LED2001PHR datasheet(27/42 Pages) STMICROELECTRONICS

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LED2001

Application information

7.3

Thermal considerations

The dissipated power of the device is tied to three different sources:

•

Conduction losses due to the RDSON, which are equal to:

Equation 33

where D is the duty cycle of the application. Note that the duty cycle is theoretically given by

the ratio between VOUT (nLED

∗V

LED + 100 mV) and VIN, but in practice it is substantially

higher than this value to compensate for the losses in the overall application. For this

reason, the conduction losses related to the RDSON increase compared to an ideal case.

•

Switching losses due to turn-ON and turn-OFF. These are derived using the following

equation:

Equation 34

where TRISE and TFALL represent the switching times of the power element that cause the

switching losses when driving an inductive load (see Figure 17). TSW is the equivalent

switching time.

Figure 17. Switching losses

•

Quiescent current losses.

Equation 35

Example (see Section Example 1):

RDSON_HS

OUT

()

⋅

OFF

RDSON_LS

OUT

()

⋅

–

()

⋅

OUT

RISE

FALL

()

----------------------------------------- FSW VIN

OUT

SW_EQ

⋅⋅

⋅

AM12908v1

⋅

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