ZXLD1322

ZXLD1322

Document number: DS32166 Rev. 3 - 2

13 of 17

www.diodes.com

April 2010

© Diodes Incorporated

A Product Line of

Diodes Incorporated

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