zxld1322 Zetex Semiconductors plc., zxld1322 Datasheet - Page 15

• Manufacturer Part Number: zxld1322
• Description: Buck/boost Mode Dc-dc Converter For Led Driving With 700ma Output And Current Control
• Manufacturer: Zetex Semiconductors plc.

point the transistor switches off and the coil current continues to flow in the LED(s) via the
Schottky diode D1.
With a buck converter, the LED is in series with the coil, so no coil current can flow until the supply
voltage exceeds the LED forward drop. The circuit will not work if the supply is less than this.
With a boost converter, there is always a path from supply to ground through the coil, Schottky
diode and LED in series, so if the supply voltage is greater than the LED and Schottky forward
drops, unlimited current will flow in the LED. The circuit will not work if the supply is greater than
this. Thus neither circuit will work for both conditions, where the supply could be either higher
or lower than the LED forward drop, for example when using 3 cells to supply it.
Although it looks like a boost circuit, taking the LED cathode to the supply means that no current
can flow in the LED even if the supply is greater than the forward drop. However, because the coil
is still connected straight across the supply during the ON phase, the current can still be
established when the supply is less than the LED forward drop. Hence this circuit will work at
supply voltages above and below the forward LED drop.
This mode is useful for example when using 3 cells and a white LED, where the voltage of 3 fully
charged alkaline cells is more than the LED forward drop, but the voltage of 3 partly discharged
rechargeable NiCd cells is less than the LED forward drop.
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% power.
Bill of materials
Issue 1 - January 2008
© Zetex Semiconductors plc 2008
ADJ
Reference
. Hence making R3 = 100mΩ and V
U1
D1
C1
C2
C3
R1
R2
R3
R4
R5
L1
L1
L1
ADJ
results in 100mV across R3 which equals 1 Amp. Making VADJ = 10mV gives a LED
GRM31MR71E225K
GRM31CR71H475K
NPIS64D100MTRF
MSS7341-103ML
Thermistor NTC
744 777910
ZXCS2000
ZXLD1322
Generic
Generic
Generic
Generic
Generic
Part No
TADJ
= 74mV which will start to reduce the LED power above 105°C. By
ADJ
ADJ
results in 10mV drop across R3 which equals 100mA.
Schottky diode
= 500mV gives a LED current of 1 Amp because the
LED Driver
4.7µF 50V
2.2µF 25V
10nF 10V
10µH 2A
10µH 2A
10µH 2A
100mΩ
25mΩ
1.5kΩ
5.1kΩ
Value
10k
15
TADJ
= 50mV which gives zero power. This will
Manufacturer
Generic 0603
Generic 0805
Generic 0603
Generic 0805
Generic 0603
Murata 1206
Murata 1206
103kt1608
Coilcraft
Wurth
Zetex
Zetex
NIC
ZXLD1322
www.zetex.com
www.coilcraft.com
www.murata.com
www.murata.com
www.wurth.co.uk
www.zetex.com
Contact Details
www.niccomp