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ZXLD1322 Datasheet(PDF) 14 Page - Diodes Incorporated |
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ZXLD1322 Datasheet(HTML) 14 Page - Diodes Incorporated |
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14 / 17 page  ZXLD1322 ZXLD1322 Document number: DS32166 Rev. 3 - 2 14 of 17 www.diodes.com April 2010 © Diodes Incorporated A Product Line of Diodes Incorporated the ON phase, the current can still be established when the supply is less than the LED forward drop. Hence this circuit will work at supply voltages above and below the forward LED drop. This mode is useful for example when using 3 cells and a white LED, where the voltage of 3 fully charged alkaline cells is more than the LED forward drop, but the voltage of 3 partly discharged rechargeable NiCd cells is less than the LED forward drop. The LED current is sensed by R3 and the controller varies this until the drop in R3 equals 20% of VADJ. Hence making R3 = 100m Ω and VADJ = 500mV gives a LED current of 1 Amp because the 500mV VADJ results in 100mV across R3 which equals 1 Amp. Making VADJ = 10mV gives a LED current of 100mA because the 50mV VADJ results in 10mV drop across R3 which equals 100mA. The power is controlled by the chip backing off the peak coil current, so it is necessary to calculate the coil inductance and current to guarantee slightly more than 100% LED power, so the circuit can control it effectively. The internal control loop is compensated by C1, which is normally 10nF. If the thermistor (R5) is used, the power will be backed off progressively as the TADJ pin goes low. With the TADJ pin above 75mV, power is 100% and this is reduced to zero when the TADJ pin reaches 50mV. Making R4 = 5kΩ and using a 103KT1608 thermistor, the thermistor will reach 869 Ω at 105°C giving VTADJ = 74mV which will start to reduce the LED power above 105°C. By 125°C the thermistor will reach 547 Ω giving VTADJ = 50mV which gives zero power. This will protect the LED from damage. These temperature values can be set by the customer by using a different thermistor or a different value of R4. If protection is not required, leaving the TADJ pin open circuit will make it float to a high voltage and always give 100% power. Bill of materials Reference Part No Value Manufacturer Contact Details U1 ZXLD1322 LED Driver Zetex www.zetex.com D1 ZHCS2000 Schottky diode Zetex L1 MSS7341-103ML 10µH 2A Coilcraft www.coilcraft.com L1 NPIS64D100MTRF 10µH 2A NIC www.niccomp L1 744 777910 10µH 2A Wurth www.wurth.co.uk C1 Generic 10nF 10V Generic 0603 C2 GRM31CR71H475K 4.7µF 50V Murata 1206 www.murata.com C3 GRM31MR71E225K 2.2µF 25V Murata 1206 www.murata.com R1 Generic 25m Ω Generic 0805 R2 Generic 1.5k Ω Generic 0603 R3 Generic 100m Ω Generic 0805 R4 Generic 5.1k Ω Generic 0603 R5 Thermistor NTC 10k 103kt1608 |
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