MAX17062 Maxim Integrated Products, MAX17062 Datasheet - Page 9

• Manufacturer Part Number: MAX17062
• Description: TFT-LCD Step-Up DC-DC Converter
• Manufacturer: Maxim Integrated Products

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AC characteristics of the inductor core material and the
ratio of inductor resistance to other power-path resis-
tances, the best LIR can shift up or down. If the induc-
tor 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 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 efficien-
cy improvements in typical operating regions.
Calculate the approximate inductor value using the typ-
ical input voltage (V
(I
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
servation of energy and the expected efficiency at that
operating point (η
in the Typical Operating Characteristics :
Calculate the ripple current at that operating point and
the peak current required for the inductor:
MAIN(MAX)
L
I
=
RIPPLE
I
⎛
⎜
⎝
IN DC MAX
V
I
(
), the expected efficiency (η
PEAK
MAIN
V
IN
,
=
⎞
⎟
⎠
MIN
V
_______________________________________________________________________________________
2
=
IN MIN
)
⎛
⎜
⎜
⎝
(
I
I
IN
) taken from an appropriate curve
IN DC MAX
MAIN MAX
=
), the maximum output current
L
(
V
I
)
MAIN MAX
MAIN
×
(
,
V
×
IN MIN
V
(
MAIN
(
(
V
)
MAIN
−
)
TFT-LCD Step-Up DC-DC Converter
×
+
)
V
)
IN
×
f
× η
OSC
I
×
IN(MIN)
RIPPLE
−
f
OSC
V
2
TYP
MIN
V
MAIN
⎞
⎟
⎟
⎠
IN MIN
⎛
⎜
⎝
(
) taken from
η
using con-
LIR
TYP
)
)
⎞
⎟
⎠
The inductor’s saturation current rating and the
MAX17062’s LX current limit (I
I
exceed I
inductor with less than 0.1Ω series resistance.
Considering the typical operating circuit (Figure 1), the
maximum load current (I
15V output and a typical input voltage of 5V. Choosing
an LIR of 0.5 and estimating efficiency of 85% at this
operating point:
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
The ripple current and the peak current are:
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s equivalent series resistance (ESR):
and:
where I
Inductor Selection section). For ceramic capacitors,
the output-voltage ripple is typically dominated by
V
teristics of the output capacitor must also be considered.
PEAK
RIPPLE(C)
I
RIPPLE
L
, and the inductor’s DC current rating should
=
PEAK
⎛
⎝ ⎜
I
IN(DC,MAX)
IN DC MAX
I
PEAK
15
. The voltage rating and temperature charac-
5
V
(
V
V
RIPPLE
RIPPLE
V
V
=
R R IPPLE ESR
⎞
⎠ ⎟
is the peak inductor current (see the
,
2
2 7
=
⎛
⎝ ⎜
.
4 5
0 6
(
.
μ
=
C C
2 35
.
)
(
Output Capacitor Selection
. For good efficiency, choose an
H
.
)
V
15
V
A
RIPPLE C
≈
=
×
×
V
×
)
A
C
I
MAIN
4 5
≈
1 2
−
0 6
15
OUT
(
15
MAIN(MAX)
.
.
+
.
I
PEAK ESR COUT
5
V
MHz
V
( )
V
A
V
0 97
⎛
⎝ ⎜
×
.
×
−
×
V
2
+
V
R
MAIN
⎞
⎠ ⎟
1 2
MAIN OSC
V
0 85
4 5
15
A
.
LIM
⎛
⎝ ⎜
RIPPLE ESR
.
.
0 85
0 50
MHz
V
V
) is 600mA with a
.
.
≈
(
) should exceed
)
−
f
≈
2 84
V
⎞
⎠ ⎟
(
.
IN
≈
≈
2 35
)
⎞
⎠ ⎟
2 7
.
A
)
.
0 97
.
μH
A
A
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