sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 9

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

ensure, since this is a soft discharge, that there are no
dangerous negative voltage excursions to be concerned
about. In order for the soft discharge circuitry to
function correctly, the chip supply must be present.
Output Voltage Selection
The output voltage (OUT2 shown) is set by the feedback
resistors R9 & R13 of Figure 2 below. The internal
reference is 1.5V, so the voltage at the feedback pin is
multiplied by three to match the 1.5V reference.
Therefore the output can be set to a minimum of 0.5V.
The equation for setting the output voltage is:
Current Limit Circuit
Current limiting of the SC483 can be accomplished in
two ways. The on-state resistance of the low-side MOSFET
can be used as the current sensing element or sense
resistors in series with the low-side source can be used
if greater accuracy is desired. R
efficient and less expensive. In both cases, the R
resistor between the ILIM pin and LX pin set the over
current threshold. This resistor R
10
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 RILIM resistor,
positive current limit will activate. The high side MOSFET
will not be turned on until the voltage drop across the
POWER MANAGEMENT
V
OUT
2005 Semtech Corp.
µ
A current source within the SC483 which is turned
1
C15
56p
0402
Figure 2: Setting The Output Voltage
R
R
13
VOUT2
VOUT
9
R9
20k0
0402
R13
14k3
0402
0
5 .
VSSA2
10
11
12
13
14
8
9
U1
EN/PSV2
TON2
VOUT2
VCCA2
FB2
PGD2
VSSA2
DS(ON)
ILIM
SC483 OUT2
is connected to a
sensing is more
PGND2
VDDP2
ILIM2
BST2
DH2
DL2
LX2
21
20
19
18
17
16
15
ILIM
9
sense element (resistor or MOSFET) falls below the
voltage across the R
current situation, the top MOSFET will never turn back
on and eventually the part will latch off due to output
undervoltage (see Output Undervoltage Protection).
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-to-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:
The equation for the current limit threshold is as follows:
Where (referring to Figure 8 on Page 16) R
R
For resistor sensing, a sense resistor is placed between
the source of Q4 and PGND. The current through the
source sense resistor develops a voltage that opposes
the voltage developed across R
developed across the R
drop across R
high side MOSFET will not be allowed to turn on. When
using an external sense resistor R
of the sense resistor.
I
LIMIT
SENSE
is the R
10e
6 -
Figure 3: Valley Current Limiting
Valley Current-Limit Threshold Point
DS(ON)
R
ILIM
R
SENSE
, a positive over-current exists and the
ILIM
of Q4.
A
ILIM
SENSE
TIME
resistor. In an extreme over-
resistor reaches the voltage
ILIM
SENSE
. When the voltage
is the resistance
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ILIM
SC483
is R10 and
I
I
I
PEAK
LOAD
LIMIT