LT1507 データシートの表示（PDF） - Linear Technology

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LT1507

500kHz Monolithic Buck Mode Switching Regulator

Linear Technology 

LT1507

APPLICATIONS INFORMATION

pin (RDIV = R1/R2 ≤ 4k). The net result is that reductions

in frequency and current limit are affected by output

voltage divider impedance. Although divider impedance is

not critical, caution should be used if resistors are

increased beyond the suggested values and short-circuit

conditions will occur with high input voltage. High

frequency pickup will also increase and the protection

accorded by frequency and current foldback will decrease.

CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR

For most applications the value of the inductor will fall in

the range of 2µH to 10µH. Lower values are chosen to

reduce physical size of the inductor. Higher values allow

more output current because they reduce peak current

seen by the LT1507 switch, which has a 1.5A limit. Higher

values also reduce output ripple voltage and reduce core

loss. Graphs in the Typical Performance Characteristics

section show maximum output load current versus induc-

tor size and input voltage. A second graph shows core loss

versus inductor size for various core materials.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allowable

component height, output voltage ripple, EMI, fault cur-

rent in the inductor, saturation and, of course, cost. The

following procedure is suggested as a way of handling

these somewhat complicated and conflicting requirements.

1. Choose a value in microhenries from the graphs of

Maximum Load Current and Inductor Core Loss for

3.3V Output. If you want to double check that the

chosen inductor value will allow sufficient load current,

go to the next section, Maximum Output Load Current.

Choosing a small inductor with lighter loads may result

in discontinuous mode of operation, but the LT1507 is

designed to work well in either mode. Keep in mind that

lower core loss means higher cost, at least for closed-

core geometries like toroids. Type 52 powdered iron,

Kool Mµ and Molypermalloy are old standbys for tor-

oids in ascending order of price. A newcomer, Metglas,

gives very low core loss with high saturation current.

Assume that the average inductor current is equal to

load current and decide whether or not the inductor

must withstand continuous fault conditions. If maxi-

mum load current is 0.5A, for instance, a 0.5A inductor

may not survive a continuous 1.5A overload condition.

Dead shorts (VOUT ≤ 1V) will actually be more gentle on

the inductor because the LT1507 has foldback current

limiting (see graph in Typical Performance Character-

istics).

2. Calculate peak inductor current at full load current to

ensure that the inductor will not saturate. Peak current

can be significantly higher than output current, espe-

cially with smaller inductors and lighter loads, so don’t

omit this step. Powdered iron cores are forgiving

because they saturate softly, whereas ferrite cores

saturate abruptly. Other core materials fall in between

somewhere. The following formula assumes a con-

tinuous mode of operation, but it errs only slightly on

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

for all conditions.

IPEAK

=

IOUT

+

VOUT (VIN – VOUT)

2(f)(L)(VIN)

VIN = Maximum input voltage

f = Switching frequency = 500kHz

3. Decide if the design can tolerate an “open” core geom-

etry like ferrite rods or barrels, which have high mag-

netic field radiation or whether it needs a closed core

like a toroid to prevent EMI problems. One would not

want an open core next to a magnetic storage media for

instance! 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. The following is an example

of just how subtle the “B” field problems can be with

open geometry cores.

We had selected an open drum shaped ferrite core for

the LTC1376 demonstration board because the induc-

tor was extremely small and inexpensive. It met all the

requirements for current and the ferrite core gave low

core loss. When the boards came back from assembly,

many of them had somewhat higher than expected

output ripple voltage. We removed the inductors and

output capacitors and found them to be no different

than the good boards. After much head scratching and

hours of delicate low level ripple measurements on the

good and bad boards, I realized that the problem must

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