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Application notes - alternative topologies

LED5000

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Doc ID 023951 Rev 1

6.2

Positive buck-boost

Positive buck-boost fits those applications that require a buck-boost topology (i.e. the input

voltage range crosses the output voltage value) and where the inverting buck-boost is not

suitable because of the main constraints for the final application (refer to

Chapter 6.1). As a

consequence the inverting buck-boost is the preferred option because it requires less

components and it has higher efficiency compared to the positive buck-boost topology.

Figure 28.

Positive buck-boost

The positive buck-boost implementation (

Figure 28) requires one more diode and an

external power switch than inverting buck-boost. The device is not floating, referred to GND,

and it is supplied with the input voltage of the application (the input voltage in inverting buck-

boost topology is instead VIN-VOUT, refer to Chapter 6.1 for details). LED5000 does not see

the output voltage during the switching activity so VOUT can be higher than the maximum

input voltage.

The equations for the positive buck-boost are similar to those seen for the inverting.

Equation 58

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

Equation 59

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 60

while its average current level is equal to:

VIN

VREF

ISW

AM13512v1

OUT

IDEAL

–

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

⋅

IDEAL

OUT

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

LOAD

REAL

–

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

RIPPLE

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

LOAD

REAL

–

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

⋅

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

---------

⋅

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