LTC4366 datasheet(17/32 Pages) AD

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LTC7862

Rev 0

For more information www.analog.com

The values for R1 and R2 are:

R1||R2

; R2

R1•RD

−RD

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

and will occur in continuous mode at the maximum input

voltage:

PLOSSR1=

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 mod-

estly higher with a DCR network than with a sense resis-

tor, due to the extra switching losses incurred through

R1. However, DCR sensing eliminates a sense resistor,

reduces conduction 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 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 addi-

tion to this basic trade-off, the effect of inductor value

on ripple current and low current operation must also be

considered.

APPLICATIONS INFORMATION

The inductor value has a direct effect on ripple current.

The inductor ripple current, ∆IL, decreases with higher

inductance or higher frequency and increases with higher

VIN:

ΔIL =

(f)(L)

VOUT 1−

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of ∆IL allows the use of low induc-

tances, 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.

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,

forcing the use of more expensive ferrite or molypermal-

loy cores. Actual core loss is independent of core size

for a fixed inductor value, but it is very dependent on

inductance 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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