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SC5010 View Datasheet(PDF) - Semtech Corporation

Part Name
Description
MFG CO.
'SC5010' PDF : 36 Pages View PDF
SC5010
Components Selection (continued)
Current Sensing Resistor Selection
The switch current is sensed via the current sensing resis-
tor, R . The sensed voltage at this pin is used to set the
SNS
peak switch current limit and also used for steady state
regulation of the inductor current. The current limit com-
parator has a trip voltage of 0.4V. R value is chosen to
SNS
set the peak inductor and switch current using the follow-
ing equation.
In order to implement inrush current limiting, a 10nF
capacitor is connected between the gate and source ter-
minal of the MOSFET.
If isolation is not required, then the DRVP pin can be left
floating. Connect the VIN directly to the inductor.
ISW(Peak) = 0.4/RSNS
The power dissipation in R can be calculated using the
SNS
following equations.
PR_SNS
=
I2
RMS
x
RSNS
IRMS = D x [IO/(1-D)]2
IO = Output DC Current, D = Duty Cycle
For the typical application circuit shown in the Detailed
Application Circuit (page 21), the power dissipation on the
sensing resistor is shown by the following equations.
Assuming V (Min) = 5V and V = 30V, thus D = 84%,
IN
OUT
PR_SNS = [(0.12/0.16)2 X 0.1] X 0.84 = 0.047(W)
For this example, a 0.1 Ω 1% thick-film chip resistor rated
at 0.125W can be used.
PLL Filter Component Selection
The Detailed Application Circuit on page 21 shows the
optimal R/C filter components for the PLL compensation.
These are optimized for internal 1MHz switching fre-
quency. Please contact Semtech application group if a
different switching frequency is selected.
Isolation MOSFET Selection
The external p-channel MOSFET provides load disconnec-
tion during shutdown or fault condition. Select a MOSFET
with low R to limit the power loss.
DS(on)
26
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