A5974D datasheet(38/46 Pages) STMICROELECTRONICS

38 / 46 page

Application information

A5974D

38/46

DocID018774 Rev 2

The device is powered from the output voltage, so the maximum voltage drop across the

LEDs and the resistor sense is 36 V.

The output voltage is given by:

Equation 46

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

Equation 47

As for positive and inverting buck-boost (see Section 8.7 and Section 8.8) the measured

real duty cycle has to 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 6 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 29 shows a tested circuit to implement a boost current source for high current LED

driving (350 mA). To implement a boost conversion the LEDs 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 a DC or AC thanks to the input bridge rectifier. In case of

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

capacitor value can be used for C5.

VOUT

VIN

–

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

VOUT VIN

–

VOUT

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

ISW

ILOAD

–

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

IRIPPLE

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

ILOAD

–

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

VIN



-----------

fSW

---------



ISW

ILOAD

–

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

IOUT MAX ISW MAX 1D

–







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