AN44 ZETEX [Zetex Semiconductors], AN44 Datasheet - Page 6

• Manufacturer Part Number: AN44
• Description: A high power LED driver for low voltage halogen replacement
• Manufacturer: ZETEX [Zetex Semiconductors]

AN44
Useful formulae for calculations
The input power from the battery during TON (assuming discontinuous operation mode) is V
I
of T
It can be seen from this how the average battery current will increase at lower V
larger compared to the fixed 1.7µs T
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 * I
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
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
As the battery voltage increases, the T
power will be substantially constant and it will just draw a battery current ramping from zero to
I
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
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
the overall percentage loss will depend on the ratio of the T
voltages where T
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.
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PEAK
PEAK
I
----------------
T
ON
PEAK
ON
/2. The average input current from the battery is therefore this current multiplied by the ratio
during T
2
to the total cycle time:
=
PEAK
×
-------------------------------------------- -
(
-------------------------------- -
T
F
V
ON
of the LED is 6V and V
BATT
I
2
ON
PEAK
T
[Joules]
×
ON
. At higher battery voltages, T
ON
T
–
OFF
is a large proportion of the cycle, the Schottky loss will not be significant. The
×
V
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L1
LED
)
F
of the Schottky is 0.3V, the efficiency loss of energy which is
OFF
. This is logical, as the fixed (approximately) LED power
ON
necessary to reach I
ON
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ON
6
will have a lower proportional of the total cycle
period and therefore does not cause a loss, so
PEAK
ON
and T
PEAK
can be calculated such that the
OFF
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will decrease, but the LED
periods. For low battery
© Zetex Semiconductors plc 2006
PEAK
/2.
IN
Issue 2 - July 2006
as T
ON
becomes
IN
*