ML0308 Preliminary

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5/9

Rev. B, July 2005

APPLICATION INFORMATION

Choosing an Inductor

Several recommended inductors that work well

with the ML0308. Consult each manufacturer for

more detailed information and for their entire

selection of related parts. Many different sizes and

shapes are available. Use the equations and

recommendations in the next few sections to find

the correct inductance value for your design.

Inductor Selection—Boost Regulator

The formula below calculates the appropriate

inductor value to be used for a boost regulator

using the ML0308 (or at least provides a good

starting point). This value provides a good

tradeoff in inductor size and system performance.

Pick a standard inductor close to this value. A

larger value can be used to slightly increase the

available output current, but limit it to around

twice the value calculated below, as too large of

an inductance will increase the output voltage

ripple without providing much additional output

current. A smaller value can be used (especially

for systems with output voltages greater than 12V)

to give a smaller physical size. Inductance can be

calculated as:

OFF

LIMIT

D

MIN

IN

OUT

t

I

V

V

V

L

+

−

=

)

(

For most systems with output voltages below 7V,

a 4.7uH inductor is the best choice, even though

the equation above might specify a smaller value.

This is due to the inductor current overshoot that

occurs when very small inductor values are used

(see Current Limit Overshoot section). For higher

output voltages, the formula above will give large

inductance values. For a 3V to 16V converter

(typical LCD Bias application), a 22uH inductor is

called for with the above equation, but a 10uH

inductor

could

be

used

without

excessive

reduction in maximum output current.

Current Limit Overshoot

For a fixed duty ratio PFM control scheme of the

ML0308, the power switch is turned off after 3/4

of a cycle or the 180mA current limit is reached.

There is a 100ns delay between the time when the

current limit is reached and when the switch

actually turns off. During this delay, the inductor

current exceeds the current limit by a small

amount. The peak inductor current can be

calculated by

d

DSSAT

MAX

IN

LIMIT

PEAK

t

L

V

V

I

I













−

+

=

)

(

evident for systems with high input voltages and

for systems where smaller inductor values are

used. This overshoot can be beneficial as it helps

increase the amount of available output current for

smaller inductor values. This will be the peak

current seen by the inductor (and the diode) during

normal

operation.

For

designs

using

small

inductance values (especially at input voltages

greater than 5V), the current limit overshoot can

be quite high. Although it is internally current

limited to 180mA, the power switch of the

ML0308 can handle larger currents without

problem, but the overall efficiency will suffer.