LED5000 datasheet(35/52 Pages) STMICROELECTRONICS

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LED5000

Application notes - alternative topologies

Doc ID 023951 Rev 1

35/51

Since the maximum operating voltage of the LED5000 is 48 V, according to

Equation 51 the

maximum input voltage of the application is 48-18.7=29.3 V

The output voltage is given by:

Equation 52

where the ideal duty cycle DIDEAL for the buck-boost converter is:

Equation 53

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:

Equation 54

while its average current level is equal to:

Equation 55

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 (see

Section Table 5.: Electrical characteristics for details) while the average

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

As a consequence, the maximum output current is:

Equation 56

where ISW MAX represents the rated current of the device.

The current capability is reduced by the term (1-DREAL) and so, for example, with a duty

cycle of 0.5, and considering an average current through the switch of 3 A, the maximum

output current deliverable to the load is 1.5 A.

The

Figure 24 shows the schematic circuit for an LED current source based on inverting

buck-boost topology. The input voltage ranges from 10 to 26 V and it can drive a string

composed of 10 LEDs with1 A DC.

OUT

–

IDEAL

–

----------------------------

⋅

IDEAL

–

OUT

–

------------------------------

LOAD

REAL

–

---------------------------

RIPPLE

--------------------

LOAD

REAL

–

---------------------------

⋅

-------------

---------

⋅

LOAD

REAL

–

---------------------------

LOAD MAX

SW MAX

REAL

–

()

⋅

≅

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