Rainbow Electronics MAX15023 User Manual
Page 17
MAX15023
Setting the Output Voltage
Set the MAX15023 output voltage on each channel by
connecting a resistive divider from the output to FB_ to
SGND (Figure 3). Select R
2
(FB_ to SGND resistor) less
than or equal to 16k
Ω. Calculate R
1
(OUT_ to FB_ resis-
tor) with the following equation:
where V
FB_
= 0.6V (typ) (see the
Electrical Characteristics
table) and V
OUT_
can range from 0.6V to (0.85 x V
IN
).
Resistor R
1
also plays a role in the design of the Type
III compensation network. If a Type III compensation
network is used, make sure to review the values of R
1
and R
2
according to the
Type III Compensation
Network (See Figure 5)
section.
Setting the Switching Frequency
The switching frequency, f
SW
, for each channel is set
by a resistor (R
T
) connected from RT to SGND. The
relationship between f
SW
and R
T
is:
where f
SW
is in kHz, R
T
is in k
Ω, and 24806 is in
1/farad. For example, a 600kHz switching frequency is
set with R
T
= 27.05k
Ω. Higher frequencies allow
designs with lower inductor values and less output
capacitance. Consequently, peak currents and I
2
R
losses are lower at higher switching frequencies, but
core losses, gate-charge currents, and switching loss-
es increase.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15023: inductance value (L),
inductor saturation current (I
SAT
), and DC resistance
(R
DC
). To select inductance value, the ratio of inductor
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
IN
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions). The
switching frequency is set by R
T
(see the
Setting the
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
SAT
) must be high enough
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
LIM
). On the other hand, these tolerances
should not prevent the converter from delivering the
rated load current (I
LOAD(MAX)
). Combining these con-
ditions, the inductor saturation current (I
SAT
) should be
such that:
where R
DS(ON,MAX)
and R
DS(ON,TYP)
are the maximum
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
DS(ON,MAX)
, the inductor saturation current should be
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).
I
1
I
SAT
RDS(ON,MAX)
RDS(ON,TYP)
LOAD(MAX)
>
Ч +
⎛
⎝⎜
⎞
⎠⎟
Ч
LIR
2
L
V
V
V
V f
I
LIR
OUT
IN
OUT
IN SW OUT
=
−
(
)
R
f
T
SW
=
24806
1 0663
(
)
.
R
R
V
V
OUT
FB
1
2
1
=
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
−
_
_
FB_
R
1
OUT_
R
2
MA15023
Figure 3. Adjustable Output Voltage
Wide 4.5V to 28V Input, Dual-Output
Synchronous Buck Controller
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