SC2446A Ver la hoja de datos (PDF) - Semtech Corporation
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SC2446A Datasheet PDF : 27 Pages
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SC2446A
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.
Figure 5. Free running frequency vs. ROSC.
Inductor (L) and Ripple Current
Both step-down controllers in the SC2446A 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
IL,rms = Io
1
+
δ2
12
.
Output Capacitor (Co) and Vout Ripple
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 (Resr) and
ESL (Lesl) (Figure 6).
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.
The first term is the DC voltage across Co at time t=0.
The second term is the voltage variation caused by the
2005 Semtech Corp.
11
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