SC2447 查看數據表（PDF） - Semtech Corporation
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SC2447 Datasheet PDF : 26 Pages
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SC2447
POWER MANAGEMENT
Application Information (Cont.)
they should be calculated using the following formulae.
Ceq(ω) :=
(R1a + R1b )2 ω2C1a2C1b2 + (C1a + C1b )2
(R1a2C1a + R1b2C1b )ω2C1aC1b + (C1a + C1b )
Req (ω)
:=
R1aR1b (R1a
(R1a
+ R1b )ω2C1a2C1b2 + (R1bC1b2 + R1aC1a2 )
+ R1b )2 ω2C1a2C1b2 + (C1a + C1b )2
where R1a and C1a are the ESR and capacitance of
electrolytic capacitors, and R1b and C1b are the ESR and
capacitance of the ceramic capacitors respectively.
(Figure 10)
Figure 11. A Simple Model for the Converter Input
C1a
C1b
R1a
R1b
Ceq
Req
Figure 10. Equivalent RC Branch
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 R1a = R1b =
R1 and C1a = C1b = C1, then Req and Ceq will be frequency-
independent and
Req = 1/2 R1 and Ceq = 2C1.
Figure 12. Typical Waveforms at Converter Input
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 11.
In Figure 11 the DC input power supply has an internal
impedance Rin and the input capacitor Cin has an ESR of
Resr. The MOSFET and the input capacitor current
waveforms, the ESR voltage ripple and the input voltage
ripple are shown in Figure 12.
It can be seen that high di/dt pulse current flows in the
input capacitor. 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 inductance
around the pulse current loop.
The RMS value of the capacitor current is approximately
ICin = Io
D[(1+
δ2
12
)(1−
D)2
η
+
D
η2
(1−
D)
].
The power dissipated in the input capacitor is then
PCin = ICin2Resr.
© 2006 Semtech Corp.
18
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