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LTC3899

3899fa

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C1 is usually selected to be in the range of 0.1μF to 0.47μF.

This forces R1|| R2 to around 2k, reducing error that might

have been caused by the SENSE+ pin’s ±1μA current.

The equivalent resistance R1||R2 is scaled to the tempera-

ture inductance and maximum DCR:



R1R2=

(DCR at 20°C)•C1

The sense resistor values are:



R1=

R1R2

; R2 =

R1•RD

1−RD

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

PLOSS R1=

VIN(MAX)− VOUT

(

)•VOUT

For the boost controller, the maximum power loss in R1

will occur in continuous mode at VIN = 1/2 • VOUT:

PLOSS R1=

VOUT(MAX)− VIN

(

)•VIN

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing or

sense resistors. Light load power loss can be modestly

higher with a DCR network than with a sense resistor,

due to the extra switching losses incurred through R1.

However,DCRsensingeliminatesasenseresistor,reduces

conduction losses and provides higher efficiency at heavy

loads.Peakefficiencyisaboutthesamewitheithermethod.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because of

MOSFET switching and gate charge losses. In addition to

this basic trade-off, the effect of inductor value on ripple

currentandlowcurrentoperationmustalsobeconsidered.

The inductor value has a direct effect on ripple current.

The inductor ripple current, ∆IL, decreases with higher

inductance or higher frequency. For the buck controllers,

∆IL increases with higher VIN:

∆IL =

( ) L

( )

VOUT 1−

VOUT

VIN

















For the boost controller, ∆IL increases with higher VOUT:

∆IL =

( ) L

( )

VIN 1−

VIN

VOUT

















Accepting larger values of ∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ∆IL = 0.3(IMAX). The maximum

∆IL occurs at the maximum input voltage for the bucks

and VIN = 1/2 • VOUT for the boost.

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.

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