Datasheet

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
15.14 10
R
f (1 x 10 ) (f )
×
=
+
where f
SW
is in Hz and R
RT
is in Ω. For example, a
300kHz switching frequency is set with R
RT
= 49.9kΩ.
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 oper-
ation 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 current (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 inductance for a given pack-
age 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 current 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
=
×× ×
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 values minimize
size and cost, but also improve transient response and
reduce efficiency due to higher peak currents. On the
other hand, higher inductance increases efficiency by
reducing the RMS current.
Find a low-loss inductor with the lowest possible DC resis-
tance 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 (ISAT) of:
I
SAT
≥ 1.35 ×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, 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 mini-
mum 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
VR I 1
2
−

> ××


where R
DS(ON,MAX)
in Ω 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
RR
[1 TC (T - T )]
= ×
+×
where R
DS(ON)
(in Ω 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/°C, 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.
Connect an external resistor (R
LIM
) from LIM to GND
to adjust the current-limit threshold, which is temper-
ature-compensated with a temperature coefficient of
2300ppm/°C. The relationship between the current-limit
threshold (V
ITH
) and R
LIM
is:
ITH
LIM
-6
MAX AMB)
10 V
R
ppm
50 10 1 2300 (T - T
C
×
=

× ×+ ×

°

where R
LIM
is in Ω, V
ITH
is in V, T
MAX
and T
AMB
are in
°C.
MAX15046 40V, High-Performance, Synchronous
Buck Controller
www.maximintegrated.com
Maxim Integrated │ 14