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AN44 Datasheet(PDF) 2 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) 2 Page - Zetex Semiconductors

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AN44
www.zetex.com
2
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
So TON can be calculated, as the voltage across L1 is obtained by subtracting the forward LED
voltage drop from VIN. Therefore, if L1 is smaller, TON will be smaller for the same peak current
IPEAK and the same battery voltage VIN. Note that, while the inductor current is ramping up to
IPEAK, the current is flowing through the LED and so the average current in the LED is the sum of
the ramps during the TON ramping up period and the TOFF ramping down period.
Off period, TOFF
The TOFF of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7µs. Note that, if relying on
this for current ramp calculations, the limits are 1.2µs min., 3.2µs max.
In order to minimize the conductive loss and switching loss, TON should not be much smaller than
TOFF. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300
and 310 are operated only up to 200 kHz. Given the fixed TOFF of 1.7µs, this gives a TON of (5µs -
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 TOFF, the energy stored in the inductor will be transferred to the LED, with some loss in the
Schottky diode. The energy stored in the inductor is:
½ x L x IPEAK2 [Joules]
Continuous and discontinuous modes (and average LED current)
If TOFF is exactly the time required for the current to reach zero, the average current in the LED
will be IPEAK/2. In practice, the current might reach zero before TOFF is complete and the average
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.
Figure 2


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