MAX15026_12 Datasheet, PDF (13/23 Pages) Maxim Integrated Products – Low-Cost, Small, 4.5V to 28V Wide Operating Range, DC-DC Synchronous Buck Controller

Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
OUT
R1
FB
R2
MAX15026
Figure 2. Adjustable Output Voltage
Setting the Switching Frequency
An external resistor connecting RT to GND sets the
switching frequency (f_SW). The relationship between
f_SW and R_RT is:
R_RT
=
f_SW
17.3 × 10⁹
+ (1x10^−7)x(f_SW2)
where f_SW is in Hz and R_RT is in Ω. For example, a
600kHz switching frequency is set with R_RT = 27.2kΩ.
Higher frequencies allow designs with lower inductor
values and less output capacitance. Peak currents and
I²R losses are lower at higher switching frequencies,
but core losses, gate-charge currents, and switching
losses increase.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15026: inductance value (L),
inductor saturation current (I_SAT), and DC resistance
(R_DC). To determine the inductance value, select the
ratio of inductor peak-to-peak AC current to DC average
current (LIR) first. For LIR values which are too high, the
RMS currents are high, and therefore I²R losses are
high. Use high-valued inductors to achieve low LIR val-
ues. Typically, inductance is proportional to resistance
for a given package type, which again makes I²R losses
high for very low LIR values. A good compromise
between size and loss is a 30% peak-to-peak ripple cur-
rent to average-current ratio (LIR = 0.3). The switching
frequency, input voltage, output voltage, and selected
LIR determine the inductor value as follows,
L = ^{VOUT (VIN − VOUT )}
V_INf_SWI_OUTLIR
where V_IN, V_OUT, and I_OUT are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R_RT (see the Setting the
Switching Frequency section). The exact inductor value
is not critical and can be adjusted to make trade-offs
among size, cost, and efficiency. Lower inductor values
minimize size and cost, but also improve transient
response and reduce efficiency due to higher peak cur-
rents. On the other hand, higher inductance increases
efficiency by reducing the RMS current.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The satura-
tion current rating (I_SAT) must be high enough to ensure
that saturation can occur only above the maximum cur-
rent-limit value (I_CL(MAX)), given the tolerance of the on-
resistance of the low-side MOSFET and of the LIM
reference current (I_LIM). Combining these conditions,
select an inductor with a saturation current (I_SAT) of:
I_SAT ≥ 1.35 x I_CL(TYP)
where I_CL(TYP) is the typical current-limit set-point. The
factor 1.35 includes R_DS(ON) variation of 25% and 10%
for the LIM reference current error. A variety of inductors
from different manufacturers are available to meet this
requirement (for example, Coilcraft MSS1278-142ML
and other inductors from the same series).
Setting the Valley Current Limit
The minimum current-limit threshold must be high
enough to support the maximum expected load current
with the worst-case low-side MOSFET on-resistance
value as the R_DS(ON) of the low-side MOSFET is used
as the current-sense element. The inductor’s valley cur-
rent occurs at I_LOAD(MAX) minus one half of the ripple
current. The minimum value of the current-limit thresh-
old voltage (V_ITH) must be higher than the voltage on
the low-side MOSFET during the ripple-current valley:
V_ITH
>
R_DS(ON,MAX)
× I_LOAD(MAX)
×
⎛⎝⎜_1−
LIR ⎞
2 ⎠⎟
where R_DS(ON) is the on-resistance of the low-side
MOSFET in ohms. Use the maximum value for R_DS(ON)
from the data sheet of the low-side MOSFET.
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