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ZXLD1320
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 a buck LED driver
Used when the input voltage is higher than the LED voltage, this circuit has an ON phase, where
the LED(s) and coil are connected in series from the supply to ground and an OFF phase, where
the coil current circulates through the LED via a Schottky diode. Thus current flows in the LED(s)
during the ON phase and during at least part of the OFF phase.
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 = 33m Ω gives a peak coil current of 1.5 Amps.
The internal power transistor turns on until the coil current builds up to the peak value, this
Issue 1 - January 2008
? Zetex Semiconductors plc 2008
14
www.zetex.com
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