SC488ML View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC488ML Datasheet PDF : 24 Pages
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SC488
POWER MANAGEMENT
Application Information
Referencing Figure 2, the equation for setting the output The schematic of RDSON sensing circuit is shown in Figure
voltage is:
4 with RILIM = R1 and RDSON of Q2.
Vout
⎛⎜⎝1
+
R2
R3
⎞⎟⎠
⋅
1.5
+5V
+VIN
+
D1
C1
Current Limit Circuit
Current limiting of the SC488 can be accomplished in two
ways. The on-state resistance of the low-side MOSFETs
can be used as the current sensing element, or a sense
resistor in the low-side source can be used if greater ac-
curacy is desired. RDSON sensing is more efficient and
less expensive. In both cases, the R resistor between
ILIM
the ILIM pin and LX sets the over-current threshold. This
resistor RILIM is connected to a 10μA current source within
the SC488 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 RILIMresis-
tor, current limit will activate. The high-side MOSFET will
not be allowed to turn on until the voltage drop across the
sense element (resistor or MOSFET) falls below the voltage
across the RILIM resistor.
The current sensing circuit actually regulates the inductor
valley current, see Figure 3. 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.
BST
DH
LX
ILIM
VDDP
DL
PGND
SC488
C2
R1
Q1
L1
D2
Q2
+
C3
Vout
Figure 4
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 over-current exists and the high-side
MOSFET will not be allowed to turn on. The over-current
equation when using an external sense resistor is:
ILOC Valley
10ƫA x RILIM
RSENSE
Schematic of resistor sensing circuit is shown in Figure 5
with RILIM = R1 and RSENSE = R4.
+5V
+VIN
I PEAK
+
D1
C1
I LOAD
I LIMIT
TIME
Valley Current - Limit Threshold Point
BST
DH
LX
ILIM
VDDP
DL
PGND
SC488
Q1
C2
L1
D2
Q2
R1
R4
+
C3
Vout
Figure 3
© 2006 Semtech Corp.
10
Figure 5
www.semtech.com
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