TLE4309

Datasheet

9

Rev. 1.0, 2007-03-20

Application Note: Thermal Considerations

1)

By describing an example, this section shows how the power dissipation and the needed

thermal resistance can be estimated.

For a typical application circuit as shown in Figure 1, the following parameters shall

apply:

•

Number of LEDs in series: 3

•

LED current desired:

I

•

LED minimum forward voltage:

V

•

LED maximum forward voltage:

V

•

Supply minimum DC voltage:

V

•

Supply maximum DC voltage:

V

•

Maximum ambient temperature:

T

1. Selecting the reference resistor:

The reference resistor

R

the value from the typical performance graph “Output Current versus Reference

Resistor” on Page 7. In order to obtain a LED forward current of

I

shows an reference resistor of

R

2. Verifying the minimum supply voltage:

In order to be able to drive the LEDs with a constant current, the minimum supply voltage

needed is:

V

whereas

V

series,

V

Since the power dissipation is low at a small voltage drop accross the regulator,

V

a minimum supply voltage of 11 V is sufficient for the example given above.

3. Determining the minimum and maximum LED forward current:

The TLE4309 has an accuracy of < 4 %. With a reference resistor of

R

output current

I

1) This information is given as a hint for the implementation of the device only and shall not be regarded as a

description or warranty of a certain functionality, condition or quality of the device.