MAX1808EUB+ Maxim Integrated, MAX1808EUB+ Datasheet - Page 13

• Manufacturer Part Number: MAX1808EUB+
• Description: Current & Power Monitors & Regulators
• Manufacturer: Maxim Integrated

An external resistor connecting RT to GND sets the
switching frequency (f
f
where f
600kHz switching frequency is set with R
Higher frequencies allow designs with lower inductor
values and less output capacitance. Peak currents and
I
but core losses, gate-charge currents, and switching
losses increase.
Three key inductor parameters must be specified for
operation with the MAX15026: inductance value (L),
inductor saturation current (I
(R
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 I
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 I
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
Figure 2. Adjustable Output Voltage
Maxim Integrated
SW
2
R losses are lower at higher switching frequencies,
DC
and R
). To determine the inductance value, select the
Low-Cost, Small, 4.5V to 28V Wide Operating
SW
RT
Range, DC-DC Synchronous Buck Controller
is in Hz and R
is:
Setting the Switching Frequency
R
RT
MAX15026
=
f
SW
SW
+
17.3 10
(
). The relationship between
1x10 )x(f
RT
FB
SAT
×
is in Ω. For example, a
−
Inductor Selection
7
), and DC resistance
9
SW
2
)
OUT
2
R
R
RT
R losses are
2
1
= 27.2kΩ.
2
R losses
frequency, input voltage, output voltage, and selected
LIR determine the inductor value as follows,
where V
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R
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
that saturation can occur only above the maximum cur-
rent-limit value (I
resistance of the low-side MOSFET and of the LIM
reference current (I
select an inductor with a saturation current (I
where I
factor 1.35 includes R
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).
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
as the current-sense element. The inductor’s valley cur-
rent occurs at I
current. The minimum value of the current-limit thresh-
old voltage (V
the low-side MOSFET during the ripple-current valley:
where R
MOSFET in ohms. Use the maximum value for R
from the data sheet of the low-side MOSFET.
V
CL(TYP)
ITH
IN
DS(ON)
, V
>
OUT
R
ITH
DS ON MAX
Setting the Valley Current Limit
DS(ON)
is the typical current-limit set-point. The
LOAD(MAX)
CL(MAX)
(
I
is the on-resistance of the low-side
, and I
L
) must be higher than the voltage on
SAT
=
SAT
LIM
,
V
≥ 1.35 x I
DS(ON)
OUT IN
of the low-side MOSFET is used
V f
) must be high enough to ensure
). Combining these conditions,
IN SW OUT
OUT
), given the tolerance of the on-
)
×
(
V
minus one half of the ripple
I
LOAD MAX
I
are typical values (so that
MAX15026
RT
variation of 25% and 10%
−
CL(TYP
V
(see the Setting the
(
LIR
OUT
)
)
)
×
⎛
⎝ ⎜
1
−
SAT
LIR
2
) of:
⎞
⎠ ⎟
DS(ON)
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