ALED6000 datasheet(28/45 Pages) STMICROELECTRONICS

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Application notes – alternative topologies

Thanks to the wide input voltage range and the adjustable compensation network, the ALED6000 device is

suitable to implement boost and buck-boost topologies.

8.1

Inverting buck-boost

The buck-boost topology fits the application with an input voltage range that overlaps the output voltage, VOUT,

which is the voltage drop across the LEDs and the sensing resistor.

The inverting buck-boost (see Figure 22. Inverting buck-boost topology ) requires the same components count as

the buck conversion and it is more efficient than the positive buck-boost, since no additional power MOSFET is

required.

Figure 22. Inverting buck-boost topology

ALED6000

VIN

A current generator based on this topology implies two main application constraints:

•

the output voltage is negative so the LEDs must be reversed

•

the device GND floats with the negative output voltage. The device is supplied between VIN and VOUT (<

0). As a consequence:

VIN+VOUT <VIN,MAX

(27)

VIN,MAX = 61 V is the maximum operating input voltage for the ALED6000, as shown in Table 5. Electrical

characteristics .

In buck-boost topology working in continuous conduction mode (CCM), the HS MOS on-time interval, D, is given

by:

D= VOUT

VIN+VOUT

(28)

However, due to power losses (mainly switching and conduction losses), the real duty cycle is always higher than

this. The real value (which can be measured in the application) should be used in the following formulas.

The peak current flowing in the embedded switch is:

ALED6000

Application notes – alternative topologies

DS13018 - Rev 4

page 28/45

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