MAX15023 Maxim Integrated Products, MAX15023 Datasheet - Page 17

• Manufacturer Part Number: MAX15023
• Description: Dual-Output Synchronous Buck Controller
• Manufacturer: Maxim Integrated Products

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Set the MAX15023 output voltage on each channel by
connecting a resistive divider from the output to FB_ to
SGND (Figure 3). Select R
than or equal to 16kΩ. Calculate R
tor) with the following equation:
where V
table) and V
Resistor R
III compensation network. If a Type III compensation
network is used, make sure to review the values of R
and R
Network (See Figure 5) section.
The switching frequency, f
by a resistor (R
relationship between f
where f
1/farad. For example, a 600kHz switching frequency is
set with R
designs with lower inductor values and less output
capacitance. Consequently, peak currents and I
losses are lower at higher switching frequencies, but
core losses, gate-charge currents, and switching loss-
es increase.
Figure 3. Adjustable Output Voltage
2
FB_
SW
according to the Type III Compensation
1
T
= 0.6V (typ) (see the Electrical Characteristics
OUT_
also plays a role in the design of the Type
is in kHz, R
Setting the Switching Frequency
= 27.05kΩ. Higher frequencies allow
MA15023
T
R
can range from 0.6V to (0.85 x V
) connected from RT to SGND. The
______________________________________________________________________________________
1
R
=
Setting the Output Voltage
T
R
SW
=
2
⎡
⎢
⎢
⎣
FB_
(
⎛
⎜
⎝
and R
f
T
SW
2
V
SW
V
24806
OUT
is in kΩ, and 24806 is in
(FB_ to SGND resistor) less
FB
Wide 4.5V to 28V Input, Dual-Output
)
1 0663
, for each channel is set
.
_
T
_
is:
⎞
⎟
⎠
1
−
(OUT_ to FB_ resis-
1
⎤
⎥
⎥
⎦
OUT_
R
R
1
2
Synchronous Buck Controller
IN
).
2
R
1
Three key inductor parameters must be specified for
operation with the MAX15023: inductance value (L),
inductor saturation current (I
(R
peak-to-peak AC current to DC average current (LIR)
must be selected first. A good compromise between
size and loss is a 30% peak-to-peak ripple current to
average-current ratio (LIR = 0.3). The switching fre-
quency, input voltage, output voltage, and selected LIR
then determine the inductor value as follows:
where V
efficiency is optimum for typical conditions). The
switching frequency is set by R
Switching Frequency section). The exact inductor value
is not critical and can be adjusted in order to make
trade-offs among size, cost, efficiency, and transient
response requirements. 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, but requires more output
capacitance to meet load-transient specifications.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The
inductor’s saturation rating (I
to ensure that saturation can occur only above the max-
imum current-limit value, given the tolerance of the low-
side MOSFET’s on-resistance and of the LIM_ reference
current (I
should not prevent the converter from delivering the
rated load current (I
ditions, the inductor saturation current (I
such that:
where R
and typical on-resistance of the low-side MOSFET. For
a given inductor type and value, choose the LIR corre-
sponding to the worst-case inductor tolerance.
For LIR = 0.4, and a +25% on the low-side MOSFET’s
R
about 50% greater than the converter’s maximum load
current. A variety of inductors from different manufac-
turers can be chosen to meet this requirement (for
example, Coilcraft MSS1278 series).
DS(ON,MAX)
DC
). To select inductance value, the ratio of inductor
I
SAT
IN
DS(ON,MAX)
LIM
, V
>
R DS(ON,MAX)
R DS(ON,TYP)
, the inductor saturation current should be
OUT
). On the other hand, these tolerances
L
, and I
=
LOAD(MAX)
and R
V
OUT IN
V f
IN SW OUT
OUT
×
(
DS(ON,TYP)
V
⎛
⎜
⎝
1
are typical values (so that
SAT
I
SAT
+
−
). Combining these con-
Inductor Selection
LIR
V
) must be high enough
2
T
OUT
), and DC resistance
LIR
(see the Setting the
⎞
⎟ ×
⎠
)
are the maximum
I
LOAD(MAX)
SAT
) should be
17