Rainbow Electronics MAX15046B User Manual
Page 14
40V, High-Performance, Synchronous
Buck Controller
MAX15046
14 _____________________________________________________________________________________
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:
9
RT
-7
2
SW
SW
17.3 10
R
f
(1x10 ) x (f
)
×
=
+
where f
SW
is in kHz and R
RT
is in kI. For example, a
300kHz switching frequency is set with R
RT
= 49.9kI.
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 MAX15046: inductance value (L),
inductor saturation current (I
SAT
), and DC resistance
(R
DC
). To determine the inductance, select the ratio of
inductor peak-to-peak AC current to DC average cur-
rent (LIR) first. For LIR values that are too high, the RMS
currents are high, and therefore I
2
R losses are high.
Use high-valued inductors to achieve low LIR values.
Typically, inductor resistance is proportional to induc-
tance 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:
OUT IN
OUT
IN
SW
OUT
V
(V - V
)
L
V
f
I
LIR
=
Ч
Ч
Ч
where V
IN
, V
OUT
, and I
OUT
are typical values. The
switching frequency is set by R
T
(see 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 val-
ues 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 with the lowest possible DC
resistance that fits in the allotted dimensions. The
saturation current rating (I
SAT
) must be high enough to
ensure that saturation cannot occur below the maximum
current-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:
≥
×
SAT
CL(TYP)
I
1.35 I
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, Vishay IHLP-4040DZ-1-5 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 current occurs at
I
LOAD(MAX)
minus one half of the ripple current. The
minimum value of the current-limit threshold voltage
(V
ITH
) must be higher than the voltage on the low-side
MOSFET during the ripple-current valley,
ITH
DS(ON,MAX)
LOAD(MAX)
LIR
V
R
I
1
2
−
>
Ч
Ч
where R
DS(ON,MAX)
in I is the maximum on-resistance
of the low-side MOSFET at maximum load current
I
LOAD(MAX)
and is calculated from the following equation:
DS(ON,MAX)
DS(ON)
MOSFET
MAX
AMB
R
R
[1 TC
(T
- T
)]
=
Ч +
Ч
where R
DS(ON)
(in I is the on-resistance of the low-
side MOSFET at ambient temperature T
AMB
(in degrees
Celsius), TC
MOSFET
is the temperature coefficient of
the low-side MOSFET in ppm/NC, and T
MAX
(in degrees
Celsius) is the temperature at maximum load current
I
LOAD(MAX).
Obtain the R
DS(ON)
and TC
MOSFET
from the
MOSFET data sheet.