R5975D datasheet(39/47 Pages) STMICROELECTRONICS

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R5975D

Application information

where the ideal duty cycle D for the boost converter is:

Equation 47

As for positive and inverting buck-boost (see Section 8.6 on page 35 and Section 8.7 on

page 37.) the measured real duty cycle must be used to calculate the switch current level.

The peak current flowing in the embedded switch is:

Equation 48

while its average current is equal to:

Equation 49

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 Table 4 on page 8 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 50

where ISW MAX represents the rated current of the device.

Figure 25 shows a tested circuit to implement a boost current source for high current LED

driving (350 mA). To implement a boost conversion the LED string must be composed of

a minimum device number having a total voltage drop larger than maximum input voltage.

The input voltage can be either DC or AC thanks to the input bridge rectifier. In the case of

a DC voltage source D1, D2, D3, D4, C1, and C2 can be removed from the circuit and 1 F

capacitor value can be used for C5.

VOUT VIN

–

VOUT

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

ISW

ILOAD

–

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

IRIPPLE

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

ILOAD

–

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

VIN



-----------

fSW

---------



ISW

ILOAD

–

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

IOUT MAX ISW MAX 1D

–







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