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ZXLD1320
current also passing through the LED(s). At this point the transistor switches off and the coil
current continues to flow in the LED(s) via Schottky diode D1.
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 TADJ 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%.
Bill of materials
Reference
U1
D1
L1
Part No
ZXLD1320
ZXCS2000
MSS7341-103ML
Value
LED Driver
Schottky diode
10μH 2A
Manufacturer
Zetex
Zetex
Coilcraft
Contact Details
www.zetex.com
www.coilcraft.co
m
L1
L1
C1
C2
C3
R1
R2
R3
R4
NPIS64D100MTRF
744 777910
Generic
GRM31CR71H475K
GRM31MR71E225K
Generic
Generic
Generic
Generic
10μH 2A
10μH 2A
10nF 10V
4.7μF 50V
2.2μF 25V
33m Ω
1.5 κΩ
100m Ω
5.1k Ω
NIC
Wurth
Generic0603
Murata 1206
Murata 1206
Generic 0805
Generic 0603
Generic 0805
Generic 0603
www.niccomp
www.wurth.co.uk
www.murata.com
www.murata.com
Issue 1 - January 2008
? Zetex Semiconductors plc 2008
15
www.zetex.com
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