Rainbow Electronics MAX17061 - Manual (Page 21)

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 controller operates in DCM mode when LIR is
higher than 2.0, and it works in CCM mode when LIR is
lower than 2.0. The best trade-off between inductor size
and converter 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 ripples can
be accepted to reduce the number of required turns and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can reduce losses throughout the power path. If
extremely thin high-resistance inductors are used, as is
common for LCD panel applications, LIR higher than 2.0
can be chosen for DCM operating mode.
Once a physical inductor is chosen, higher and lower val-
ues of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detail
design procedure for CCM can be described as follows:
Inductor Selection in CCM Operation
1) Calculate the approximate inductor value using the
typical input voltage (V IN ), the maximum output cur-
rent (I OUT(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:
The MAX17061 has a minimum inductor value limitation
for a stable operation in CCM mode at low input voltage
because of the internal fixed-slope compensation. The
minimum inductor value for stability is calculated with
the following equation:
where 24.7mV is a scale factor from slope compensa-
tion, and the R
S is the equivalent current-sensing scale
factor (12m
Ω typ):
1) Choose an available inductor value from an appropri-
ate inductor family. Calculate the maximum DC input
current at the minimum input voltage V
IN(MIN), using
conservation of energy and the expected efficiency
at that operating point (
η
MIN ) taken from an appropri-
ate curve in the Typical Operating Characteristics :
2) Calculate the ripple current at that operating point
and the peak current required for the inductor:
Inductor Selection in DCM Operation
When DCM operating mode is chosen to minimize the
inductor value, the calculations are different from those
above in CCM mode. The maximum inductor value for
DCM mode is calculated with the following equation:
The peak-inductor current in DCM is calculated with fol-
lowing equation:
The inductor’s saturation current rating 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.
Inductor Selection Design
Examples:
Considering the Typical Operating Circuit with four
10-LED strings and 25mA LED full-scale current, the
maximum load current (I
OUT(MAX)
) is 100mA with a
35.9V output and a minimal input voltage of 7V.
Choosing a CCM operating mode with LIR = 1 at 1MHz
and estimating efficiency of 85% at this operating point:
In CCM, the inductor has to be higher than L CCM(MIN):
A10µH inductor is chosen, which is higher than the
minimum L that guarantees stability in CCM.
L
V V V m
mV MHz
H
CCM MIN ( )
. .
. .
. =
+ − Ч
( )
Ч
Ч Ч
=
35 9 0 4 2 7 12
2 24 7 0 9
6 0
Ω
μ
L
V
V
V V
mA MHz
H = ⎛
⎝⎜
⎞
⎠⎟ −
×
⎛
⎝⎜ ⎞
⎠⎟ ⎛
⎝⎜ ⎞
⎠⎟
=
7
35 9
35 9 7
100 1
0 85
1
9 44
2
.
. .
. μ
I I
I
PEAK IN DCMAX
RIPPLE = +
( , )
2
L
V
V V
V
f V I
DCM MAX
IN MIN
OUT MAX DIODE
IN MIN
OSC MAX OUT MAX OUT MAX
( )
( )
( )
( )
( ) ( ) ( )
= −
+
⎛
⎝
⎜
⎞
⎠
⎟
Ч
Ч
Ч Ч Ч
1
2
2 η
I
V V V
L V f
RIPPLE
IN MIN OUT MAX IN MIN
OUT MAX OSC
=
Ч −
( )
Ч Ч
( ) ( ) ( )
( )
I
I V
V
IN DCMAX
OUT MAX OUT
IN MIN MIN
( , )
( )
( )
=
Ч
Ч η
L
V V V R
mV f
CCM MIN
OUT MAX DIODE IN MIN S
OSC MIN
( )
( ) ( )
( )
.
=
+ − Ч
( )
Ч
Ч Ч
2
2 24 7
L
V
V
V V
I f LIR
IN MIN
OUT
OUT IN MIN
OUT MAX OSC
TYP =
⎛
⎝⎜ ⎞
⎠⎟ −
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝⎜ ⎞
⎠⎟
_ _
( )
2
η
MAX17061
8-String White LED Driver with
SMBus for LCD Panel Applications
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