Diodes ZXLD1322 - Manual (Page 13)

ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
13 of 17
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April 2010
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Typical Operating Conditions
Inductive converters can operate in either CONTINUOUS mode, where current always flows ithe 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 buck / boost LED driver
Used when the input voltage can go higher or 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. The important difference is that this circuit has the LED cathode taken to VIN
instead of ground.
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 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