LTC3707IGN datasheet(14/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

Selection of Operating Frequency

The LTC3707 uses a constant frequency architecture with

the frequency determined by an internal oscillator capacitor.

This internal capacitor is charged by a ﬁxed current plus

an additional current that is proportional to the voltage

applied to the FREQSET pin.

A graph for the voltage applied to the FREQSET pin vs

frequency is given in Figure 5. As the operating frequency

is increased the gate charge losses will be higher, reducing

efﬁciency (see Efﬁciency Considerations). The maximum

switching frequency is approximately 310kHz.

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 and increases with higher VIN:

ΔI

L =

(f)(L)

OUT

1–

OUT

⎛

⎝⎜

⎞

⎠⎟

OPERATING FREQUENCY (kHz)

120

170

220

270

320

3707 F05

2.5

2.0

1.5

1.0

0.5

Figure 5. FREQSET Pin Voltage vs Frequency

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

ΔI = 30% • IOUT(MAX) or higher for

good load transient response and sufﬁcient ripple current

signal in the current loop. Remember, 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,

forcing the use of more expensive ferrite, molypermalloy,

or Kool Mμ® 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!

Molypermalloy (from Magnetics, Inc.) is a very good, low

loss core material for toroids, but it is more expensive

than ferrite. A reasonable compromise from the same

manufacturer is Kool Mμ. Toroids are very space efﬁcient,

especially when you can use several layers of wire. Because

they generally lack a bobbin, mounting is more difﬁcult.

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