LTC3104 datasheet(12/26 Pages) LINER

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LTC3630

3630fb

APPLICATIONS INFORMATION

well-controlled, the inductor value must be chosen so that

it is larger than a minimum value which can be computed

as follows:

VIN(MAX) •tON(MIN)

IPEAK

•1.2

where VIN(MAX) is the maximum input supply voltage when

switching is enabled, tON(MIN) is 150ns, IPEAK is the peak

current, and the factor of 1.2 accounts for typical inductor

tolerance and variation over temperature. Inductor values

that violate the above equation will cause the peak current

to overshoot and permanent damage to the part may occur.

Although the above equation provides the minimum in-

ductor value, higher efficiency is generally achieved with

a larger inductor value, which produces a lower switching

frequency. The inductor value chosen should also be large

enough to keep the inductor current from going very nega-

tive which is more of a concern at higher VOUT (>~12V). For

a given inductor type, however, as inductance is increased,

DC resistance (DCR) also increases. Higher DCR trans-

lates into higher copper losses and lower current rating,

both of which place an upper limit on the inductance. The

recommended range of inductor values for small surface

mount inductors as a function of peak current is shown

in Figure 4. The values in this range are a good compromise

between the trade-offs discussed above. For applications

where board area is not a limiting factor, inductors with

larger cores can be used, which extends the recommended

range of Figure 4 to larger values.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of the more expensive ferrite cores. Actual

core loss is independent of core size for a fixed inductor

value but is very dependent of the inductance selected.

As the inductance increases, core losses decrease. Un-

fortunately, increased inductance requires more turns of

wire and therefore copper losses will increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing satura-

tion. Ferrite core material saturates “hard,” which means

that inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequently output voltage

ripple. Do not allow the core to saturate!

Figure 4. Recommended Inductor Values for Maximum Efficiency

Figure 3. Switching Frequency for VOUT = 3.3V

VIN INPUT VOLTAGE (V)

400

500

600

3630 F03

300

200

100

VOUT = 3.3V

ISET OPEN

L = 4.2μH

L = 10μH

L = 22μH

L = 47μH

L = 100μH

PEAK INDUCTOR CURRENT (mA)

100

1000

3630 F04

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