LTC1530CS8 datasheet(13/24 Pages) LINER

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LTC1530

Inductor Selection

The inductor is often the largest component in an LTC1530

design and must be chosen carefully. Choose the inductor

value and type based on output slew rate requirements

and expected peak current. The required output slew rate

primarily controls the inductor value. The maximum rate

of rise of inductor current is set by the inductor’s value, the

input-to-output voltage differential and the LTC1530’s

maximum duty cycle. In a typical 5V input, 2.8V output

application, the maximum rise time will be:

MAX

OUT

−









185

where L is the inductor value in

µH. With proper frequency

compensation, the combination of the inductor and output

capacitor values determine the transient recovery time. In

general, a smaller value inductor improves transient

response at the expense of ripple and inductor core

saturation rating. A 2

µH inductor has a 0.9A/µs rise time

in this application, resulting in a 5.5

µs delay in responding

to a 5A load current step. During this 5.5

µs, the difference

between the inductor current and the output current is

made up by the output capacitor. This action causes a

temporary voltage droop at the output. To minimize this

effect, the inductor value should usually be in the 1

µH to

µH range for most 5V input LTC1530 circuits. Different

combinations of input and output voltages and expected

loads may require different values.

Once the required inductor value is selected, choose the

inductor core type based on peak current and efficiency

requirements. Peak current in the inductor is equal to the

maximum output load current plus half of the peak-to-

peak inductor ripple current. Inductor ripple current is set

by the inductor’s value, the input voltage, the output

voltage and the operating frequency. If the efficiency is

high, ripple current is approximately equal to:

RIPPLE

OUT

OSC

−

()( )

()( )( )

where

fOSC = LTC1530 oscillator frequency

LO = Inductor value

Solving this equation for a typical 5V to 2.8V application

with a 2

µH inductor, ripple current is:

056

300

kHz

()( )

P-P

Peak inductor current at 11.2A load:

11 2

12 2

The ripple current should generally fall between 10% and

40% of the output current. The inductor must be able to

withstand this peak current without saturating, and the

copper resistance in the winding should be kept as low as

possible to minimize resistive power loss. Note that in

circuits not employing the current limit function, the

current in the inductor may rise above this maximum

under short circuit or fault conditions; the inductor should

be sized accordingly to withstand this additional current.

Inductors with gradual saturation characteristics (example:

powdered iron) are often the best choice.

Input and Output Capacitors

A typical LTC1530 design places significant demands on

both the input and the output capacitors. During normal

steady load operation, a buck converter like the LTC1530

draws square waves of current from the input supply at the

switching frequency. The peak current value is equal to the

output load current plus 1/2 the peak-to-peak ripple cur-

rent. Most of this current is supplied by the input bypass

capacitor. The resulting RMS current flow in the input

capacitor heats it and causes premature capacitor failure

in extreme cases. Maximum RMS current occurs with

50% PWM duty cycle, giving an RMS current value equal

to IOUT/2. A low ESR input capacitor with an adequate

ripple current rating must be used to ensure reliable

operation. Note that capacitor manufacturers’ ripple cur-

rent ratings are often based on only 2000 hours (3 months)

lifetime at rated temperature. Further derating of the input

capacitor ripple current beyond the manufacturer’s speci-

fication is recommended to extend the useful life of the

circuit. Lower operating temperature has the largest effect

on capacitor longevity.

APPLICATIO S I FOR ATIO

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