ALED6000 datasheet(33/45 Pages) STMICROELECTRONICS

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8.3

Floating boost

The floating-boost topology fits the application with an input voltage range always lower than the output voltage,

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

The floating boost (see Figure 30. Floating-boost topology ) requires the same components count as the buck

conversion.

Figure 30. Floating-boost topology

ALED6000

VIN

In this topology the output voltage is referred to VIN and not to GND. The device is supplied by VOUT as a

consequence the maximum allowed voltage drop across the LEDs string is 61 V.

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

given by:

D=VOUT−VIN

VOUT

(35)

However, due conduction losses, the real duty cycle is always higher than the ideal one. The real value should be

used in the following formulas.

The peak current flowing in the embedded switch is:

ISW=IOUT1−D+IRIPPLE2=IOUT1−D+ VIN⋅D

2⋅L⋅fSW

(36)

While its average current level is equal to:

ISW=IOUT1−D

(37)

This is due to the fact that the current flowing through the internal power switch is delivered to the output only

during the OFF phase.

The switch peak current must be lower than the minimum current limit of the overcurrent protection and the

average current must be lower than the rated DC current of the device.

In addition to these constraints, the thermal considerations summarized in Section 6.4 Thermal considerations

must also be evaluated.

ALED6000

Floating boost

DS13018 - Rev 4

page 33/45

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