Rainbow Electronics MAX17062 - Manual (Page 9)

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
IN ), the maximum output current
(I MAIN(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
IN(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
MAX17062’s 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
Ω series resistance.
Considering the typical operating circuit (Figure 1), the
maximum load current (I
MAIN(MAX) ) is 600mA with a
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:
Output Capacitor Selection
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 PEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors,
the output-voltage ripple is typically dominated by
V
RIPPLE(C) . The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
V V V
V
RIPPLE RIPPLE C RIPPLE ESR
RIPPLE
( ) ( )
(
= +
C C
MAIN
OUT
MAIN IN
MAIN OSC
I
C
V V
V f
V
)
≈
− ⎛
⎝⎜ ⎞
⎠⎟
R RIPPLE ESR PEAK ESR COUT
I R
( ) ( )
≈
I
V V V
H V MHz
A
I A
A
A
RIPPLE
PEAK
. ( . )
. .
.
.
.
.
=
× −
μ Ч Ч
≈
= + ≈
4 5 15 4 5
2 7 15 1 2
0 97
2 35
0 97
2
2 84
I
A V
V
A
IN DC MAX ( , )
.
. .
.
=
Ч
Ч
≈
0 6 15
4 5 0 85
2 35
L
V
V
V V
A MHz . .
= ⎛
⎝⎜
⎞
⎠⎟
−
×
⎛
⎝⎜
⎞
⎠⎟
5
15
15 5
0 6 1 2
2
.
.
.
0 85
0 50
2 7
⎛
⎝⎜
⎞
⎠⎟
≈ μH
I I
I
PEAK IN DC MAX
RIPPLE
( , )
= +
2
I
V V V
L V f
RIPPLE
IN MIN MAIN IN MIN
MAIN OSC
( )
( ) ( )
=
Ч −
Ч Ч
I
I V
V
IN DC MAX
MAIN MAX MAIN
IN MIN MIN
( , )
( )
( )
=
Ч
Ч η
L
V
V
V V
I f LIR
IN
MAIN
MAIN IN
MAIN MAX OSC
TYP
( )
=
⎛
⎝
⎜
⎞
⎠
⎟
−
×
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
η
MAX17062
TFT-LCD Step-Up DC-DC Converter
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