SC403BEVB データシートの表示（PDF） - Semtech Corporation

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SC403BEVB

6A EcoSpeed® Integrated FET Regulator with Programmable LDO

Semtech Corporation 

SC403B

Applications Information (continued)

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

setting the frequency) using the following equation.

RTON

(tON  10ns) u VIN

25pF u VOUT

To select RTON, use the maximum value for VIN, and for tON

use the value associated with maximum VIN.

t ON

V OUT

VINMAX u f SW

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

Substituting for RTON results in the following solution.

RTON = 129.9kΩ, 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 effi-

ciency. Higher inductor values will reduce the ripple current

and ripple 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.

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

efficiency, and transient performance.

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

(VIN - VOUT). The equation for determining inductance is

shown next.

L (VIN  VOUT ) u tON

IRIPPLE

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 VIN and tON values that corre-

spond to VINMAX.

L (13.2  1.5) u 379ns 1.48PH

3A

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

decrease the typical IRIPPLE to 2.7A.

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 VIN conditions is also

checked using the following equations.

tON _ VINMIN

25pF u RTON u VOUT  10ns

VINMIN

461ns

IRIPPLE

(VIN  VOUT ) u tON

L

IRIPPLE _ MIN

(10.8  1.5) u 461ns

1.5PHu (1 0.2)

2.38A

IRIPPLE _ MAX

(10.8  1.5) u 379ns

1.5PHu (1 0.2)

3.7A

The value of L has been adjusted by +20% for the equa-

tions above assuming an inductor tolerance of +20%.

The inductor value is typically selected to provide a ripple

current that is between 25% to 50% of the maximum load

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