MAX8740 Maxim Integrated Products, MAX8740 Datasheet - Page 8

• Manufacturer Part Number: MAX8740
• Description: TFT-LCD Step-Up DC-DC Converter
• Manufacturer: Maxim Integrated Products

TFT-LCD Step-Up DC-DC Converter
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 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:
8
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OUT(MAX)
_______________________________________________________________________________________
L
I
=
RIPPLE
I



IN DC MAX
), the expected efficiency (η
V
I
(
PEAK
V
OUT
IN
,
=



2
MIN
V
=
IN MIN



)
I
OUT MAX
(
I
IN
) taken from an appropriate curve
IN DC MAX
=
), the maximum output current
L
(
V
OUT
(
I
)
OUT MAX
×
V
,
×
IN MIN
V
(
(
(
)
OUT
−
V
OUT
×
)
V
)
+
IN
)
f
OSC
×
× η
×
I
−
IN(MIN)
RIPPLE
f
OSC
V
MIN



V
TYP
2
OUT
IN MIN



(
η
) taken from
LIR
TYP
using con-
)
)



The inductor’s saturation current rating and the
MAX8740’s LX current limit (I
and the inductor’s DC current rating should exceed
I
with less than 0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
and a 5V typical input voltage. Choosing an LIR of 0.35
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):
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.
IN(DC,MAX)
RIPPLE(C)
L
I
RIPPLE
=
V
RIPPLE C
V
PEAK
I



IN DC MAX
RIPPLE
13 5
V
. The voltage rating and temperature charac-
(
RIPPLE ESR
5
I
. For good efficiency, choose an inductor
.
PEAK
V
=
V
,
( )
OUT(MAX)
is the peak inductor current (see the



2 7
2
(
.
=
4 5
≈



)
.
=
µ
0 9
V
Output Capacitor Selection
H
.
V
=
RIPPLE C
C
)
13 5
I
3 2
OUT
A
OUT
×
.
×
) is 900mA with a 13.5V output
≈
.
0 9
4 5
A
×
.
13 5
.
( .
V
12 5
I
PEAK
A
V
+
( )



1 2
.
−
LIM
.
V
×
×
V
V
OUT
V
0 93
5
MHz
OUT OSC
+
.
V
) should exceed I
×
3 5
0 85
2
−
R
.
.
V
1 2
A
ESR COUT
RIPPLE ESR
−
f



4 5
V
.
.



MHz
≈
V
(
V
≈
0 85
0 35
IN
)
.
.
3 7
3 2



(
.
,
.

 ≈

A
≈
and
)
A
)
0 93
.
2 7
PEAK
.
A
µ
H
,