LM3743 datasheet(16/23 Pages) NSC

16 / 23 page

Application Information (Continued)

be followed to create many other designs with varying input

voltages, output voltages, load currents, and switching fre-

quency.

Switching Frequency

Selection of the operating switching frequency is based on

trade-offs between size, cost, efficiency, and response time.

For example, a lower frequency will require larger more

expensive inductors and capacitors. While a higher switch-

ing frequency will generally reduce the size of these compo-

nents, but will have a reduction in efficiency. Fast switching

converters allow for higher loop gain bandwidths, which in

turn have the ability to respond quickly to load and line

transients. For the example application we have chosen a

300 kHz switching frequency because it will reduce the

switching power losses and in turn allow for higher conduc-

tion losses considering the same power loss criteria, thus it

is possible to sustain a higher load current.

Output Inductor

The output inductor is responsible for smoothing the square

wave created by the switching action and for controlling the

output current ripple (

∆I

OUT) also called the AC component

of the inductor current. The DC current into the load is equal

to the average current flowing in the inductor. The induc-

tance is chosen by selecting between trade-offs in efficiency,

size, and response time. The recommended percentage of

AC component to DC current is 30% to 40%, this will provide

the best trade-off between energy requirements and size,

(read AN-1197 for theoretical analysis). Another criteria is the

ability to respond to large load transient responses; the

smaller the output inductor, the more quickly the converter

can respond. The equation for output inductor selection is:

Here we have plugged in the values for input voltage, output

voltage, switching frequency, and 30% of the maximum load

current. This yields an inductance of 1.34 µH. The output

inductor must be rated to handle the peak current (also equal

to the peak switch current), which is (I

OUT + (0.5 x

∆I

OUT)) =

11.5A, for a 10A design and a AC current of 3A.

The Coiltronics DR125–1R5 is 1.5 µH, is rated to 13.8A RMS

current, and has a direct current resistance (DCR) of 3 m

Ω.

After selecting the Coiltronics DR125–1R5 for the output

inductor, actual inductor current ripple must be re-calculated

with the selected inductance value. This information is

needed to determine the RMS current through the input and

output capacitors. Re-arranging the equation used to select

inductance yields the following:

IN(MAX) is assumed to be 10% above the steady state input

voltage, or 5.5V at V

IN = 5.0V. The re-calculated current

ripple will then be 2.69A. This gives a peak inductor/switch

current will be 11.35A.

Output Capacitor

The output capacitor in a switching regulator is selected on

the basis of capacitance, equivalent series resistance (ESR),

size, and cost. In this example the output current is 10A and

the expected type of capacitor is an aluminum electrolytic, as

with the input capacitors. An important specification in

switching converters is the output voltage ripple

∆V

OUT.At

300 kHz the impedance of most capacitors is very small

compared to ESR, hence ESR becomes the main selection

criteria. In this design the load requires a 2% ripple , which

results in a

∆V

OUT of 36 mVP-P. Thus the maximum ESR is

then:

ESR

MAX is 13 m

Ω. Aluminum electrolytic (Al-E), tantalum

(Ta), solid aluminum, organic, and niobium (Nb) capacitors

are all popular in switching converters. In general, by the

time enough capacitors have been paralleled to obtain the

desired ESR, the bulk capacitance is more than enough to

supply the load current during a transient from no-load to full

load. The number and type of capacitors used depends

mainly on their size and cost. One exception to this is

multi-layer ceramic capacitors. MLCCs have very low ESR,

but also low capacitance in comparison with other types.

This makes them attractive for lower power designs. For

higher power or for fast load transients the number of ML-

CCs needed often increases the size and cost to unaccept-

able levels. Because the load could transition quickly from 0

to 10A, more bulk capacitance is needed than the MLCCs

can provide. One compromise is a solid electrolytic POS-

CAP from Sanyo or SP-caps from Panasonic. POSCAP and

SPcaps often have large capacitances needed to supply

currents for load transients, and low ESRs. The 6TPD470M

by Sanyo has 470 µF, and a maximum ESR of 10 m

Ω. Solid

electrolytics have stable ESR relative to temperature, and

capacitance change is relatively immune to bias voltage.

Tantalums (Ta), niobium (Nb), and Al-E are good solutions

for ambient operating temperatures above 0˚C, however

their ESR tends to increase quickly below 0˚C ambient op-

erating temperature, so these capacitor types are not rec-

ommended for this area of operation.

Input Capacitor

The input capacitors in a buck converter are subjected to

high RMS current stress. Input capacitors are selected for

their ability to withstand the heat generated by the RMS

current and the ESR as specified by the manufacturer. Input

RMS ripple current is approximately:

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