sc1470 Semtech Corporation, sc1470 Datasheet - Page 8

• Manufacturer Part Number: sc1470
• Description: Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

Output Voltage Selection
The output voltage is set by the feedback resistors R3 &
R7 of Figure 2 below. The internal reference is 1.5V, so
the voltage at the feedback pin is multiplied by three to
match
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 SC1470 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 sets 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
C5
56p
0402
A current source within the SC1470 which is turned
1
Figure 2: Setting The Output Voltage
the
VOUT
R3
20k0
0402
R7
14k3
0402
R
R
3
7
1.5V
0
5 .
reference.
1
2
3
4
5
6
7
U1
EN/PSV
TON
VOUT
VCCA
FB
PGD
VSSA
DS(ON)
ILIM
is connected to a
Therefore
sensing is more
SC1470
PGND
VDDP
ILIM
BST
DH
DL
LX
14
13
12
11
10
9
8
the
ILIM
8
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 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 4 on Page 15) R
R
For resistor sensing, a sense resistor is placed between
the source of Q2 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 Q2.
A
ILIM
SENSE
TIME
resistor. In an extreme over-
resistor reaches the voltage
ILIM
SENSE
. When the voltage
is the resistance
www.semtech.com
SC1470
ILIM
is R4 and
I
I
I
PEAK
LOAD
LIMIT