SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
POWER MANAGEMENT
Application Information (Cont.)
SC2446
C1a
R1a
C1b
R1b
Ceq
Req
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 R = R =
1a
1b
R1 and C1a = C1b = C1, then Req and Ceq will be frequency-
independent and
Figure 9. Typical waveforms at converter input.
R = 1/2 R and C = 2C .
eq
1
eq
1
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.
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
MOSFET’s 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) ].
The power dissipated in the input capacitors is then
PCin = ICin2Resr.
Figure 8. A simple model for the converter input
In Figure 8 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
9.
For reliable operation, the maximum power dissipation in
the capacitors should not result in more than 10oC of
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. For higher power applications,
multiple capacitors are placed in parallel to increase the
ripple current handling capability.
2004 Semtech Corp.
14
www.semtech.com
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