ZXLD1321

Issue 1 - January 2008

14

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© Zetex Semiconductors plc 2008

Typical operating conditions

Inductive converters can operate in either CONTINUOUS mode, where current always flows in

the inductor, but rises during the ON period and falls during the OFF period, or DISCONTINUOUS

mode, where the current falls to zero during the OFF period. The mode depends on several

factors, including supply voltage, output (LED) voltage and the choice of peak current and

inductor value. Calculations need to be done to determine which mode the converter will be in.

The circuit should be designed to give slightly more LED current than required under the lowest

supply voltage, so the control loop can regulate the current accurately. If the theoretical LED

current is less than that required, the control loop will not be able to reach the required value. The

calculations will give an idea of the ON and OFF times and hence the operating frequency, but

bear in mind that the control loop will reduce the peak current to achieve the exact programmed

LED current and this will raise the operating frequency. In general, values in the discontinuous

mode are simpler to calculate because the current can go from zero to the theoretical maximum

during the ON period and fall to zero during the OFF period. In continuous mode the current will

start from some value, so the ON time will be lower to reach the theoretical maximum and lower

still when the control loop reduces the peak current below the maximum.

Circuit operation

Operation of boost LED driver

The input voltage must always be lower than the LED voltage. This circuit has an ON phase,

where the coil is connected from the supply to ground and an OFF phase, where the coil current

flows through the LED via a Schottky diode. The current therefore only flows into the LED circuit

during the OFF phase, although the reservoir capacitor C3 should keep current flowing in the

LED(s) continuously.

ADJ is set between 50mV and 500mV to give between 10% and 100% power respectively. Making

R2 = zero gives a base current to the output transistor of 50mA nominal and making R2 = 1.68k

Ω

gives 10mA nominal. The reduced base current will lower supply current and hence improve

efficiency in lower power applications. Making R1 = 25m

Ω gives a peak coil current of 2 Amps.

The internal power transistor turns on until the coil current builds up to the peak value. At this

point the transistor switches off and the coil current continues to flow in the LED(s) via Schottky

diode D1.