SC508 datasheet(23/32 Pages) SEMTECH

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SC508/SC508A

Applications Information (continued)

Inductor Selection

In order to determine the inductance, the ripple current

must first be defined. Low inductor values result in smaller

size but create higher ripple current which can reduce

efficiency. Higher inductor values will reduce the ripple

current/voltage and for a given DC resistance are more

efficient. However, larger inductance translates directly

into larger packages and higher cost. Cost, size, output

ripple, and efficiency are all used in the selection process.

The ripple current will also set the boundary for power-

save operation. The switching will typically enter power-

save mode when the load current decreases to 1/2 of the

ripple current. For example, if ripple current is 4A then

Power-save operation will typically start for loads less than

2A. If ripple current is set at 40% of maximum load current,

then power-save will start for loads less than 20% of

maximum current.

The inductor value is typically selected to provide a ripple

current that is between 25% to 60% of the maximum load

current. This provides an optimal trade-off between cost,

efficiency, and transient performance.

During the DH on-time, voltage across the inductor is

IN - VOUT). The following equation for determining induc-

tance is shown.

RIPPLE

OUT

)

(

In this example the inductor ripple current is set approxi-

mately equal to 50% of the maximum load current. Thus

ripple current target will be 50% x 8A or 4A.

To find the minimum inductance needed, use the V

IN and

ON values that correspond to VINMAX.

A slightly smaller value of 1.8µH is selected. This will

increase the maximum I

RIPPLE to 4.3A.

Note that the inductor must be rated for the maximum DC

load current plus 1/2 of the ripple current.

The ripple current under minimum V

IN conditions is also

checked using the following equations.

)

(

OUT

RIPPLE

Capacitor Selection

The output capacitors are chosen based on required ESR

and capacitance. The maximum ESR requirement is con-

trolled by the output ripple requirement and the DC toler-

ance. The output voltage has a DC value that is equal to

the valley of the output ripple plus 1/2 of the peak-to-peak

ripple. Change in the output ripple voltage will lead to a

change in DC voltage at the output.

The design goal is for the output voltage regulation to be

±4% under static conditions. The internal 600mV refer-

ence tolerance is 1%. Allowing 1% tolerance from the FB

resistor divider, this allows 2% tolerance due to V

OUT ripple.

Since this 2% error comes from 1/2 of the ripple voltage,

the allowable ripple is 4%, or 72mV for a 1.8V output.

The maximum ripple current of 4.3A creates a ripple

voltage across the ESR. The maximum ESR value allowed

is shown by the following equations.

ESR

MAX = 16.7 mΩ

The output capacitance is chosen to meet transient

requirements. A worst-case load release, from maximum

load to no load at the exact moment when inductor

current is at the peak, determines the required capaci-

tance. If the load release is instantaneous (load changes

from maximum to zero in < 1µs), the output capacitor

must absorb all the inductor’s stored energy. This will

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