LTC3859 datasheet(21/40 Pages) LINER

21 / 40 page

LTC3859

3859f

APPLICATIONS INFORMATION

The maximum power loss in R1 is related to duty cycle. For

the buck controllers, the maximum power loss 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,

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 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 gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current and

low current operation must also be considered.

The inductor value has a direct effect on ripple current.

The inductor ripple current

ΔIL decreases with higher

inductance or frequency. For the buck controllers,

ΔIL

increases with higher VIN:

ΔIL =

(f)(L)

VOUT 1−

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

For the boost controller, the inductor ripple current

ΔIL

increases with higher VOUT:

ΔIL =

(f)(L)

VIN 1−

VIN

VOUT

⎛

⎝⎜

⎞

⎠⎟

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 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 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,

forcing the use of more expensive ferrite or molypermalloy

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

ﬁxed inductor value, but it is very dependent on inductance

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

at 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 and Schottky Diode

(Optional) Selection

Two external power MOSFETs must be selected for each

controller in the LTC3859: 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).

LTC3859 Datasheet(PDF) 21 Page - Linear Technology

LTC3859 Datasheet(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL