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 2004 Semtech Corp.

www.semtech.com

SC1485

POWER MANAGEMENT

Applications Information (Cont.)

Current Limit Circuit

Current limiting of the SC1485 can be accomplished in

two ways. First, the device can implement on-state re-

sistance of the low-side MOSFET as the current sensing

element (RDS

a resistive element in the low-side source (R

sensing). The second method offers greater accuracy of

the current limit threshold over RDS

added expense of a sense resistor and associated effi-

ciency loss.

Whether RDS

a scaling resistor between LX and ILIM is required. This

resistor, R

within the SC1485 through the ILIM pin. This sets a volt-

age drop equal to 10uA times R

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 over-cur-

rent trip signal is issued. Current limiting will prevent the

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

frequency. As the frequency decreases, the output volt-

age 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:

()

ON

ILIM

OC

RDS

R

A

10

Valley

IL

•

µ

=

()

(

)

2

I

Valley

IL

Average

IL

L

OC

OC

∆

+

=

FIGURE 2

FIGURE 3

The schematic of RDS

ure 3 with R

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

tor reaches voltage drop across R

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

OC

R

R

A

10

Valley

IL

•

µ

=

Schematic of resistor sensing circuit is shown in Figure 4

with R

R4

C2

D1

R1

+

C3

+5V

D2

SC1485

FBK

Q1

Q2

+VIN

R3

1

2

3

4

5

6

7

PGND

DL

VDDP

ILIM

LX

DH

BST

+

C1

L1

0.5V - 5.5V

R2

FIGURE 4

TIME

Valley Current-Limit Threshold Point

C2

0.5V - 5.5V

Q1

SC1485

R2

+

C1

R1

1

2

3

4

5

6

7

PGND

DL

VDDP

ILIM

LX

DH

BST

L1

FBK

+VIN

Q2

D1

+5V

+

C3

R3

D2