SC1480
POWER MANAGEMENT
Applications Information (Cont.)
a scaling resistor between LX and ILIM is required. This
resistor, RILIM, is connected to a 10µA current source
within the SC1480 through the ILIM pin. This sets a volt-
age drop equal to 10µA times RILIM. As the current in-
creases through the lower MOSFET, the phase pin volt-
age will decrease until the offset voltage caused by RILIM
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.
+5V
+VIN
+
D1
C2
C1
Q1
Q2
14
13
12
11
10
9
BST
DH
L1
LX
VOUT
ILIM
VDDP
DL
8
R1
PGND
D2
+
C3
SC1480
FBK
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:
FIGURE 3
Schematic of RDSON sensing circuit is shown in Figure 3
with 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 RILIM.
When the voltage developed across the RSENSE resistor
reaches voltage drop across RILIM, an overcurrent will be
issued. The overcurrent equation when using an exter-
nal sense resistor is:
RILIM
ILOC (Valley)=10µA•
RDSON
∆IL
ILOC (Average)= ILOC (Valley)+
2
RILIM
ILOC (Valley)=10µA•
RSENSE
Schematic of resistor sensing circuit is shown in Figure 4
with RILIM = R1 and RSENSE = R2.
IPEAK
ILOAD
ILIMIT
+5V
+VIN
+
D1
C2
C1
Q1
14
13
12
11
10
9
BST
DH
L1
TIME
LX
VOUT
ILIM
VDDP
DL
Valley Current-Limit Threshold Point
8
Q2
R2
PGND
+
C3
SC1480
D2
R1
FIGURE 2
FBK
FIGURE 4
2006 Semtech Corp.
8
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SEMTECH [ SEMTECH CORPORATION ]
