Zxld1320 – Diodes ZXLD1320 User Manual
Page 15
ZXLD1320
© Zetex Semiconductors plc 2008
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
Part No
Value
Manufacturer
Contact Details
U1
ZXLD1320
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
Generic0603
C2
GRM31CR71H475K
4.7µF 50V
Murata 1206
www.murata.com
C3
GRM31MR71E225K
2.2µF 25V
Murata 1206
www.murata.com
R1
Generic
33m
Ω
Generic 0805
R2
Generic
1.5
κΩ
Generic 0603
R3
Generic
100m
Ω
Generic 0805
R4
Generic
5.1k
Ω
Generic 0603