SC1480
POWER MANAGEMENT
Applications Information (Cont.)
a scaling resistor between LX and ILIM is required. This
resistor, R
ILIM
, is connected to a 10
µA
current source
within the SC1480 through the ILIM pin. This sets a volt-
age drop equal to 10
µA
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:
+5V
+VIN
D1
+
C1
BST
DH
LX
ILIM
VDDP
DL
PGND
14
13
12
11
10
9
8
C2
Q1
L1
VOUT
R1
Q2
SC1480
D2
+
C3
FBK
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:
IL
OC
(
Valley
)
=
10
µ
A
•
R
ILIM
RDS
ON
∆
I
L
2
IL
OC
(
Average
)
=
IL
OC
(
Valley
)
+
IL
OC
(
Valley
)
=
10
µ
A
•
INDUCTOR CURRENT
I
PEAK
I
LOAD
+5V
R
ILIM
R
SENSE
Schematic of resistor sensing circuit is shown in Figure 4
with R
ILIM
= R1 and R
SENSE
= R2.
+VIN
I
LIMIT
D1
+
C1
TIME
Valley Current-Limit Threshold Point
BST
DH
LX
ILIM
VDDP
DL
PGND
14
13
12
11
10
9
8
C2
Q1
L1
VOUT
Q2
+
R1
D2
R2
FBK
C3
SC1480
FIGURE 2
FIGURE 4
2006 Semtech Corp.
8
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SEMTECH [ SEMTECH CORPORATION ]
