SC488 Semtech Corporation, SC488 Datasheet - Page 10

• Manufacturer Part Number: SC488
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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Referencing Figure 2, the equation for setting the output
voltage is:
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 effi cient and
less expensive. In both cases, the R
the ILIM pin and LX sets the over-current threshold. This
resistor R
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 R
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 R
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.
© 2006 Semtech Corp.
POWER MANAGEMENT
Application Information
ILIM
is connected to a 10μA current source within
ILIM
Valley Current - Limit Threshold Point
resistor.
V out
Figure 3
⎛
⎜
⎝
TIME
1
+
R2
R3
⎞
⎟
⎠
⋅
1.5
ILIM
resistor between
I
I
I
PEAK
LOAD
LIMIT
ILIM
resis-
10
The schematic of RDS
4 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
voltage developed across the R
drop across R
MOSFET will not be allowed to turn on. The over-current
equation when using an external sense resistor is:
Schematic of resistor sensing circuit is shown in Figure 5
with R
SC488
SC488
PGND
VDDP
ILIM
PGND
ILIM
VDDP
BST
DH
DL
ILIM
LX
BST
DH
ILIM
DL
LX
= R1 and R
= R1 and RDS
ILIM,
IL
OC
an over-current exists and the high-side
+5V
+5V
Valley
D1
C2
D1
C2
SENSE
R1
R1
ON
Figure 4
Figure 5
sensing circuit is shown in Figure
= R4.
ON
+VIN
+VIN
of Q2.
10
Q1
Q2
Q1
Q2
R4
SENSE
A
+
+
C1
C1
D2
D2
R
resistor reaches voltage
R
SENSE
L1
L1
ILIM
+
+
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C3
C3
ILIM
SC488
. When the
Vout
Vout