LT1432CS8 datasheet(17/28 Pages) LINER

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LT1432

function of duty cycle. Care must be taken to ensure that

the sum of output current plus 1/2 inductor p-p ripple

current does not exceed the switch current limit at the

highest duty cycle (lowest input voltage).

Duty Cycle (maximum)

Vf = Diode forward voltage

1/2 p-p Ripple Current

(Use minimum VIN +2V)

A 2.5A design using an LT1271 at 60kHz, with a minimum

input voltage of 7V and a 50

µH inductor, would have a

maximum duty cycle of (5 + 0.5)/7 = 79%. 1/2 p-p ripple

current would be:

5 7

2–5

2 7 + 2 60E

50E

0.37A

3–6

() +

()

() 





Output current plus 1/2 ripple current = 2.5 + 0.37 = 2.9A.

The switch current rating for the LT1271 is shown on the

data sheet as 4A for duty cycle below 50% and 2.67 (2–

DC) for duty cycles greater than 50%. With DC = 79%,

switch current rating would be 2.67 (2 – 0.79) = 3.23A, so

this meets the guidelines. It should be noted that if normal

running load current conditions result in switch currents

that are close to the maximum switch ratings, efficiency

will drop. Switch voltage loss at maximum switch current

rating is typically 0.7V, and this represents a significant

loss, especially at low input voltages. In most laptop

computer designs, surge currents from hard or floppy

disks require an oversized switcher, so normal running

currents are typically less than one half rated switch

current and efficiency is high except during the short

surge periods.

2. Ferrite designs have very low core loss, so design

goals can concentrate on copper loss and preventing

saturation. The downside is that the finished unit will

almost surely be larger than a molypermalloy toroid de-

sign because of the basic topological limitations of the

ferrite/bobbin arrangement. Newer low-profile ferrite cores

are even less space efficient than older configurations.

Cost may also be higher. Ferrite core material saturates

“hard,” which means that inductance collapses abruptly

when peak design current is exceeded. This may be a

problem in current limit or if peak load requirements are

not well characterized.

3. Molypermalloy (from Magnetics, Inc.) is a very

good, low loss core material for toroids, but it is (naturally)

rather expensive. A reasonable substitute is KoolM

µ(same

manufacturer). Toroids are very space efficient, especially

when you can convince the manufacturer to use several

layers of wire. Because they generally lack a bobbin,

mounting is more difficult. Newer designs for surface

mount are available (Coiltronics), which are nested in a

ring that does not increase the height significantly.

Catch Diode

The catch diode carries load current only during switch

“off” time. Its average current is therefore dependent on

switch duty cycle. At high input voltages, the diode con-

ducts most of the time, and as VIN approaches VOUT, it

conducts only a small fraction of the time. The current

rating of the diode should be higher than maximum load

current for two reasons. First, conservative diode current

improves efficiency because the diode forward voltage is

lower, and second, short circuit conditions result in near

100% diode duty cycle at currents higher than full load

unless some form of foldback current limiting is used.

Schottky diodes are a must for their low forward drop and

fast switching times.

Maximum diode reverse voltage is equal to maximum

input voltage. However, do not over-specify the diode for

breakdown voltage. Schottky diodes are made with lighter

silicon doping as breakdown ratings increase. This gives

higher forward voltage and degrades regulator efficiency.

An MBR350 (3A, 50V) has almost 100mV higher forward

voltage than the MBR330 (3A, 30V).

Diode current ratings are predicated on proper thermal

mounting techniques. Check the manufacturers assump-

tions carefully before assuming that a 3A diode is actually

capable of carrying 3A continuously. Pad size may have to

be larger than normal to meet the mounting requirements

for full current capability.

APPLICATI

I FOR ATIO

()

VVf

OUT

IN MIN

= ()(

)

()( )( )

VV – V

f L

OUT

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