MAX1518 Maxim Integrated Products, MAX1518 Datasheet - Page 19

• Manufacturer Part Number: MAX1518
• Description: (MAX1516 - MAX1518) TFT-LCD DC-DC Converters
• Manufacturer: Maxim Integrated Products

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The minimum inductance value, peak current rating,
and series resistance are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output load capability,
transient-response time, and output voltage ripple. Size
and cost are also important factors to consider.
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
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increases size and can increase
I
decrease the size but increase the current ripple and
peak current. Finding the best inductor involves choos-
ing the best compromise between 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 cur-
rent. The best trade-off between inductor size and cir-
cuit efficiency for step-up regulators generally has an
LIR between 0.3 and 0.5. However, depending on the
AC characteristics of the inductor core material and
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 section, and an estimate of LIR based
on the above discussion:
4
I
Industry, Inc.
2
2
MAIN(MAX)
R is a registered trademark of Instruments for Research and
R losses in the inductor. Low inductance values
), the expected efficiency (η
______________________________________________________________________________________
IN
), the maximum output current
Main Step-Up Regulator
Design Procedure
Inductor Selection
TFT-LCD DC-DC Converters with
TYP
) taken from
2
R
®
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:
The inductor’s saturation current rating and the
MAX1516/MAX1517/MAX1518s’ LX current limit (I
should exceed I
rating should exceed I
choose an inductor with less than 0.1Ω series resistance.
Considering the Typical Operating Circuit, the maximum
load current (I
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 80% at that operating point:
The ripple current and the peak current are:
Operational Amplifiers
L
L
=
=
I
I
I
I
RIPPLE
PEAK
RIPPLE
PEAK






I
IN DCMAX
13
V
5
MAIN
(
V
MAIN(MAX)
I
V
IN DCMAX
V
IN
=
=



(
,
2
1 8
=
I
PEAK
=
IN DCMAX
MIN






.
V
,
3 3
(
2
0 5
IN MIN
A
.
4 5



.
)
) taken from the appropriate curve
.
I
(
µ ×
+
, and the inductor’s DC current
13
MAIN MAX
=
,
A
IN(DC,MAX)
H
) is 500mA with a 13V output and
V
)
0 74
I
L V
MAIN MAX
V
×
=
V
.
×
)
×
1 2
2
MAIN
V
−
13
0 5
(
)
×
(
.
4 5
IN MIN
13
.
5
+
A
.
(
MAIN
MHz
V
(
(
V
V
V
I
A
V
RIPPLE
MAIN
≈
×
−
−
×
)
×
2 2
1 2
2
×
)
V
13
4 5
.
.
0 8
. For good efficiency,



)
×
IN
× η
.
f



.
OSC
×
MHz
A
f
V
−
OSC
0 85
V
IN(MIN)
V
0 5
MIN
V
.
)
≈
MAIN
.
IN MIN
1 8



(
.
≈




 ≈

η
0 74
A
LIR
TYP
) using con-
.
3 3
)
)
.
A
µ



H
LIM
19
)