paid to the PC

board layout to ensure a stable, noise-free and efficient

application circuit. Figures 6 and 7 show representative

PC board layouts for each package option to outline some

of the primary considerations. A few key guidelines are

listed below.

1. Theparasiticinductanceandresistanceofallcirculating

high current paths should be minimized. This can be

accomplished by keeping the routes to the inductor,

output capacitor and PVIN1/2 bypass capacitors as

short and as wide as possible. Capacitor ground con-

nections should via down to the ground plane by way

of the shortest route possible. The bypass capacitors

on PVIN1, PVIN2 and VCC should be placed as close to

theICaspossibleandshouldhavetheshortestpossible

return paths to ground.

2. The exposed pad in both packages provides one of the

primary paths for heat generated within the package.

The IC must be soldered down to the backpad and the

backpad area should be filled with vias connecting it

to the ground plane.

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