Diodes ZXLD1321 - Manual (Page 17)

ZXLD1321
Issue 1 - January 2008 17 www.zetex.com
© Zetex Semiconductors plc 2008
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.
Note that in Bootstrap mode, the input current will be 2 or 3 times larger than the LED current and
the duty cycle will be such that TON is larger than TOFF, due to the fact that the supply voltage
charging the coil is low. Because of this, large LED currents can not be programmed at very low
supply voltages, as the transistor current would need to exceed 2 Amps.
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 = 5k
Ω
and using a 103KT1608 thermistor, the thermistor will reach
869
Ω
at 105°C giving TADJ = 74mV which will start to reduce the LED power above 105°C. By
125°C the thermistor will reach 547
Ω
giving 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.
Reference Part No Value Manufacturer Contact Details
U1 ZXLD1321 LED Driver Zetex www.zetex.com
D1 ZXCS2000 Schottky diode Zetex
L1 MSS7341-103ML 10µH 2A Coilcraft www.coilcraft.co
m
L1 NPIS64D100MTRF 10µH 2A NIC www.niccomp
L1 744 777910 10µH 2A Wurth www.wurth.co.uk
C1 Generic 10nF 10V Generic
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 Generic 0603
R3 Generic 100m
Ω
Generic 0805
R4 Generic 5.1k Ω Generic 0603
R5 Thermistor NTC 10k 103kt1608