Datasheet

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 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 smaller 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 efficiency
improvements in typical operating regions.
Calculate the approximate inductor value using the
typical input voltage (V
VIN
), the maximum output cur-
rent (I
AVDD(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
MAX17113’s LX1 current limit should exceed I
AVDD
_
PEAK
and the inductor’s DC current rating should exceed
I
VIN(DC,MAX)
. For good efficiency, choose an inductor
with less than 0.05Ω series resistance.
Considering the typical operating circuit in Figure 1, the
maximum load current (I
AVDD(MAX)
) is 1.0A with a 16V
output and a typical 12V input voltage. Choosing an
LIR of 0.6 and estimating efficiency of 90% at this oper-
ating point:
Using the circuit’s minimum input voltage (10.8V) and
estimating efficiency of 90% at that operating point:
Choosing a 4.7μH inductor, 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 dis-
charging of the output capacitance, and the ohmic rip-
ple due to the capacitor’s ESR:
and:
where I
AVDD
_
PEAK
is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors, the
output-voltage ripple is typically dominated by
V
AVDD
_
RIPPLE(C)
. The voltage rating and temperature
characteristics of the output capacitor must also be
considered. Note that all ceramic capacitors typically
have large temperature coefficient and bias voltage
coefficients. The actual capacitor value in circuit is typi-
cally significantly less than the stated value.
Input Capacitor Selection
The input capacitor reduces the current peaks drawn
from the input supply and reduces noise injection into
the IC. Two 10μF ceramic capacitors are used in the typ-
ical operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source impedance
VIR
AVDD RIPPLE ESR AVDD PEAK ESR AVDD_() _ _
≈
V
I
C
VV
Vf
AVDD RIPPLE C
AVDD
AVDD
AVDD VIN
AVDD S
_()
≈
-
WW
⎛
⎝
⎜
⎞
⎠
⎟
VV V
AVDD RIPPLE AVDD RIPPLE C AVDD RIPPLE E__()_(
=+
SSR)
IA
A
A
PEAK
=+≈164
12
2
224.
.
.
I
VV V
μH V kHz
RIPPLE
=
×
()
××
≈
10 8 16 10 8
4 7 16 600
1
..
.
-
..2A
I
AV
V
A
VIN DC MAX(, )
.
..
.=
×
×
≈
10 16
10 8 0 9
164
L
V
V
VV
AkHz
AVDD
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
12
16
16 12
1 600
0
2
-.990
06
56
.
.
⎛
⎝
⎜
⎞
⎠
⎟
≈ μH
II
I
AVDD PEAK VIN DC MAX
AVDD RIPPLE
_(,)
_
=+
2
I
VVV
L
AVDD RIPPLE
VIN MIN AVDD VIN MIN
AV
_
() ()
=
×
()
-
DDD AVDD SW
Vf××
I
IV
V
VIN DC MAX
AVDD MAX AVDD
VIN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
I
AVDD
VIN
AVDD
AVDD VIN
AVDD MAX
=
⎛
⎝
⎜
⎞
⎠
⎟
×
2
-
()
ffLIR
SW
TYP
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
η
MAX17113
Low-Cost, Multiple-Output
Power Supply for LCD TVs
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