Rainbow Electronics MAX15026 User Manual
Page 13
MAX15026
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
______________________________________________________________________________________
13
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:
where f
SW
is in kHz and R
RT
is in k
Ω. For example, a
600kHz switching frequency is set with R
RT
= 27.2k
Ω.
Higher frequencies allow designs with lower inductor
values and less output capacitance. Peak currents and
I
2
R 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 I
2
R 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 I
2
R 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,
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:
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.
V
R
I
LIR
ITH
DS ON MAX
LOAD MAX
>
Ч
Ч ⎛
⎝⎜
⎞
⎠⎟
−
(
,
)
(
)
1
2
L
V
V
V
V f
I
LIR
OUT
IN
OUT
IN SW OUT
=
−
(
)
R
17.3 10
f
1x10 )x(f
RT
9
SW
7
SW
=
×
+
−
(
)
2
FB
R
1
OUT
R
2
MAX15026
Figure 2. Adjustable Output Voltage