SC4508A Просмотр технического описания (PDF) - Semtech Corporation
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SC4508A Datasheet PDF : 21 Pages
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
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)
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
η
)2
+
D
η2
(1 −
D)]
The power dissipated in the input capacitors is then:
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 3.
PCin = ICin2Resr
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 (RMS) 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.
Figure 3. A simple model for the converter input
In Figure 3 the DC input voltage source has an internal
impedance R and the input capacitor C has an ESR of
in
in
Resr. MOSFET and input capacitor current waveforms, ESR
voltage ripple and input voltage ripple are shown in Figure
4.
Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is:
∆v ESR
= Resr (1+
δ
2
)Idc
The peak-to-peak input voltage ripple due to the capacitor
is:
∆v C
≈
DIdc
Cinfs
From these two expressions, CIN can be found to meet
the input voltage ripple specification.
Power MOSFETs Selection
Main considerations in selecting the MOSFETs are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFETs are directly related to
the total gate charge (Cg) and channel on-resistance
(Rds(on)). In order to judge the performance of MOSFETs,
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 5.
Figure 4. Typical waveforms at converter input.
2007 Semtech Corp.
10
www.semtech.com
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