參數(shù)資料
型號(hào)： MAX1518ETJ
廠商： MAXIM INTEGRATED PRODUCTS INC
元件分類： 穩(wěn)壓器
英文描述： TFT-LCD DC-DC Converters with Operational Amplifiers
中文描述： 1.6 A SWITCHING REGULATOR, 1380 kHz SWITCHING FREQ-MAX, PQCC32
封裝： 5 X 5 MM, 0.80 MM HEIGHT, MO-220-WHHD2, TQFN-32
文件頁(yè)數(shù)： 19/26頁(yè)
文件大小： 919K
代理商： MAX1518ETJ
M
TFT-LCD DC-DC Converters with
Operational Amplifiers
______________________________________________________________________________________
19
Design Procedure
Main Step-Up Regulator
Inductor Selection
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
2
R
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
2
R losses in the inductor. Low inductance values
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
IN
), the maximum output current
(I
MAIN(MAX)
), the expected efficiency (
η
TYP
) taken from
an appropriate curve in the
Typical Operating
Characteristics
section, 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
IN(MIN)
) using con-
servation of energy and the expected efficiency at that
operating point (
η
MIN
) taken from the 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
MAX1516/MAX1517/MAX1518s’ LX current limit (I
LIM
)
should exceed I
PEAK
, and the inductor’s DC current
rating should exceed I
IN(DC,MAX)
. For good efficiency,
choose an inductor with less than 0.1
series resistance.
Considering the
Typical Operating Circuit
, the maximum
load current (I
MAIN(MAX)
) is 500mA with a 13V 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 80% at that operating point:
The ripple current and the peak current are:
4 5
3 3
.
I
V
0 74
.
V
V
V
MHz
A
I
A
A
A
RIPPLE
PEAK
=
×
μ ×
×
≈
=
+
≈
13
13
4 5
1 2
.
0 74
.
1 8
.
2
2 2
.
.
(
.
)
I
A
V
V
A
IN DCMAX
(
,
)
.
.
.
.
=
×
×
≈
0 5
4 5
13
0 8
1 8
L
V
V
V
×
V
MHz
A
H
=
1 2
.
≈
μ
5
13
13
5
0 5
.
0 85
0 5
.
3 3
.
2
.
I
V
V
V
L V
f
I
I
I
RIPPLE
IN MIN
(
MAIN
IN MIN
(
MAIN
RIPPLE
+
OSC
PEAK
IN DCMAX
(
=
×
×
=
)
)
,
)
(
)
2
I
I
V
V
IN DCMAX
(
MAIN MAX
MAIN
IN MIN
(
MIN
,
)
(
)
)
=
×
×
η
L
V
V
V
V
×
I
f
LIR
IN
MAIN
MAIN
(
IN
OSC
MAIN MAX
TYP
η
=
2
)
I
2
R is a registered trademark of Instruments for Research and
Industry, Inc.
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