MAX15039_11 Datasheet, PDF (14/19 Pages) Maxim Integrated Products – 6A, 2MHz Step-Down Regulator with Integrated Switches Open-Drain, Power-Good Output

6A, 2MHz Step-Down Regulator
with Integrated Switches
Inductor Selection
Choose an inductor with the following equation:
L = ^{VOUT × (VIN − VOUT )}
f_S × V_IN × LIR × I_OUT(MAX)
where LIR is the ratio of the inductor ripple current to full
load current at the minimum duty cycle. Choose LIR
between 20% to 40% for best performance and stability.
Use an inductor with the lowest possible DC resistance
that fits in the allotted dimensions. Powdered iron ferrite
core types are often the best choice for performance.
With any core material, the core must be large enough
not to saturate at the current limit of the MAX15039.
Output-Capacitor Selection
The key selection parameters for the output capacitor are
capacitance, ESR, ESL, and voltage-rating requirements.
These affect the overall stability, output ripple voltage,
and transient response of the DC-DC converter. The out-
put ripple occurs due to variations in the charge stored
in the output capacitor, the voltage drop due to the
capacitor’s ESR, and the voltage drop due to the
capacitor’s ESL. Estimate the output-voltage ripple due
to the output capacitance, ESR, and ESL:
V_RIPPLE = V_RIPPLE(C) + V_RIPPLE(ESR) + V_RIPPLE(ESL)
where the output ripple due to output capacitance,
ESR, and ESL is:
V_RIPPLE(C)
=
8
x
I_P−P
C _OUT
x
f_S
V_RIPPLE(ESR) = I_P−P x ESR
V_RIPPLE(ESL) =
I_P−P
t _ON
x ESL
or:
V_RIPPLE(ESL)
=
I_P−P
t _OFF
x
ESL
or whichever is larger.
The peak-to-peak inductor current (I_P-P) is:
I_{P − P}
=
V_IN − V_OUT
f_S × L
x
V_OUT
V_IN
Use these equations for initial output-capacitor selec-
tion. Determine final values by testing a prototype or an
evaluation circuit. A smaller ripple current results in less
output-voltage ripple. Since the inductor ripple current
is a factor of the inductor value, the output-voltage rip-
ple decreases with larger inductance. Use ceramic
capacitors for low ESR and low ESL at the switching
frequency of the converter. The ripple voltage due to
ESL is negligible when using ceramic capacitors.
Load-transient response depends on the selected out-
put capacitance. During a load transient, the output
instantly changes by ESR x ΔI_LOAD. Before the con-
troller can respond, the output deviates further,
depending on the inductor and output capacitor val-
ues. After a short time, the controller responds by regu-
lating the output voltage back to its predetermined
value. The controller response time depends on the
closed-loop bandwidth. A higher bandwidth yields a
faster response time, preventing the output from deviat-
ing further from its regulating value. See the Compen-
sation Design section for more details.
Input-Capacitor Selection
The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching
noise in the IC. The total input capacitance must be
equal or greater than the value given by the following
equation to keep the input-ripple voltage within specifi-
cation and minimize the high-frequency ripple current
being fed back to the input source:
C_{IN _ MIN}
=
D x T_S x I_OUT
V_{IN -RIPPLE}
where V_IN-RIPPLE is the maximum allowed input ripple
voltage across the input capacitors and is recommend-
ed to be less than 2% of the minimum input voltage. D
is the duty cycle (V_OUT/V_IN) and T_S is the switching
period (1/f_S).
The impedance of the input capacitor at the switching
frequency should be less than that of the input source so
high-frequency switching currents do not pass through
the input source, but are instead shunted through the
input capacitor. The input capacitor must meet the ripple
current requirement imposed by the switching currents.
The RMS input ripple current is given by:
I_RIPPLE = I_LOAD ×
V_OUT × (V_IN − V_OUT )
V_IN
where I_RIPPLE is the input RMS ripple current.
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