ZXLD1320

Issue 1 - January 2008

15

www.zetex.com

© 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

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

125°C the thermistor will reach 547

Ω 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

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