SC411EVB SEMTECH [Semtech Corporation], SC411EVB Datasheet - Page 10

• Manufacturer Part Number: SC411EVB
• Description: Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
• Manufacturer: SEMTECH [Semtech Corporation]

Output Voltage Selection
The output voltage is set by the feedback resistors R3 &
R5 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 SC411 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
fi cient and less expensive. In both cases, the R
tor between the ILIM pin and LX pin sets the over current
threshold. This resistor RILIM is connected to a 10μA cur-
rent source within the SC411 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 sense element (resistor
or MOSFET) falls below the voltage across the R
© 2007 Semtech Corp.
POWER MANAGEMENT
Application Information (Cont.)
56p
0402
C5
Figure 2: Setting The Output Voltage
VOUT
R3
20k0
0402
R5
14k3
0402
V
OUT
1
2
3
4
= ( 1 + ――― ) • 0.5
VOUT
VCCA
PGD
FB
U1
R3
R5
SC411
DS(ON)
sensing is more ef-
VDDP
ILIM
DH
LX
12
11
10
9
ILIM
ILIM
resis-
resis-
10
tor. In an extreme over-current situation, the top MOSFET
will never turn back on and eventually the part will latch
off due to output under-voltage (see Output Under-voltage
Protection).
The current sensing circuit actually regulates the induc-
tor 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 17) 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
oped across the R
across R
side MOSFET will not be allowed to turn on. When using
an external sense resistor R
sense resistor.
SENSE
is the R
ILIM,
Figure 3: Valley Current Limiting
a positive over-current exists and the high
DS(ON)
I
LIMIT
of Q2.
SENSE
= 10μA × R
resistor reaches the voltage drop
ILIM
SENSE
. When the voltage devel-
ILIM
is the resistance of the
/ R
SENSE
www.semtech.com
ILIM
(Amps)
SC411
is R4 and