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參數(shù)資料
| 型號： | AN44 |
| 廠商： | Zetex Semiconductor |
| 元件分類： | LED驅(qū)動器 |
| 英文描述： | A high power LED driver for low voltage halogen replacement |
| 中文描述： | 高功率LED低壓鹵素燈更換驅(qū)動程序 |
| 文件頁數(shù)： | 6/6頁 |
| 文件大?。?/td> | 123K |
| 代理商： | AN44 |
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
Streitfeldstrae 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 V
IN
*
I
PEAK
/2. The average input current from the battery is therefore this current multiplied by the ratio
of T
ON
to the total cycle time:
It can be seen from this how the average battery current will increase at lower V
IN
as T
ON
becomes
larger compared to the fixed 1.7μs T
OFF
. 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:
* L1 * I
PEAK2
[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 I
PEAK
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 I
PEAK
/2.
As the battery voltage increases, the T
ON
necessary to reach I
PEAK
will decrease, but the LED
power will be substantially constant and it will just draw a battery current ramping from zero to
I
PEAK
during T
ON
. At higher battery voltages, T
ON
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 V
F
of the LED is 6V and V
F
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 T
ON
period and therefore does not cause a loss, so
the overall percentage loss will depend on the ratio of the T
ON
and T
OFF
periods. For low battery
voltages where T
ON
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
2
----------------
T
×
T
ON
T
OFF
---------------------------------
×
T
ON
I
V
BATT
V
LED
–
(
)
--------------------------------------------
=
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