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AN44 Datasheet(PDF) 2 Page  Zetex Semiconductors 

AN44 Datasheet(HTML) 2 Page  Zetex Semiconductors 
2 / 6 page 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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