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LTC7800

7800f

For more information www.linear.com/LTC7800

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

perature inductance and maximum DCR:

R1|| R2

DCR at 20

°C

(

) •C1

The resistor values are:

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

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

totheextraswitchinglossesincurredthroughR1.However,

DCR sensing eliminates a sense resistor, reduces conduc-

tion losses and provides higher efficiency at heavy loads.

Peak efficiency is about the same with either method.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related n 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,

ΔIL, decreases with higher induc-

tance or higher frequency and increases with higher VIN:

ΔIL =

( ) L()

VOUT 1–

VOUT

VIN













Accepting larger values of

ΔIL 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

ΔIL = 0.3(IMAX). The maximum

ΔIL occurs at the maximum input voltage.

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 determined by RSENSE. Lower

inductor values (higher

ΔIL) 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!

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