LED5000 datasheet(41/52 Pages) STMICROELECTRONICS

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LED5000

Application notes - alternative topologies

Doc ID 023951 Rev 1

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Equation 66

where VOP_MIN is the minimum operating voltage.

The equations for the floating boost are:

Equation 67

The ideal duty cycle DIDEAL for the boost converter is:

Equation 68

As seen for the buck-boost topologies (

Chapter 6.1 and Chapter 6.2), due to power losses

the real duty cycle is always higher than the ideal. The real value (that can be measured in

the application) should be used in the following formulas to estimate the switch current.

The peak current flowing in the embedded switch is:

Equation 69

while its average current level is equal to:

Equation 70

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 depends on the application conditions:

Equation 71

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.

IN_START

OP_MIN

DIODE

5.5V

DIODE

OUT

IDEAL

–

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

IDEAL

OUT

–

OUT

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

LOAD

REAL

–

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

RIPPLE

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

LOAD

REAL

–

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

⋅

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

---------

⋅

LOAD

REAL

–

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

LOAD MAX

SW MAX

REAL

–

()

⋅

≅

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