LED7707 datasheet(37/48 Pages) STMICROELECTRONICS

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LED7707

Application information

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6.4.9

Power dissipation estimate

As explained in section 5.2, there are several contributions to the total power dissipation.

Neglecting the power dissipated by the LDO (surely less significant compared with the other

contributions), equation (18), (20), (21) and (22) help to estimate the overall power

dissipation.

Before starting the power dissipation estimate it is important to highlight that the following

calculations are considering the worst case (the actual value of the dissipated power would

require measurements). Therefore the power dissipation is estimated according to the

following assumptions:

Minimum input voltage (10.8 V), which leads to maximum input current (and also D will

have the higher value, see Section 6.4.4);

Maximum RDS(on) of the internal power MOSFET;

LEDs in the row of the leading generator will have the maximum forward voltage,

whereas all other LEDs in the other rows will have the minimum forward voltage.

100 % dimming signal duty cycle is considered.

The conduction and switching losses on the internal power switch can be calculated as:

Equation 41

Equation 42

where tr = tf = 15 ns

The power dissipation related to the current generators is given by:

Equation 43

Equation 44

Equation 45

The junction temperature can be estimated by equation (18) considering TA = 25 °C:

Equation 46

216

DIM

DSon

cond

⋅

233

)

(

DIM

OUT

⋅

DIM

IFB

ROW

Master

GEN

⋅

()

(

)

630

DIM

LEDs

IFB

ROWs

ROW

GEN

⋅

−

⋅

GEN

Master

GEN

cond

tot

≅

tot

Amb

⋅

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