Datasheet

The maximum output current in DCM is governed by the
following equation:
( )
( )
2
LIM SW OUT DIODE
OUT_DCM(MAX)
OUT OUT DIODE IN
LI f V V
I
2V V V V
× × ×η× +
=
×× + −
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when
selecting an inductor. These factors affect the converter’s
operating mode, efficiency, maximum output load capability,
transient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output
voltage, and switching frequency determine the
inductor 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 is chosen, the
ripple current and the peak current of the inductor can
be minimized. If a small-size inductor is required, DCM
can be chosen. In DCM, 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 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
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
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
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
IN(MIN) OUT IN(MIN)
TYP
OUT OUT(MAX) SW
V VV
L
V I f LIR

−

η


=




×



The MAX17127 has a minimum inductor-value limitation
for stable operation in CCM operating 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 2V R
L
2 SF f
+ −× ×
=
××
where SF is a scale factor from slope compensation,
and R
S
is the equivalent current-sensing scale factor
(15mΩ typ).
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 appropriate curve in
the Typical Operating Characteristics:
OUT(MAX) OUT
IN(DC,MAX)
IN(MIN) MIN
IV
I
V
×
=
×η
www.maximintegrated.com
Maxim Integrated │ 17
MAX17127 Six-String WLED Driver with
Integrated Step-Up Converter