Rainbow Electronics MAX8728 User Manual
Page 24
MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
24
______________________________________________________________________________________
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 increase physical size and can
increase I
2
R losses in the inductor. Low inductance val-
ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing 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 0.2 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
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
MAX8728’s LX2 current limit should exceed I
AVDD
_
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 in Figure 1,
the maximum load current (I
AVDD(MAX)
) is 500mA with
a 13.5V output and a typical input voltage of 12V.
Choosing an LIR of 0.3 and estimating efficiency of
95% at this operating point:
Using the circuit’s minimum input voltage (10.8V) and
estimating efficiency of 90% 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 dis-
charging of the output capacitance, and the ohmic rip-
ple due to the capacitor’s ESR:
where I
AVDD
_
PEAK
is the peak-inductor current (see the
Inductor Selection section). For ceramic capacitors, the
V
V
V
V
I
C
V
V
V
x f
and
V
I
x R
AVDD RIPPLE
AVDD RIPPLE C
AVDD RIPPLE ESR
AVDD RIPPLE C
AVDD
AVDD
AVDD
IN
AVDD
SW
AVDD RIPPLE ESR
AVDD PEAK
ESR
_
_
( )
_
(
)
_
( )
_
(
)
_
_
,
=
+
≈
−
⎛
⎝⎜
⎞
⎠⎟
≈
I
V
V
V
H
V
MHz
A
I
A
A
A
RIPPLE
PEAK
.
.
.
.
.
.
.
.
.
.
=
Ч
−
(
)
Ч
Ч
≈
=
+
≈
10 8
13 5
10 8
6 4
13 5
1 5
0 23
0 69
0 23
2
0 81
μ
I
A
V
V
A
IN DC MAX
(
,
)
.
.
.
.
.
=
Ч
Ч
≈
0 5
13 5
10 8
0 9
0 69
L
V
V
V
V
A
MHz
H
AVDD
.
.
.
.
.
.
.
= ⎛
⎝⎜
⎞
⎠⎟
−
×
⎛
⎝⎜
⎞
⎠⎟
⎛
⎝⎜
⎞
⎠⎟
≈
12
13 5
13 5
12
0 5
1 5
0 95
0 5
6 4
2
μ
I
V
V
V
L
V
f
I
I
I
AVDD RIPPLE
IN MIN
AVDD
IN MIN
AVDD
AVDD
SW
AVDD PEAK
IN DC MAX
AVDD RIPPLE
_
(
)
(
)
_
(
,
)
_
=
Ч
−
(
)
Ч
Ч
=
+
2
I
I
V
V
IN DC MAX
AVDD MAX
AVDD
IN MIN
MIN
(
,
)
(
)
(
)
=
Ч
Ч η
L
V
V
V
V
I
f
LIR
AVDD
IN
AVDD
AVDD
IN
AVDD MAX
SW
TYP
(
)
=
⎛
⎝⎜
⎞
⎠⎟
−
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝⎜
⎞
⎠⎟
2
η