SC401B datasheet(23/32 Pages) SEMTECH

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SC401B

Applications Information (continued)

There are two values of load current to evaluate — con-

tinuous load current and peak load current. Continuous

load current relates to thermal stresses which drive the

selection of the inductor and input capacitors. Peak load

current determines instantaneous component stresses

and filtering requirements such as inductor saturation,

output capacitors, and design of the current limit circuit.

The following values are used in this design.

IN = 12V + 10%

OUT = 1.5V + 4%

VDD = 5V

SW = 300kHz

Load = 15A maximum

Frequency Selection

Selection of the switching frequency requires making a

trade-off between the size and cost of the external filter

components (inductor and output capacitor) and the

power conversion efficiency.

The desired switching frequency is 300kHz which results

from using components selected for optimum size and

cost.

A resistor (R

TON) is used to program the on-time (indirectly

setting the frequency) using the following equation.

TON

INMAX

OUT

Use the value associated with maximum V

IN for TON.

ON = 379 ns at 13.2VIN, 1.5VOUT, 300kHz

Substituting for R

TON results in the following solution.

TON = 133.3kΩ, use RTON = 130kΩ

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 PSAVE

operation. The switching will typically enter PSAVE mode

when the load current decreases to 1/2 of the ripple

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

operation will typically start for loads less than 2A. If ripple

current is set at 40% of maximum load current, then PSAVE

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 50% of the maximum load

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

efficiency, and transient performance.

During the on-time, voltage across the inductor is

IN - VOUT). The equation for determining inductance is

shown next.

RIPPLE

OUT

)

(

Example

In this example, the inductor ripple current is set equal to

30% of the maximum load current. Therefore ripple

current will be 30% x 15A or 4.5A. To find the minimum

inductance needed, use the V

IN and TON values that corre-

spond to V

INMAX.

379

)

(

A slightly larger value of 1µH is selected. This will decrease

the maximum I

RIPPLE to 4.43A.

Note that the inductor must be rated for the maximum DC

load current plus 1/2 of the ripple current.

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