zxld1320 Zetex Semiconductors plc., zxld1320 Datasheet - Page 15

• Manufacturer Part Number: zxld1320
• Description: Buck Mode Dc-dc Converter For Led Driving With 1.5a Output And Current Control
• Manufacturer: Zetex Semiconductors plc.

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
500mV V
current of 100mA because the 50mV V
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 = 5kΩ and using a 103KT1608 thermistor, the thermistor will reach
869Ω at 105°C giving V
125°C the thermistor will reach 547Ω giving V
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
Issue 1 - January 2008
© Zetex Semiconductors plc 2008
Reference
ADJ
. Hence making R3 = 100mΩ and V
U1
D1
L1
L1
L1
C1
C2
C3
R1
R2
R3
R4
ADJ
results in 100mV across R3 which equals 1 Amp. Making V
GRM31MR71E225K
GRM31CR71H475K
NPIS64D100MTRF
MSS7341-103ML
744 777910
ZXLD1320
ZXCS2000
Part No
Generic
Generic
Generic
Generic
Generic
TADJ
= 74mV which will start to reduce the LED power above 105°C. By
ADJ
ADJ
Schottky diode
results in 10mV drop across R3 which equals 100mA.
LED Driver
4.7µF 50V
2.2µF 25V
= 500mV gives a LED current of 1 Amp because the
10nF 10V
10µH 2A
10µH 2A
10µH 2A
100mΩ
Value
33mΩ
1.5κΩ
5.1kΩ
15
TADJ
= 50mV which gives zero power. This will
Manufacturer
Generic 0805
Generic 0603
Generic 0805
Generic 0603
Generic0603
Murata 1206
Murata 1206
Coilcraft
Wurth
Zetex
Zetex
NIC
ADJ
= 10mV gives a LED
ZXLD1320
www.murata.com
www.murata.com
www.wurth.co.uk
Contact Details
www.coilcraft.co
www.zetex.com
www.zetex.com
www.niccomp
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