SC2544 Datasheet, PDF (12/24 Pages) Semtech Corporation – High Performance Wide Input Range Dual Synchronous Buck Controller

SC2544
POWER MANAGEMENT
Applications Information (Cont.)
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where R_1a and C_1a are the ESR and capacitance of
electrolytic capacitors, and R_1b and C_1b are the ESR
and capacitance of the ceramic capacitors, respectively
(Figure 5).
In Figure 6 the DC input voltage source has an internal
impedance R_in and the input capacitor C_in has an ESR
of R_esr. MOSFET and input capacitor current
waveforms, ESR voltage ripple and input voltage ripple
are shown in Figure 7.
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Figure 5. 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_1a = R_1b = R₁ and C_1a = C_1b = C₁,
then R_eq and C_eq will be frequency-independent and
R_eq = 1/2 R₁ and C_eq = 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 6.
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Figure 7. 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
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The power dissipated in the input capacitors is then

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Figure 6. A simple model for the converter input.
P_Cin = I_Cin2R_esr.
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10^oC
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. It is common
pratice that multiple capacitors are placed in parallel
to increase the ripple current handling capability.
ã 2005 Semtech Corp.
12
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