SC2542TETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2542TETRT Datasheet PDF : 27 Pages
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SC2542
POWER MANAGEMENT
Applications Information (Cont.)
Req and Ceq are both functions of frequency. For rigor-
ous 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 14.
L1
Rin Resr
VDC
Cin
Q1
D1
Co
Ro
Figure 15. 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.
Figure 14. A simple model for the converter input.
The RMS value of the capacitor current is approximately:
In Figure 14 the DC input voltage source has an internal
impedance Rin and the input capacitor Cin has an ESR
of Resr. MOSFET and input capacitor current waveforms,
ESR voltage ripple and input voltage ripple are shown in
Figure 15.
ICin = IO
D
(1 +
δ2
12
)(1 −
D
η
)2
+
(1 −
D)
The power dissipated in the input capacitors is then:
PCin = ICin 2 Resr
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10°C of
temperature rise. Many manufacturers specify the maxi-
mum allowable ripple current (ARMS) rating of the ca-
pacitor at a given ripple frequency and ambient tempera-
ture. The input capacitance should be high enough to
handle the ripple current. It is common practice that mul-
tiple capacitors are placed in parallel to increase the
ripple current handling capability.
Sometimes meeting tight input voltage ripple specifica-
tions may require the use of larger input capacitance. At
full load, the peak-to-peak input voltage ripple due to
2005 Semtech Corp.
17
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