sc2545 Semtech Corporation, sc2545 Datasheet - Page 12

• Manufacturer Part Number: sc2545
• Description: Sc2545 High Performance Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

Input Capacitor (Cin)
Input Capacitor (Cin)
where R
electrolytic capacitors, and R
capacitance of the ceramic capacitors, respectively
(Figure 5).
Req and Ceq are both functions of frequency. For rigorous
design, the equivalent ESR should be evaluated at the
ripple frequency for voltage ripple calculation when both
ceramic and electrolytic capacitors are used. If R
= R
independent and
Input Capacitor (Cin)
Input Capacitor (Cin)
Input Capacitor (Cin)
The input supply to the converter usually comes from a
pre-regulator. Since the input supply is not ideal, input
capacitors are needed to filter the current pulses at the
switching frequency. A simple buck converter is shown in
Figure 6.
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
C
C
R
R
eq
eq
1
eq
eq
(
(
and C
ω
ω
Figure 6. A simple model for the converter input.
(
(
ω
ω
)
)
Rin
: )
: )
VDC
=
=
1a
Figure 5. Equivalent RC branch.
=
=
1a
R
R
and C
R
= C
a 1
a 1
eq
R
R
Resr
b 1
b 1
= 1/2 R
1b
Cin
R (
R (
R (
R (
= C
2
1a
C1a
R1a
a 1
a 1
a 1
a 1
are the ESR and capacitance of
+
+
1
+
+
, then R
R
R
R
R
1
1
b 1
b 1
b 1
b 1
Q1
and C
)
)
)
)
2
2
ω
ω
C1b
R1b
ω
ω
2
2
2
2
C
C
eq
1b
C
C
D1
a 1
a 1
a 1
a 1
eq
and C
and C
2
2
C
C
2
2
= 2C
C
C
L1
b 1
b 1
b 1
b 1
2
2
2
2
+
+
eq
1b
+
+
1
R (
R (
.
will be frequency-
(
(
are the ESR and
C
C
b 1
b 1
a 1
a 1
C
C
+
+
b 1
b 1
2
2
C
C
Co
+
+
b 1
b 1
Ceq
Req
R
R
)
)
2
2
1a
a 1
a 1
C
C
= R
a 1
a 1
Ro
2
2
1b
)
)
12
In Figure 6 the DC input voltage source has an internal
impedance R
R
voltage ripple and input voltage ripple are shown in Figure
7.
It can be seen that the current in the input capacitor
pulses with high di/dt. Capacitors with low ESL should be
used. It is also important to place the input capacitor
close to the MOSFETs on the PC board to reduce trace
inductances around the pulse current loop.
The RMS value of the capacitor current is approximately
The power dissipated in the input capacitors is then
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10
temperature rise. Many manufacturers specify the
maximum allowable ripple current (ARMS) rating of the
capacitor at a given ripple frequency and ambient
temperature. The input capacitance should be high
enough to handle the ripple current. It is common pratice
that multiple capacitors are placed in parallel to increase
the ripple current handling capability.
esr
. MOSFET and input capacitor current waveforms, ESR
i
i
i
i
V
V
V
V
Figure 7. Typical waveforms at converter input.
Q1
Q1
Cin
Cin
esr
esr
Cin
Cin
I
Cin
in
=
and the input capacitor C
I
o
D
P
Cin
[(
1
= I
+
12
δ
Cin
2
2
)(
R
1
esr
−
.
D
η
)
2
+
η
D
2
1 (
in
−
www.semtech.com
SC2545
has an ESR of
D
]. )
o
C of