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LTC3852

3852f

APPLICATIONS INFORMATION

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant

frequency architectures by preventing sub-harmonic

oscillations at high duty cycles. It is accomplished internally

by adding a compensating ramp to the inductor current

signal. Normally, this results in a reduction of maximum

inductor peak current for duty cycles >40%. However, the

LTC3852 uses a novel scheme that allows the maximum

inductor peak current to remain unaffected throughout

all duty cycles.

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. 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

DIL decreases with higher

inductance or frequency and increases with higher VIN:

ΔI

L =

f• L

OUT 1–

OUT

⎛

⎝⎜

⎞

⎠⎟

VIN2

VIN

INTVCC

BOOST

GND2

INDUCTOR

DCR

SENSE+

SENSE–

LTC3852

VOUT

3852 F03

R1**

*PLACE C1 NEAR SENSE+, SENSE– PINS

**PLACE R1 NEAR INDUCTOR

C1*

R1||R2 • C1 =

RSENSE(EQ) = DCR

DCR

R1 + R2

Accepting larger values of

DIL 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

DIL = 0.3(IMAX). The maximum

DIL 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

≈10% of the current limit determined by RSENSE. Lower

inductor values (higher

DIL) 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 increase.

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 the

LTC3852 controller: one N-channel MOSFET for the top

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

(synchronous) switch.

Figure 3. Current Mode Control Using the Inductor DCR

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