SC1480ITSTRT datasheet(8/22 Pages) SEMTECH

8 / 22 page

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SC1480

POWER MANAGEMENT

Applications Information (Cont.)

a scaling resistor between LX and ILIM is required. This

resistor, R

ILIM, is connected to a 10µA current source

within the SC1480 through the ILIM pin. This sets a volt-

age drop equal to 10µA times R

ILIM. As the current in-

creases through the lower MOSFET, the phase pin volt-

age will decrease until the offset voltage caused by R

ILIM

is reached and ILIM < PGND. At this point an overcurrent

trip signal is issued. Current limiting will prevent the firing

of a DH on-pulse, thereby reducing the switching fre-

quency. As the frequency decreases, the output voltage

will drop until an under-voltage shutdown is reached.

The current sensing circuit actually limits the inductor

valley current (see Figure 2). This means that if the cur-

rent limit is set to 10A, the peak current through the

inductor would be 10A plus the peak ripple current, and

the average current through the inductor would be 10A

plus 1/2 the peak-to-peak ripple current. The equations

for setting the valley current and calculating the average

current through the inductor are shown below:

()

ILIM

RDS

Valley

•

()

(

)

Valley

Average

∆

ILIMIT

ILOAD

IPEAK

TIME

Valley Current-Limit Threshold Point

FIGURE 2

SC1480

+5V

PGND

VDDP

ILIM

BST

VOUT

FBK

+VIN

FIGURE 3

Schematic of RDS

ON sensing circuit is shown in Figure 3

with RDS

ON of Q2.

Similarly, for resistor sensing, the current through the

lower MOSFET and the source sense resistor develops a

voltage that opposes the voltage developed across R

ILIM.

When the voltage developed across the R

SENSE resistor

reaches voltage drop across R

ILIM, an overcurrent will be

issued. The overcurrent equation when using an exter-

nal sense resistor is:

()

SENSE

ILIM

Valley

•

Schematic of resistor sensing circuit is shown in Figure 4

with R

ILIM = R1 and RSENSE = R2.

+5V

SC1480

PGND

VDDP

ILIM

BST

+VIN

VOUT

FBK

FIGURE 4

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