LM3753 Datasheet, PDF (22/38 Pages) National Semiconductor (TI) – Scalable 2-Phase Synchronous Buck Controllers with Integrated FET Drivers and Linear Regulator Controller

By defining the maximum input voltage ripple, the minimum
requirement for the input capacitance can be calculated as:
The converter exhibits a negative input impedance which is
lowest at the minimum input voltage:
The damping factor for the input filter is given by:
For multi-phase operation, the general equation for the input
capacitor rms current is approximated as:
This is valid for D < 1 / N and repeats for a total of N times.
I_O represents the total output current and N is the number of
phases. Figure 12 shows the input capacitor rms current as
a function of the output current, duty cycle and number of
phases.
Where R_LIN is the input wiring resistance and R_CIN is the series
resistance of the input capacitors. The term Z_S / Z_IN will always
be negative due to Z_IN.
When δ = 1, the input filter is critically damped. This may be
difficult to achieve with practical component values. With δ <
0.2, the input filter will exhibit significant ringing. If δ is zero or
negative, there is not enough resistance in the circuit and the
input filter will sustain an oscillation.
When operating near the minimum input voltage, an alu-
minum electrolytic capacitor across C_IN may be needed to
damp the input for a typical bench test setup. Any parallel
capacitor should be evaluated for its rms current rating. The
current will split between the ceramic and aluminum capaci-
tors based on the relative impedance at the switching fre-
quency. Using a square wave approximation, the rms current
in each capacitor is found from:
30091948
FIGURE 12. Input Capacitor RMS Current as a Function
of Output Current
For multi-phase operation the maximum rms current can be
approximated as:
I_CIN(RMS)MAX ≈ 0.5 x I_O / N
In most applications for point-of-load power supplies, the in-
put voltage is the output of another switching converter. This
output often has a lot of bulk capacitance, which may provide
adequate damping.
When the converter is connected to a remote input power
source through a wiring harness, a resonant circuit is formed
by the line impedance and the input capacitors. If step input
voltage transients are expected near the maximum rating of
the LM3753/54, a careful evaluation of the ringing and pos-
sible overshoot at the device VIN pin should be completed.
To minimize overshoot make C_IN > 10 x L_IN. The characteristic
source impedance and resonant frequency are:
Input Capacitor Design Procedure
Ceramic capacitors are sized to support the required rms cur-
rent. An aluminum electrolytic capacitor is used for damping.
Find the minimum value for the ceramic capacitors from:
Allowing ΔV_IN = 0.6V for the design example, the minimum
value is C_IN = 34.7 μF. Find the rms current rating from:
I_CIN(RMS)MAX ≈ 0.5 x I_O / N
Using the same criteria, the result is 12.5A rms. Manufacturer
data for 4.7 μF, 25V, X7R capacitors in a 1210 package allows
for 4A rms with a 20°C temperature rise. For the design ex-
ample, using two ceramic capacitors for each phase will meet
both the input voltage ripple and rms current target. Since the
series resistance is so low at about 4 mΩ per capacitor, a
parallel aluminum electrolytic is used for damping. A good
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