SC1486ITSTRT datasheet(10/17 Pages) SEMTECH

10 / 17 page

 2002 Semtech Corp.

www.semtech.com

SC1486

POWER MANAGEMENT

Vout

•











 +

Current Limit Circuit

Current limiting of the SC1486 can be accomplished in

two ways. The on-state resistance of the low-side

MOSFETs can be used as the current sensing element

or sense resistors in the low-side sources can be used if

greater accuracy is desired. RDSON sensing is more effi-

cient and less expensive. In both cases, the R

ILIM resis-

tors between the ILIM pin and LX set the over current

threshold. This resistor R

ILIM is connected to a 10uA cur-

rent source within the SC1486 which is turned on when

the low side MOSFET turns on. When the voltage drop

across the sense resistor or low side MOSFET equals the

voltage across the R

ILIMresistor, current limit will activate.

The high side will not be allowed to turn on until the volt-

age drop across the sense element (resistor or MOSFET)

falls below the voltage across the R

ILIM resistor.

The current sensing circuit actually regulates the induc-

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

current 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:

ILIMIT

ILOAD

IPEAK

TIME

Valley Current-Limit Threshold Point

FIGURE 2

FIGURE 3

The schematic of RDS

ON sensing circuit is shown in Fig-

ure 3 with R

ILIM = R1 and RDSON 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

ILIM.When the voltage developed across the RSENSE resis-

tor reaches voltage drop across R

ILIM, an over-current ex-

ists and the high side MOSFET will not be allowed to turn

on. The over-current equation when using an external

sense resistor is:

()

SENSE

ILIM

Valley

•

Schematic of resistor sensing circuit is shown in Figure 4

with R

ILIM = R1 and RSENSE = R4.

FIGURE 4

Vout

+5V

+VIN

PGND

VDDP

ILIM

BST

Vout

+5V

PGND

VDDP

ILIM

BST

+VIN

Application Information (Cont.)

SC1486ITSTRT Datasheet(PDF) 10 Page - Semtech Corporation

SC1486ITSTRT Datasheet(HTML) 10 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL