Datasheet
MAX17014
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I
2
R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I
2
R losses in the inductor. Low inductance val-
ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing the best compromise among circuit
efficiency, inductor size, and cost.
The equations used here include a constant, LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full-load current.
The best trade-off between inductor size and circuit effi-
ciency for step-up regulators generally has an LIR
between 0.2 and 0.5. However, depending on the AC
characteristics of the inductor core material and ratio of
inductor resistance to other power path resistances, the
best LIR can shift up or down. If the inductor resistance
is relatively high, more ripple can be accepted to
reduce the number of turns required and increase the
wire diameter. If the inductor resistance is relatively low,
increasing inductance to lower the peak current can
decrease losses throughout the power path. If extremely
thin high-resistance inductors are used, as is common
for smaller LCD panel applications, the best LIR can
increase to between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions.
Calculate the approximate inductor value using the
typical input voltage (V
VIN
), the maximum output cur-
rent (I
AVDD(MAX)
), the expected efficiency (
η
TYP
) taken
from an appropriate curve in the
Typical Operating
Characteristics
, and an estimate of LIR based on the
above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage V
VIN(MIN)
using con-
servation of energy and the expected efficiency at that
operating point (
η
MIN
) taken from an appropriate curve
in the
Typical Operating Characteristics
:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX17014’s LX1 current limit should exceed I
AVDD
_
PEAK
and the inductor’s DC current rating should exceed
I
VIN(DC,MAX)
. For good efficiency, choose an inductor
with less than 0.05Ω series resistance.
Considering the typical operating circuit in Figure 1, the
maximum load current (I
AVDD(MAX)
) is 1.5A with a 16V
output and a typical 12V input voltage. Choosing an
LIR of 0.25 and estimating efficiency of 90% at this
operating point:
Using the circuit’s minimum input voltage (10.8V) and
estimating efficiency of 90% at that operating point:
The ripple current and the peak current are:
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and dis-
charging of the output capacitance, and the ohmic rip-
ple due to the capacitor’s ESR:
and:
VIR
AVDD RIPPLE ESR AVDD PEAK ESR AVDD_() _ _
≈
V
I
C
VV
Vf
AVDD RIPPLE C
AVDD
AVDD
AVDD VIN
AVDD SW
_()
≈
−
⎛
⎝
⎜
⎞
⎠
⎟
VV V
AVDD RIPPLE AVDD RIPPLE C AVDD RIPPLE ESR__()_()
=+
IA
A
A
PEAK
=+ ≈247
062
2
278.
.
.
I
VV V
H V MHz
A
RIPPLE
=
×−
()
××
≈
10 8 16 10 8
47 16 12
062
..
..
.
μ
I
AV
V
A
VIN DCMAX(, )
.
..
.=
×
×
≈
15 16
10 8 0 9
247
L
V
V
VV
A MHz
H
AVDD
=
⎛
⎝
⎜
⎞
⎠
⎟
−
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
≈
12
16
16 12
15 12
090
025
47
2
..
.
.
. μ
II
I
AVDD PEAK VIN DCMAX
AVDD RIPPLE
_(,)
_
=+
2
I
VVV
LVf
AVDD RIPPLE
VIN MIN AVDD VIN MIN
AVDD AVDD SW
_
() ()
=
×−
()
××
I
IV
V
VIN DCMAX
AVDD MAX AVDD
VIN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
I f LIR
AVDD
VIN
AVDD
AVDD VIN
AVDD MAX SW
TYP
=
⎛
⎝
⎜
⎞
⎠
⎟
−
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
()
η
Low-Cost Multiple-Output
Power Supply for LCD TVs
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