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LT1766/LT1766-5

1766fc

APPLICATIONS INFORMATION

the high side for discontinuous mode, so it can be used

for all conditions.

Vf L

PEAK

OUT

LP P

OUT

=+ ()(

)

()( )( )()

()

–

EMI

Decide if the design can tolerate an open core geometry like

a rod or barrel, which have high magnetic ﬁeld radiation,

or whether it needs a closed core like a toroid to prevent

EMI problems. This is a tough decision because the rods

or barrels are temptingly cheap and small and there are

no helpful guidelines to calculate when the magnetic ﬁeld

radiation will be a problem.

Additional Considerations

After making an initial choice, consider additional factors

such as core losses and second sourcing, etc. Use the

experts in Linear Technology’s Applications department

if you feel uncertain about the ﬁnal choice. They have

experience with a wide range of inductor types and can tell

you about the latest developments in low proﬁle, surface

mounting, etc.

Maximum Output Load Current

Maximum load current for a buck converter is limited

by the maximum switch current rating (IP). The current

rating for the LT1766 is 1.5A. Unlike most current mode

converters, the LT1766 maximum switch current limit

does not fall off at high duty cycles. Most current mode

converters suffer a drop off of peak switch current for

duty cycles above 50%. This is due to the effects of slope

compensation required to prevent subharmonic oscilla-

tions in current mode converters. (For detailed analysis,

see Application Note 19.)

The LT1766 is able to maintain peak switch current limit

over the full duty cycle range by using patented circuitry*

to cancel the effects of slope compensation on peak switch

current without affecting the frequency compensation it

provides.

Maximum load current would be equal to maximum switch

current

for an inﬁnitely large inductor, but with ﬁnite

inductor size, maximum load current is reduced by one-

half peak-to-peak inductor current (ILP-P). The following

formula assumes continuous mode operation, implying

that the term on the right is less than one-half of IP.

IOUT(MAX) =

Continuous Mode

I–

= I

LP-P

−

() −

()

()( )( )

Lf V

OUT

–

For VOUT = 5V, VIN = 8V, VF(D1) = 0.63V, f = 200kHz and

L = 20μH:

OUT MAX

()

−

=−

() −

()

()(

)()

=−

5 0 63 8 5 0 63

2 20 10

200 10

0 21 1 29

.– .

••

Note that there is less load current available at the higher

input voltage because inductor ripple current increases.

At VIN = 15V, duty cycle is 33% and for the same set of

conditions:

OUT MAX

()

.– .

••

=−

() −

()

()(

)()

=−

−

5 0 63 15 5 0 63

2 20 10

200 10

15 044

1 06

To calculate actual peak switch current with a given set

of conditions, use:

VV V

Lf V

SW PEAK

OUT

() =+

+−

()

()( )( )

L-P

()

–

Reduced Inductor Value and Discontinuous Mode

If the smallest inductor value is of most importance to a

converter design, in order to reduce inductor size/cost,

discontinuous mode may yield the smallest inductor solu-

tion. The maximum output load current in discontinuous

mode, however, must be calculated and is deﬁned later

in this section.

*Patent # 6, 498, 466

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