LTM8056 datasheet(21/42 Pages) LINER

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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

25% of the current limit (30% for the boost) determined

by RSENSE. Lower inductor values (higher ∆IL) will cause

this to occur at lower load currents, which can cause a dip

in efficiency in the upper range of low current operation. In

Burst Mode operation, lower inductance values will cause

the burst frequency to decrease.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcingtheuseofmoreexpensiveferriteormolypermalloy

cores. Actual core loss is independent of core size for a

fixedinductorvalue,butitisverydependentoninductance

value selected. As inductance increases, core losses go

down. Unfortunately, increased inductance requires more

turns of wire and therefore copper losses will increase.

Ferrite designs have very low core loss and are preferred

for high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates hard, which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Power MOSFET Selection

Two external power MOSFETs must be selected for each

controller in the LTC7813: one N-channel MOSFET for the

top switch (main switch for the buck, synchronous for the

boost), and one N-channel MOSFET for the bottom switch

(main switch for the boost, synchronous for the buck).

The peak-to-peak drive levels are set by the DRVCC volt-

age. This voltage can range from 5V to 10V depending on

configurationoftheDRVSETpin.Therefore,bothlogic-level

and standard-level threshold MOSFETs can be used in

most applications depending on the programmed DRVCC

voltage. Pay close attention to the BVDSS specification for

the MOSFETs as well.

The LTC7813’s unique ability to adjust the gate drive level

between 5V to 10V (OPTI-DRIVE) allows an application

circuit to be precisely optimized for efficiency. When

adjusting the gate drive level, the final arbiter is the total

input current for the regulator. If a change is made and

the input current decreases, then the efficiency has im-

proved. If there is no change in input current, then there

is no change in efficiency.

Selection criteria for the power MOSFETs include the

on-resistance RDS(ON), Miller capacitance CMILLER, input

voltage and maximum output current. Miller capacitance,

CMILLER, can be approximated from the gate charge curve

usually provided on the MOSFET manufacturers’ data

sheet. CMILLER is equal to the increase in gate charge

along the horizontal axis while the curve is approximately

flat divided by the specified change in VDS. This result is

then multiplied by the ratio of the application applied VDS

to the gate charge curve specified VDS. When the IC is

operating in continuous mode the duty cycles for the top

and bottom MOSFETs are given by:

Buck Main Switch Duty Cycle =

VOUT

VIN

Buck Sync Switch Duty Cycle =

VIN − VOUT

VIN

Boost Main Switch Duty Cycle =

VOUT − VIN

VOUT

Boost Sync Switch Duty Cycle =

VIN

VOUT

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