Diodes ZXLD1322 - Manual (Page 14)

ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
14 of 17
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April 2010
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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 V
ADJ
. Hence making
R3 = 100m Ω and V
ADJ
= 500mV gives a LED current of 1 Amp because the 500mV V
ADJ
results in 100mV across R3 which
equals 1 Amp. Making V
ADJ
= 10mV gives a LED current of 100mA because the 50mV V
ADJ
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 T
ADJ
pin reaches 50mV. Making R4 = 5k Ω and using a
103KT1608 thermistor, the thermistor will reach 869 Ω at 105°C giving V
TADJ
= 74mV which will start to reduce the LED
power above 105°C. By 125°C the thermistor will reach 547 Ω giving V
TADJ
= 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