SC414EVB datasheet(22/29 Pages) SEMTECH

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SC414/SC424

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.

= 12V + 10%

OUT

= 1V + 4%

= 250kHz

Load = 6A 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 250kHz 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.

OUT

TON

400

To select R

TON

, use the maximum value for V

, and for T

use the value associated with maximum V

INMAX

OUT

ON = 303 ns at 13.2VIN, 1VOUT, 250kHz

Substituting for R

TON

results in the following solution.

TON = 130.9kΩ, 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 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 50% 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

- V

OUT

). The equation for determining inductance is

shown next.

RIPPLE

OUT

)

(

Example

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

50% of the maximum load current. Therefore ripple

current will be 50% x 6A or 3A. To find the minimum

inductance needed, use the V

and T

values that corre-

spond to V

INMAX

318

)

(

A slightly larger value of 1.5μH is selected. This will

decrease the maximum I

RIPPLE

to 2.53A.

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

conditions is also

checked using the following equations.

311

INMIN

OUT

TON

VINMIN

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