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LTC1771

The basic LTC1771 application circuit is shown in Figure

1 on the first page. External component selection is driven

by the load requirement and begins with the selection of

RSENSE. Once RSENSE is known, L can be chosen. Next, the

MOSFET and D1 are selected. The inductor is chosen

based largely on the desired amount of ripple current and

for Burst Mode operation. Finally CIN is selected for its

ability to handle the required RMS input current and COUT

is chosen with low enough ESR to meet the output voltage

ripple and transient specifications.

RSENSE Selection

RSENSE is chosen based on the required output current.

The LTC1771 current comparator has a maximum thresh-

old of 140mV/RSENSE. The current comparator threshold

sets the peak inductor current, yielding a maximum aver-

age output current IMAX equal to the peak less half the

peak-to-peak ripple current

∆IL. For best performance

when Burst Mode operation is enabled, choose

∆IL equal

to 35% of peak current. Allowing a margin for variations in

the LTC1771 and external components gives the following

equation for choosing RSENSE:

RSENSE = 100mV/IMAX

At higher supply voltages, the peak currents may be

slightly higher due to overshoot from current comparator

delay and can be predicted from the second term in the

following equation:

PEAK

SENSE

OUT

≅+









014

–

()

Inductor Value Selection

Once RSENSE is known, the inductor value can be deter-

mined. The inductance value has a direct effect on ripple

current. The ripple current decreases with higher induc-

tance and increases with higher VOUT. The ripple current

during continuous mode operation is set by the off-time

and inductance to be:

∆=









L CONT

OFF

OUT

()

where tOFF = 3.5µs. However, the ripple current at low

loads during Burst Mode operation is:

∆IL(BURST) ≈ 35% of IPEAK ≈ 0.05/RSENSE

For best efficiency when Burst Mode operation is enabled,

choose:

∆IL(CONT) ≤ ∆IL(BURST)

so that the inductor current is continuous during the burst

periods. This sets a minimum inductor value of:

LMIN = (75µH)(VOUT + VD)(RSENSE)

When burst is disabled, ripple currents less than

∆IL(BURST)

can be achieved by choosing L > LMIN. Lower ripple

current reduces output voltage ripple and core losses, but

too low of ripple current will adversely effect efficiency.

Inductor Core Selection

Once the value of 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 M

µ® cores. Actual core loss is

independent of core size for a fixed inductor value, but 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 increase in 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 manu-

facturer is Kool M

µ. Toroids are space efficient, especially

when you can use several layers of wire. Because they

generally lack a bobbin, mounting is more difficult. How-

ever, designs for surface mount are available that do not

increase the height significantly.

Kool M

µ is a registered trademark of Magnetics, Inc.

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