LED7707 datasheet(29/48 Pages) STMICROELECTRONICS

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LED7707

Application information

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6.2

Thermal considerations

In order to prevent the device from exceeding the thermal shutdown threshold (150 °C), it is

important to estimate the junction temperature through the following equation:

Equation 17

where TA is the ambient temperature, Rth,JA is the equivalent thermal resistance junction to

ambient and PD,tot is the power dissipated by the device.

The Rth,JA measured on the application demonstration board (described in Section 6.5) is

42 °C/W.

The PD,tot has several contributions, listed below.

Conduction losses due to the RDS(on) of the internal power switch, equal to:

Equation 18

where D is defined as:

Equation 19

and DDIM is the duty cycle of the PWM dimming signal.

Switching losses due to the power MOSFET turn on and off, calculated as:

Equation 20

where tr and tf are the power MOSFET rise time and fall time respectively.

Current generators losses. This contribution is strictly related to the LEDs used in

the application. Only the contribution of the leading current generator (“master”

current generator) can be predicted, regardless of the LEDs forward voltage:

Equation 21

where IROW is the current flowing through the row, whereas VIFB is the voltage across the

master current generator (typically 700 mV).

The voltages across the other current generators depend on the spread of the LEDs forward

voltage. The worst case for power dissipation (maximum forward voltage LEDs in the master

row, minimum forward voltage LEDs in all other rows) can be estimated as:

tot

Amb

⋅

DIM

DSon

cond

⋅

OUT

−

DIM

OUT

)

(

⋅

DIM

IFB

ROW

Master

GEN

⋅

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