SC1480EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1480EVB Datasheet PDF : 22 Pages
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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-
+5V
+VIN
age drop equal to 10µA times R . As the current in-
ILIM
creases through the lower MOSFET, the phase pin volt-
+
D1
C1
age will decrease until the offset voltage caused by RILIM
is reached and ILIM < PGND. At this point an overcurrent
Q1
C2
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
BST
DH
LX
ILIM
VDDP
DL
PGND
14
13
12
11
10
9
8
R1
L1
D2
+
VOUT
will drop until an under-voltage shutdown is reached.
SC1480
C3
Q2
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:
IL
OC
(Valley
)=
10µA
•
RILIM
RDSON
ILOC
(Average)
=
ILOC
(Valley ) +
∆IL
2
FIGURE 3
Schematic of RDS sensing circuit is shown in Figure 3
ON
with RDS of Q2.
ON
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 RSENSE resistor
reaches voltage drop across RILIM, an overcurrent will be
issued. The overcurrent equation when using an exter-
nal sense resistor is:
IPEAK
ILOAD
IL
OC
(Valley
)
=
10µA
•
RILIM
R SENSE
Schematic of resistor sensing circuit is shown in Figure 4
with RILIM = R1 and RSENSE = R2.
+5V
+VIN
ILIMIT
+
D1
C1
TIME
Valley Current-Limit Threshold Point
FIGURE 2
BST
DH
LX
ILIM
VDDP
DL
PGND
14
13
12
11
10
9
8
SC1480
C2
R1
Q1
L1
Q2
D2
R2
+
C3
FBK
VOUT
2006 Semtech Corp.
FIGURE 4
8
www.semtech.com
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