LT1432CS8 datasheet(16/28 Pages) LINER

16 / 28 page

LT1432

Inductor Selection

Inductor selection would be easy if money and space didn’t

count. Unfortunately, these two factors usually count the

most, and compromises must be made. High efficiency

converters generally cannot afford the core loss found in

low cost powdered iron cores, forcing the use of more

expensive cores such as ferrite, molypermalloy, or KoolM

µ.

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 loss goes down.

Unfortunately, increased inductance requires more turns

of wire and therefore copper loss will increase. The trick is

to find the smallest inductor whose inductance is high

enough to limit core loss, and whose series resistance is

low enough to limit copper loss. Historically, inductor

manufacturers have a tendency to be ultra conservative

when designing inductors, and unless you are very specific

about your constraints and requirements, they will more

often than not come up with a unit which is 50% larger than

the optimum. Part of this is due to manufacturing consid-

erations. The trade-off of core loss and copper loss is

optimized by “filling the winding window” with wire, but

especially for toroids this can require more expensive

winding techniques than the widely used “single layer”

design. The lesson here is to spend time with the manufac-

turer exploring the cost trade-offs of different inductor

designs. The following guidelines may be helpful in this

regard.

1. For most buck converter applications using the

LT1070, LT1170, or LT1270 families of parts at 40kHz to

100kHz, inductor value will be in the range of 25

µH to

200

µH. The lower values would be used for higher output

currents and/or higher frequencies, with higher values

used for low output current, low frequency applications.

Lower inductance obviously means smaller size, but at

some point the core loss will begin to hurt, or the large

peak-to-peak inductor currents will cause high output

ripple voltage or limit available output current. The follow-

ing formula is a rough guide for picking an initial inductor

value:

MAX

= ()( )

IMAX = maximum load current, including surges

f = switching frequency

This formula assumes that a switcher IC is selected which

has a maximum switch current of 1.5 to 2.5 times maxi-

mum load current. For a 2.5A design using the LT1271 at

60kHz, L would calculate to 53

µH. This formula is very

arbitrary, so do not hesitate to modify the calculated value

by as much as 2:1 if the need arises. Keep in mind that all

the IC switchers have a peak current rating which is a

APPLICATI

I FOR ATIO

At heavy loads, switch on-resistance increases minimum

input voltage. With an LT1071 for instance, minimum

input is 6.1V at 1A load, but increases to 6.3V at 2A load.

If absolute minimum input voltage is needed, use lower

frequency devices with higher current rating than is actu-

ally needed. The LT1070, for instance, operates down to

6.15V at 2A. Current limit is defined by the LT1432, so

higher current switchers used in lower current applica-

tions do not degrade performance or reliability.

Minimum Load Current in Normal Mode

There is a minimum load current requirement in normal

mode. This is caused by the necessity to “pump” the IC

switcher floating power supply capacitor during switch

“off” time. This pumping current comes from inductor

current, so load current must not be allowed to drop too

low, or the floating bias supply for the switcher will

collapse. Minimum load current is a function of input

voltage as shown in Figure 16.

INPUT VOLTAGE (V)

LT1432 F15

510

Figure 16. Minimum Normal Mode Load Current

LT1432CS8 датащи(PDF) 16 Page - Linear Technology

LT1432CS8 датащи(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL