A5974AD datasheet(39/49 Pages) STMICROELECTRONICS

39 / 49 page

A5974AD

Application information

Doc ID 018762 Rev 1

39/49

Figure 28.

350 mA LED boost current source

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

LEDs and resistor sense is 36 V.

The output voltage is given by:

Equation 42

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

Equation 43

As for positive and inverting buck-boost (see Section 8.6 and Section 8.7) the measured

real duty cycle must be used to calculate the switch current level.

The peak current flowing in the embedded switch is:

Equation 44

while its average current is equal to:

Equation 45

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 for details) while the average current must be lower than the rated

DC current of the device.

OUT

–

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OUT

–

OUT

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

LOAD

–

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RIPPLE

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

LOAD

–

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

⋅

-----------

---------

⋅

LOAD

–

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