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Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Table 4. Component Suppliers
Murata
Nichia
Sumida
Toshiba
Vishay
SUPPLIER
PHONE
770-436-1300
248-352-6575
847-545-6700
949-455-2000
203-268-6261
WEBSITE
www.murata.com
www.nichia.com
www.sumida.com
www.toshiba.com/taec
www.vishay.com
? V IN _ MIN ? ? V OUT IN _ MIN ? ? η TYP ?
? V
L = ? ? ? ? ? ?
( V OUT ( MAX ) + V DIODE ? 2 × V IN ( MIN ) ) × R S
51 mV × f OSC ( MIN )
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when
selecting an inductor. These factors affect the conver-
ter’s operating mode, efficiency, maximum output load
capability, transient response time, output voltage ripple,
and cost.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance minimizes the current rip-
ple, and therefore reduces the peak current, which
decreases core losses in the inductor and I 2 R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increases physical size and I 2 R copper loss-
es in the inductor. Low inductor values decrease the
physical size, but increase the current ripple and peak
current. Finding the best inductor involves the compro-
mises among circuit efficiency, inductor size, and cost.
When choosing an inductor, the first step is to deter-
mine the operating mode: continuous conduction mode
(CCM) or discontinuous conduction mode (DCM). The
MAX8790A has a fixed internal slope compensation,
which requires a minimum inductor value. When CCM
mode is chosen, the ripple current and the peak cur-
rent of the inductor can be minimized. If a small-size
inductor is required, DCM mode can be chosen. In
DCM mode, the inductor value and size can be mini-
mized but the inductor ripple current and peak current
are higher than those in CCM. The controller can be
stable, independent of the internal slope compensation
mode, but there is a maximum inductor value require-
ment to ensure the DCM operating mode.
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 switches to CCM mode when LIR
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
required turns and increase the wire diameter. If the
inductor resistance is relatively low, increasing induc-
tance to lower the peak current can reduce losses
throughout the power path. If extremely thin high-resis-
tance 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
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detail
design procedure can be described as follows:
Calculate the approximate inductor value using the typ-
ical input voltage (V IN ), the maximum output current
(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:
2
? V OUT ? ? I OUT ( MAX ) × f OSC ? ? LIR ?
The MAX8790A has a minimum inductor value limitation
for stable operation in CCM mode at low input voltage
because of the internal fixed slope compensation. The
minimum inductor value for stability is calculated by the
following equation:
L CCM ( MIN ) =
where 51mV is a scale factor based on slope compen-
sation, and R S is the current-sense resistor. To deter-
mine the minimum inductor value, the R S can be
temporarily calculated using the following equation:
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,
R S _ TMP =
100 mV
,
1 . 2 × I IN ( DCMAX )
depending on the AC characteristics of the inductor
where 100mV is the current-limit sense voltage.
______________________________________________________________________________________
17
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