732

Selecting Inductors
to Drive LEDs
Application Design Example
LED lighting is an exciting and fast growing application. LEDs can provide low cost, reliable lighting for a
wide variety of applications ranging from architectural
and automotive to signage and handheld devices. The
designer faces the challenge of choosing from a vast
array of LED manufacturers and device styles. Equally
challenging can be the selection of components for
the LED driver circuit. Fortunately powerful tools are
available that greatly facilitate the power inductor
selection.
Since LED applications can use LEDs individually as
well as parallel or serial arrays, the driver circuit may
need to be a voltage step-up, step-down, or both. The
Coilcraft DC-DC Converter Inductor Selector tool
can be used to select the inductor for all of these driver
circuit configurations.
Example 1 – SEPIC Converter for LithiumIon Battery, Step-Up/Step-Down
Example 1 demonstrates the selection of coupled inductors for SEPIC converters driving a single white
LED. Table 1 lists a sampling of typical white LEDs with
forward currents of 350 mA.
These devices can be driven from a single Li-ion battery, the typical power source of handheld devices. It is
desired to operate from as wide a voltage range as
possible in order to maximize the battery life per charge.
Fsw =
Vin =
Vo =
Io =
400 kHz
2.7 to 4.2 Vdc
3.42 Vdc
350 mA
The challenge with this design is that when fully charged
or during charging, the Li-ion cell voltage is greater
than the output voltage. Conversely the cell voltage
will be less than the desired output during the end of
the discharge cycle. Therefore, neither a buck nor
boost regulator will be sufficient for this purpose and a
topology must be chosen that allows both voltage
step-up and step-down. The SEPIC topology, using
coupled inductors, is becoming increasingly popular
for this purpose. The Coilcraft DC-DC Converter
Inductor Selector tool can select the inductors for
this application.
Table 1. Typical White LEDs
Company
Part Number
Vf
Io
Color
Avago
ASMT-MW00
3.6
0.35
Cool white
www.avagotech.com
Cree
XRCWHT-L1-0000-00601
3.5
0.35
Cool white
www.cree.com
Everlight
EHP-AX08B/CT01H-P01
3.5
0.35
Cool white
www.everlight.com
Lite-on
LOPL-E011WA
3.8
0.35
white
www.us.liteon.com
Osram
LW-W5SM
3.2
0.35
White
www.osram-os.com
Philips Lumileds
LXK2-PW12-S00
3.42
0.35
White
www.philipslumileds.com
Seoul
Semiconductor
W10190
3.5
0.35
Cool white
www.seoulsemicon.com
Document 732-1
Revised 11/21/08
The inductor selection process begins with choosing
the appropriate circuit topology.
The results shown in Step 4 can be sorted for the
parameters of most interest to the user, whether the
emphasis is power efficiency, physical size, or cost.
Example 2 – Buck Converter
Example 2 demonstrates the use of the Coilcraft DCDC Converter Inductor Selector tool to select the
inductor for a buck regulator to drive a single white
LUXEON® K2 LED from Philips Lumileds. Philips
Lumileds projects that white LUXEON K2 products will
deliver, on average, 70% lumen maintenance at 50,000
hours of operation at a forward current of 1000 mA1, so
let’s examine what it takes to drive the LED at 1000 mA.
The SEPIC topology input screen requests the input/
output specifications, the switching frequency and asks
the user to select the allowed ripple current.
Assuming a switching frequency of 250 kHz and an
input voltage range of 15 to 30 Vdc, drive circuit parameters are:
Fsw =
Vin =
Vo =
Io =
250 kHz
15 to 30 Vdc
3.42 Vdc
1000 mA
This is all the information needed to proceed to the
Coilcraft DC-DC Converter Inductor Selector tool.
The first step is to identify the driver circuit topology.
The required inductance and current ratings for each
winding of the coupled inductor are then displayed in
Step 3.
Step 2 requires input of the operating parameters: Vin,
Vout, Iout, switching frequency and the selection of the
allowed peak-peak ripple current.
From this information, a list of suitable inductor part
numbers is displayed.
LUXEON® is a registered trademark of Philips Lumileds Lighting Company
1
LUXEON K2 Datasheet DS51 (6/08) http://www.philipslumileds.com/
Document 732-2
Revised 11/21/08
Note that the input voltage as well as the output current
and voltage are specified as part of the design requirement. The switching frequency may represent some
design freedom if a driver IC is not yet selected, but
generally the only degree of freedom in selecting the
inductor value is the amount of ripple current to be
allowed. In this case the design is started assuming a
40% peak-peak current ripple.
From these inputs the Coilcraft DC-DC Converter
Inductor Selector tool returns in Step 3, the required
inductance value, Isat and Irms ratings that the desired
inductor must have.
If extra margin in the current rating to prevent inductor
saturation is preferred, sorting by Isat is recommended.
From these results the tool searches and finds all
Coilcraft inductors that meet the inductance, Isat and
Irms criteria.
The Coilcraft DC-DC Converter Inductor Selector
tool features the flexibility to help the user optimize the
inductor selection based on the criteria most important
to that application.
With multiple results, the user may optimize the inductor selection based on criteria specific to the application. The tool allows a quick sort of the results by user
selected parameters. For example, for a handheld
mobile device or backlight display, component height
may be the most important criteria.
Conclusion
LED lighting is a growing and exciting application area
and Coilcraft design tools can guide the designer
quickly and easily to inductors best suited to a variety
of applications.
Document 732-3
Revised 11/21/08
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