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AN44 Datasheet(PDF) 6 Page - Zetex Semiconductors

Part No. AN44
Description  A high power LED driver for low voltage halogen replacement
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Maker  ZETEX [Zetex Semiconductors]
Homepage  http://www.diodes.com/
Logo ZETEX - Zetex Semiconductors

AN44 Datasheet(HTML) 6 Page - Zetex Semiconductors

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AN44
www.zetex.com
6
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
For international sales offices visit www.zetex.com/offices
Zetex products are distributed worldwide. For details, see www.zetex.com/salesnetwork
This publication is issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or
reproduced for any purpose or form part of any order or contact or be regarded as a representation relating to the products or services concerned.
The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service.
Europe
Zetex GmbH
Streitfeldstraße 19
D-81673 München
Germany
Telefon: (49) 89 45 49 49 0
Fax: (49) 89 45 49 49 49
europe.sales@zetex.com
Americas
Zetex Inc
700 Veterans Memorial Highway
Hauppauge, NY 11788
USA
Telephone: (1) 631 360 2222
Fax: (1) 631 360 8222
usa.sales@zetex.com
Asia Pacific
Zetex (Asia Ltd)
3701-04 Metroplaza Tower 1
Hing Fong Road, Kwai Fong
Hong Kong
Telephone: (852) 26100 611
Fax: (852) 24250 494
asia.sales@zetex.com
Corporate Headquarters
Zetex Semiconductors plc
Zetex Technology Park, Chadderton
Oldham, OL9 9LL
United Kingdom
Telephone: (44) 161 622 4444
Fax: (44) 161 622 4446
hq@zetex.com
Useful formulae for calculations
The input power from the battery during TON (assuming discontinuous operation mode) is VIN *
IPEAK/2. The average input current from the battery is therefore this current multiplied by the ratio
of TON to the total cycle time:
It can be seen from this how the average battery current will increase at lower VIN as TON becomes
larger compared to the fixed 1.7µs TOFF. This is logical, as the fixed (approximately) LED power
will require more battery current at lower battery voltage to draw the same power.
The energy which is stored in the inductor equals the energy which is transferred from the
inductor to the LED (assuming discontinuous operation) is:
½ * L1 * IPEAK
2 [Joules]
Therefore, when the input and the output voltage difference are greater, the LED will have more
energy which will be transferred from the inductor to the LED rather than be directly obtained
from the battery. If the inductor size L1 and peak current IPEAK can be calculated such that the
current just reaches zero in 1.7µs, then the power in the LED will not be too dependent on battery
volts, since the average current in the LED will always be approximately IPEAK/2.
As the battery voltage increases, the TON necessary to reach IPEAK will decrease, but the LED
power will be substantially constant and it will just draw a battery current ramping from zero to
IPEAK during TON. At higher battery voltages, TON will have a lower proportional of the total cycle
time, so that the average battery current at higher battery voltage will be less, such that power
(and efficiency) is conserved.
The forward voltage which is across the Schottky diode detracts from the efficiency. For example,
assuming VF of the LED is 6V and VF of the Schottky is 0.3V, the efficiency loss of energy which is
transferred from the inductor is 5%, i.e. the ratio of the Schottky forward drop to the LED forward
drop. The Schottky is not in circuit during the TON period and therefore does not cause a loss, so
the overall percentage loss will depend on the ratio of the TON and TOFF periods. For low battery
voltages where TON is a large proportion of the cycle, the Schottky loss will not be significant. The
Schottky loss will also be less significant at higher LED voltages (more LED's in series) as Schottky
drop becomes a lower percentage of the total voltage.
I
PEAK
2
----------------
T
ON
T
ON
T
OFF
×
---------------------------------
×
T
ON
I
PEAK
L1
×
V
BATT
V
LED
()
---------------------------------------------
=


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