Rainbow Electronics MAX17127 User Manual
Page 17
______________________________________________________________________________________ 17
MAX17127
Six-String WLED Driver with Integrated
Step-Up Converter
The maximum output current in DCM is governed by the
following equation:
(
)
(
)
2
LIM
SW
OUT
DIODE
OUT_DCM(MAX)
OUT
OUT
DIODE
IN
L I
f
V
V
I
2 V
V
V
V
Ч
Ч
Ч ηЧ
+
=
Ч
Ч
+
−
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when select-
ing an inductor. These factors affect the converter’s
operating mode, efficiency, maximum output load capa-
bility, transient response time, output voltage ripple, and
cost. 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
also require more energy storage and more turns of wire,
which increase 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 best
inductor involves 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 MAX17127
has a fixed internal slope compensation, which requires
a 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 con-
troller can be stable, independent of the internal 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 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
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 detailed
design procedure for CCM can be described as follows.
Calculate the approximate inductor value using the
typical input voltage (V
IN
), 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:
2
IN(MIN)
OUT
IN(MIN)
TYP
OUT
OUT(MAX)
SW
V
V
V
L
V
I
f
LIR
−
η
=
×
The MAX17127 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:
(
)
OUT(MAX)
DIODE
IN(MIN)
S
CCM(MIN)
SW(MIN)
V
V
2 V
R
L
2 SF f
+
− Ч
Ч
=
Ч
Ч
where SF is a scale factor from slope compensation,
and R
S
is the equivalent current-sensing scale factor
(15mI typ).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V
IN(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:
OUT(MAX)
OUT
IN(DC,MAX)
IN(MIN)
MIN
I
V
I
V
Ч
=
Ч η