MAX17112 Maxim Integrated Products, MAX17112 Datasheet - Page 10

• Manufacturer Part Number: MAX17112
• Description: DC-DC Converter
• Manufacturer: Maxim Integrated Products

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High-Performance, Step-Up, DC-DC Converter
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple, and therefore, reduce the peak current, which
decreases core losses in the inductor and I
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increase physical size and can increase I
losses in the inductor. Low inductance values decrease
the physical 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 called LIR,
which is the ratio of the inductor peak-to-peak ripple
current to the average DC inductor current at the full
load current. The best trade-off between inductor size
and circuit 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
resistances, the best LIR can shift up or down. If the
inductor resistance is relatively high, more ripple is
acceptable to reduce the number of turns required, and
to increase the wire diameter. If the inductor resistance
is relatively low, increasing inductance to lower the
peak current can decrease losses through 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 :
10
MAIN(EFF)
______________________________________________________________________________________
L
=
), the expected efficiency (η
⎛
⎝ ⎜
V
I
IN DC MAX
V
OUT
IN
(
MIN
,
⎞
⎠ ⎟
2
⎛
⎜
⎝
IN
) taken from an appropriate curve
I
MAIN EFF
)
), the maximum output current
=
V
I
MAIN
MAIN EFF
(
V
IN MIN
(
(
)
-
×
V
IN
f
)
OSC
)
× η
×
IN(MIN)
V
MIN
⎞ ⎞
⎟
⎠
OUT
TYP
⎛
⎝ ⎜
η
LIR
TYP
) taken from
2
, using con-
R losses in
⎞
⎠ ⎟
2
R
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX17112’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 typical input voltage of 5V. Choosing an LIR of 0.5
and estimating 85% efficiency at this operating point:
Using the circuit’s minimum input voltage (4.5V) and
estimating 85% efficiency at this operating point:
The ripple current and the peak current at that input
voltage 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):
IN(DC,MAX)
L
=
I
RIPPLE
I
⎛
⎝ ⎜
RIPPLE
15
V
I
IN DC MAX
. For good efficiency, choose an inductor
5
V
RIPPLE
I
PEAK
RIPPLE C
V
V
(
I
PEAK
MAIN(MAX
⎞
⎠ ⎟
=
2
,
=
⎛
⎝ ⎜
2 7
=
.
V
0 6
4 5
( )
=
=
IN MIN
.
.
2 35
Output Capacitor Selection
µH
)
V
I
.
IN DC MAX
15
(
V
RIPPLE C
A
=
≈
(
×
×
L V
×
V
C
4 5
I
)) is 600mA with a 15V output
0 6
A
MAIN
15
.
1 2
(
)
×
.
OUT
-
15
+
,
.
×
5
V
A
V
0 97
( )
V
MHz
(
MAIN
V
×
V
.
×
×
LIM
⎛
⎝ ⎜
MAIN
-
2
0 85
1 2
)
15
+
4 5
V
V
.
.
+
A
MAIN
.
MAIN OSC
⎞
⎠ ⎟
V
) should exceed I
V
×
I
MHz
RIPPLE ESR
RIPPLE
⎛
⎝ ⎜
V
=
f
-
0 85
≈
O
)
0
2 84
2
V
.
S S C
.5 5
2 35
f
.
IN MIN
-
.
≈
V
(
⎞
⎠ ⎟
(
IN
0 9
A
≈ . µH
A
.
⎞
⎟ ⎟
⎠
2 7
)
7 7 A
)
)
PEAK