ZXLD1321 Ver la hoja de datos (PDF) - Zetex => Diodes

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Fabricante

ZXLD1321

Boost mode DC-DC converter for LED driving with 1A output and current control

Zetex => Diodes 

ZXLD1321

The LED current is sensed by R3 and the controller varies this until the drop in R3 equals 20% of

VADJ. Hence making R3 = 100mΩ and VADJ = 500mV gives a LED current of 1 Amp because the

500mV VADJ results in 100mV across R3 which equals 1 Amp. Making VADJ = 10mV gives a LED

current of 100mA because the 50mV VADJ 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.

Note that in Bootstrap mode, the input current will be 2 or 3 times larger than the LED current and

the duty cycle will be such that TON is larger than TOFF, due to the fact that the supply voltage

charging the coil is low. Because of this, large LED currents can not be programmed at very low

supply voltages, as the transistor current would need to exceed 2 Amps.

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 TADJ = 74mV which will start to reduce the LED power above 105°C. By

125°C the thermistor will reach 547Ω giving 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.

Reference

U1

D1

L1

Part No

ZXLD1321

ZXCS2000

MSS7341-103ML

L1

NPIS64D100MTRF

L1

744 777910

C1

Generic

C2

GRM31CR71H475K

C3

GRM31MR71E225K

R1

Generic

R2

Generic

R3

Generic

R4

Generic

R5

Thermistor NTC

Value

LED Driver

Schottky diode

10µH 2A

10µH 2A

10µH 2A

10nF 10V

4.7µF 50V

2.2µF 25V

25mΩ

100mΩ

5.1kΩ

10k

Manufacturer

Zetex

Zetex

Coilcraft

NIC

Wurth

Generic

Murata 1206

Murata 1206

Generic 0805

Generic 0603

Generic 0805

Generic 0603

103kt1608

Contact Details

www.zetex.com

www.coilcraft.co

m

www.niccomp

www.wurth.co.uk

www.murata.com

www.murata.com

Issue 1 - January 2008

17

© Zetex Semiconductors plc 2008

www.zetex.com

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