SC2620 Semtech Corporation, SC2620 Datasheet - Page 11

• Manufacturer Part Number: SC2620
• Description: 30V Step-down Regulator
• Manufacturer: Semtech Corporation

where L is in H and f is in MHz.
Equation (3) shows that for a given
D decreases. If
L based on the nominal input voltage. Always verify
converter operation at the input voltage extremes.
The peak current limits of both SC2620 power transistors
are internally set at 3.2A. The peak current limits are duty-
cycle invariant and are guaranteed higher than 2.3A. The
maximum load current is therefore conservatively:
If
The saturation current of the inductor should be 20-30%
higher than the peak current limit (2.3A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Power Line Input Capacitor
A buck converter draws pulse current with peak-to-peak
amplitude equal to its output current I
supply. An input capacitor placed between the supply and
the buck converter filters the AC current and keeps the
current drawn from the supply to a DC constant. The input
capacitance C
input current. Its equivalent series resistance (ESR) should
be low so that power dissipated in the capacitor does not
result in significant temperature rise and degrade reliability.
For a single channel buck converter, the RMS ripple current
in the input capacitor is
POWER MANAGEMENT
Applications Information
2008 Semtech Corp.
∆
L
I
I
I
OUT
I
OUT
RMS
L
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=
(
V (
(
MAX
(
MAX
CIN
0
OUT
3 .
)
)
)
=
⋅
=
I
I
LM
I
I
OUT
LM
V (
LM
. 0
IN
, then
IN
V
45
−
should be high enough to filter the pulse
IN
D
∆
)(
2
2
1 (
0
varies over a wide range, then choose
I
I
L
L
V
2 .
In
−
=
)(
) D
I
2
LM
. 0
3 .
V
.
69
−
OUT
A
0
f )
I 3 .
2
. 0
2
LM
I
L
25
=
V
)
OUT
. 0
85
,
OUT
⋅
I
I
from its input
L
LM
increases as
.
(6)
(4)
(5)
11
Power dissipated in the input capacitor is
Equation (6) has a maximum value of
corresponding to the worst-case power dissipation
A dual-channel step-down converter with interleaved
switching reduces the RMS ripple current in the input
capacitor to a fraction of that of a single-phase buck
converter. If both power transistors in the SC2620 were
to switch on in phase, the current drawn by the SC2620
would consist of current pulses with amplitude equal to
the sum of the channel output currents. If each channel
were delivering I
the input current would switch from zero to 2I
ripple current in the input capacitor would then be I
Power dissipated in C
of a single-channel converter. The SC2620 produces the
highest RMS ripple current in C
running and delivering the maximum output current (2A).
The input capacitor therefore should have a RMS ripple
current rating of at least 1A.
Multi-layer ceramic capacitors, which have very low ESR
(a few m ) and can easily handle high RMS ripple current,
are the ideal choice for input filtering. A single 4.7 F or
10 F X5R ceramic capacitor is adequate. For high voltage
applications, a small ceramic (1 F or 2.2 F) can be placed
in parallel with a low ESR electrolytic capacitor to satisfy
both the ESR and bulk capacitance requirements.
Output Capacitor
The output ripple voltage V
expressed as
where C
Inductor ripple current
(Equation (3)). The output ripple voltage is therefore the
highest when V
results from the ESR of the output capacitor while the
I
2
OUT
4
⋅
V
ESR
OUT
OUT
in C
is the output capacitance.
I
L
IN
ESR
.
IN
OUT
is at its maximum. The first term in (7)
and operating at 50% duty cycle, then
8
IN
fC
would be
1
OUT
I
L
OUT
increases as D decreases
IN
of a buck converter can be
when only one channel is
I
2
OUT
⋅
ESR
I
OUT
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2
SC2620
I
RMS
2
, 4 times that
OUT
( at
(
CIN
. The RMS
)
⋅
D
(
ESR
1
2
OUT
(7)
)
),
.
.