SC2447 Semtech Corporation, SC2447 Datasheet - Page 16

• Manufacturer Part Number: SC2447
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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 8).
Inductor (L) and Ripple Current
Both step-down controllers in the SC2447 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 *I
The peak current in the inductor becomes (1+ /2)*Io
and the RMS current is
POWER MANAGEMENT
Application Information (Cont.)
2006 Semtech Corp.
800
700
600
500
400
300
200
100
Figure 8. Free Running Frequency vs. R
0
0
o
, the inductance value will then be
50
I
L
L
,
rms
Rosc (k Ohm)
=
100
=
V
o
I
δ
1 (
o
I
o
−
1
f
D
s
+
150
δ
12
)
.
2
.
200
250
OSC
16
Consider the following 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 next larger standard inductance value.
The inductance varies with temperature and DC current.
It is a good engineering practice to re-evaluate the
resulting 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 (C
The output capacitor filters the inductor current in the
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
9 ).
If the current through the branch is i
the terminals will then be
This basic equation illustrates the effect of ESR, ESL
and C
The first term is the DC voltage across C
The second term is the voltage variation caused by the
o
v
on the output voltage.
o
) (
Figure 9. An Equivalent Circuit of C
t
=
V
o
+
C
1
o
o
) and V
∫
0
t
i
b
) (
t
dt
Lesl
Resr
out
+
Co
L
Ripple
esl
www.DataSheet4U.com
esr
di
) and ESL (L
b
(t), the voltage across
dt
b
) (
t
+
www.semtech.com
R
SC2447
o
esr
at time t=0.
i
b
o
(
esl
t
).
) (Figure