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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
The maximum output current, input voltage, output
voltage, and switching frequency determine the induc-
tor value. Very high inductance minimizes the current
ripple, 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
Calculate the approximate inductor value using the
typical input voltage (V S ), the maximum output cur-
rent (I OUT(MAX) ), the expected efficiency ( E TYP ) taken
from an appropriate curve in the Typical Operating
Characteristics , and an estimate of LIR based on the
above discussion:
L = ? ? ? ?
? V OUT -V S(MIN) ? ? E TYP ?
? I OUT(MAX) SW ?? ? LIR ? ?
?
?
alsorequiremoreenergystorageandmoreturnsofwire,
which increases physical size and I 2 R copper losses.
Low inductor values decrease the physical size but
increase the current ripple and peak current. Finding the
? V S(MIN) ?
? V OUT ?
2
? ?
O f
( V OUT(MAX) + V DIODE -2 O V S(MIN) ) O R S
L CCM(MIN) =
best inductor involves the compromises among circuit
efficiency, inductor size, and cost.
In choosing an inductor, the first step is to determine the
operating mode: continuous-conduction mode (CCM) or
discontinuous-conduction mode (DCM). The MAX17105
has a fixed internal slope compensation, which requires
minimum inductor value. When CCM mode is chosen,
the ripple current and the peak current 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 minimized, but the inductor ripple
current and peak current are higher than those in CCM.
The controller can be stable, independent of the inter-
nal slope-compensation mode, but there is a maximum
inductor value requirement 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 MAX17105 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 with the
following equation:
2 O 25.5mV O f SW(MIN)
where 25.5mV is a scale factor from slope compensa-
tion, and R S is the equivalent current-sensing scale fac-
tor (13.7m I typ).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V S(MIN), using conserva-
tion of energy and the expected efficiency at that operat-
ing point ( E MIN ) taken from an appropriate curve in the
Typical Operating Characteristics :
the average DC inductor current at the full load current.
The controller operates in DCM mode when LIR is higher
than 2.0, and it works in CCM mode when LIR is lower
I IN(DC,MAX) =
I OUT(MAX) O V OUT
V S(MIN) O E MIN
V S(MIN) O ( V OUT(MAX) -V S(MIN) )
I RIPPLE =
I PEAK = I IN(DC,MAX) + RIPPLE
V S(MIN)
L DCM(MAX) = ? 1- ?
V OUT(MAX) DIODE ? ?
?
V S(MIN) 2 O E
2 O f SW(MAX) OUT(MAX) OUT(MAX)
O V
O I
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 resis-
tance 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
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detail
design procedure for CCM can be described as:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
L O V OUT(MAX) O f SW
I
2
When the 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:
? ?
? + V
O
22
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