AN44 ZETEX [Zetex Semiconductors], AN44 Datasheet - Page 2

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

AN44
So T
voltage drop from V
I
I
the ramps during the T
Off period, T
The T
this for current ramp calculations, the limits are 1.2µs min., 3.2µs max.
In order to minimize the conductive loss and switching loss, T
T
and 310 are operated only up to 200 kHz. Given the fixed T
1.7µs) = 3.3µs minimum. However, this is not an absolute limitation and these devices have been
operated at 2 or 3 times this frequency, but conversion efficiency can suffer under these
conditions.
During T
Schottky diode. The energy stored in the inductor is:
Continuous and discontinuous modes (and average LED current)
If T
will be I
current will be less because part of the cycle is spent with zero LED current. This is called the
‘discontinuous’ operation mode and is shown in Figure 2.
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PEAK
PEAK
OFF
½ x L x I
OFF
. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300
ON
, the current is flowing through the LED and so the average current in the LED is the sum of
OFF
and the same battery voltage V
is exactly the time required for the current to reach zero, the average current in the LED
PEAK
can be calculated, as the voltage across L1 is obtained by subtracting the forward LED
OFF
of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7µs. Note that, if relying on
PEAK
, the energy stored in the inductor will be transferred to the LED, with some loss in the
/2. In practice, the current might reach zero before T
OFF
2 [Joules]
IN
. Therefore, if L1 is smaller, T
ON
ramping up period and the T
IN
. Note that, while the inductor current is ramping up to
Figure 2
2
ON
will be smaller for the same peak current
OFF
OFF
ramping down period.
ON
of 1.7µs, this gives a T
should not be much smaller than
OFF
is complete and the average
© Zetex Semiconductors plc 2006
Issue 2 - July 2006
ON
of (5µs -