SC483ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC483ITSTRT
Semtech Corporation
'SC483ITSTRT' PDF : 27 Pages View PDF
SC483
POWER MANAGEMENT
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:
VOUT
=
1 +
R9
R13
•
0.5
sense element (resistor or MOSFET) falls below the
voltage across the RILIM resistor. In an extreme over-
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:
VOU T2
C 15
56p
0402
R9
20k0
0402
R 13
14k3
0402
8 EN/PSV2
BST2 21
9 TON2
DH2 20
10 VOUT2
LX2 19
11 VCCA2
ILIM2 18
12 FB2
VDDP2 17
13 PGD2
DL2 16
14 VSSA2
PGND2 15
U1 SC483 OUT2
VSSA2
IPEAK
ILOAD
ILIMIT
TIME
Valley Current-Limit Threshold Point
Figure 3: Valley Current Limiting
Figure 2: Setting The Output Voltage
The equation for the current limit threshold is as follows:
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. RDS(ON) sensing is more
efficient and less expensive. In both cases, the RILIM
resistor between the ILIM pin and LX pin set the over
current threshold. This resistor RILIM is connected to a
10µA current source within the SC483 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 RILIM resistor,
positive current limit will activate. The high side MOSFET
will not be turned on until the voltage drop across the
ILIMIT
= 10e-6
RILIM
• RSENSE
A
Where (referring to Figure 8 on Page 16) RILIM is R10 and
RSENSE is the RDS(ON) of Q4.
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 RILIM. When the voltage
developed across the RSENSE resistor reaches the voltage
drop across RILIM, a positive over-current exists and the
high side MOSFET will not be allowed to turn on. When
using an external sense resistor RSENSE is the resistance
of the sense resistor.
2005 Semtech Corp.
9
www.semtech.com
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