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A Product Line of
Diodes Incorporated
ZXLD1322
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 ? 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 ? and using a
103KT1608 thermistor, the thermistor will reach 869 ? 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 ? 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
U1
D1
L1
L1
L1
C1
C2
C3
R1
R2
R3
R4
R5
Part No
ZXLD1322
ZHCS2000
MSS7341-103ML
NPIS64D100MTRF
744 777910
Generic
GRM31CR71H475K
GRM31MR71E225K
Generic
Generic
Generic
Generic
Thermistor NTC
Value
LED Driver
Schottky diode
10μH 2A
10μH 2A
10μH 2A
10nF 10V
4.7μF 50V
2.2μF 25V
25m Ω
1.5k Ω
100m Ω
5.1k Ω
10k
Manufacturer
Zetex
Zetex
Coilcraft
NIC
Wurth
Generic 0603
Murata 1206
Murata 1206
Generic 0805
Generic 0603
Generic 0805
Generic 0603
103kt1608
Contact Details
www.zetex.com
www.coilcraft.com
www.niccomp
www.wurth.co.uk
www.murata.com
www.murata.com
ZXLD1322
Document number: DS32166 Rev. 3 - 2
14 of 17
www.diodes.com
April 2010
? Diodes Incorporated
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