SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
Setting the Switching Frequency
The switching frequency is set with an external resistor
connected from Pin 3 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)
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. 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 SC2446 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)
value is δ*Io, the inductance value will then be
L
=
Vo (1− D) .
δIo fs
The peak current in the inductor becomes (1+δ/2)*Io
and the RMS current is
Co
Lesl
Resr
Figure 6. An equivalent circuit of Co.
If the current through the branch is ib(t), the voltage across
the terminals will then be
∫ vo (t) =
Vo
+
1
Co
t
ib (t)dt + Lesl
0
dib (t)
dt
+
R esr ib
(t).
This basic equation illustrates the effect of ESR, ESL and
Co on the output voltage.
IL,rms = Io
1
+
δ2
12
.
The first term is the DC voltage across Co at time t=0. The
second term is the voltage variation caused by the charge
balance between the load and the converter output. The
2004 Semtech Corp.
12
www.semtech.com
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