SC2441EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441EVB
Semtech Corporation
'SC2441EVB' PDF : 35 Pages View PDF
SC2441
POWER MANAGEMENT
Applications Information (Cont.)
conversion ratio at a given switching frequency. If the
desired output voltage requires high operating duty-cycle,
then operating frequency will have to be lowered to allow
modulating headroom.
L
=
Vo (1− D) .
δIo fs
The peak current in the inductor becomes (1+δ/2)*Io
and the RMS current is
Setting the Step-down Channel Frequency
The switching frequency of both step-down controllers is
set with an external resistor from Pin 10 to the ground.
The set frequency is inversely proportional to the resistor
value (Figure 5).
800
700
600
500
400
300
200
100
0
0
50
100 150 200 250
Rosc (k Ohm)
IL,rms = Io
1
+
δ2
12
.
The followings are to be considered when choosing
inductors.
a) Inductor core material: For high efficiency applications
above 350KHz, ferrite, Kool-Mu and polypermalloy
materials should be used. Low-cost powdered iron cores
can be used for cost sensitive-applications below 350KHz
but with attendant higher core losses.
b) Select inductance value: Sometimes the calculated
inductance value is not available off-the-shelf. The designer
can choose the adjacent (larger) standard inductance
value. The inductance varies with temperature and DC
current. It is a good engineering practice to re-evaluate
the resultant current ripple at the rated DC output current.
c) Current rating: The saturation current of the inductor
should be at least 1.5 times of the peak inductor current
under all conditions.
Output Capacitor (Co) and Vout Ripple
Figure 5. Step-down Channel Free-running frequency vs.
ROSC.
Inductor (L) and Ripple Current
The output capacitor provides output current filtering in
steady state and serves as a reservoir during load transient.
The output capacitor can be modeled as an ideal capacitor
in series with its parasitic ESR (R ) and ESL (L ) (Figure
esr
esl
6).
Both step-down controllers in the SC2441 operate in
synchronous continuous-conduction mode (CCM) regardless
of the output load. The output inductor selection/design
is based on the output DC and transient requirements.
Both output current and voltage ripples are reduced with
larger inductors but it takes longer to change the inductor
current during load transients. Conversely smaller inductors
results in lower DC copper losses but the AC core losses
(flux swing) and the winding AC resistance losses are
higher. A compromise is to choose the inductance such
that peak-to-peak inductor ripple-current is 20% to 30% of
the rated output load current.
Assuming that the inductor current ripple (peak-to-peak)
is δ*Io, the inductance will then be
Co
Lesl
Resr
Figure 6. C equivalent circuit
o
If the current through the branch is ib(t), the voltage across
the terminals will then be
2005 Semtech Corp.
14
www.semtech.com
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