SC2446AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2446AEVB Datasheet PDF : 27 Pages
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SC2446A
POWER MANAGEMENT
Application Information (Cont.)
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 7)
Figure 8. A simple model for the converter input
C1a
R1a
C1b
R1b
In Figure 8 the DC input voltage source has an internal
impedance R and the input capacitor C has an ESR of
in
in
Ceq
R . MOSFET and input capacitor current waveforms, ESR
esr
voltage ripple and input voltage ripple are shown in Figure
Req
9.
Figure 7. 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.
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 8.
Figure 9. Typical waveforms at converter input.
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
ICin = Io
D[(1+
δ2
12
)(1
−
D )2
η
+
D
η2
(1−
D) ].
2005 Semtech Corp.
13
www.semtech.com
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