TLE4309 Datasheet, PDF (9/12 Pages) Infineon Technologies AG – Adjustable Linear Low Dropout LED Driver

TLE4309
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_F = 260 mA
• LED minimum forward voltage: V_F,MIN = 3.0 V
• LED maximum forward voltage: V_F,MAX = 3.4 V
• Supply minimum DC voltage: V_I,MIN = 11.0 V
• Supply maximum DC voltage: V_I,MAX = 13.0 V
• Maximum ambient temperature: T_a,max = 85 °C
1. Selecting the reference resistor:
The reference resistor R_REF can be selected by using the formular on page 2 or by taking
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_F = 260 mA, the graph
shows an reference resistor of R_REF = 0.68 Ω.
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_I,MIN = V_F,MAX,total + V_dr + V_REF
whereas V_F,MAX,total is the maximum forward voltage sum of the LEDs connected in
series, V_dr the dropout voltage and V_REF the reference voltage.
Since the power dissipation is low at a small voltage drop accross the regulator,
V_dr = 0.35 V is assumed. The maximum reference voltags is V_REF = 185mV. Therefore,
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_REF = 0.68 Ω, the
output current I_Q will be between 251 mA and 272 mA.
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.
Datasheet
9
Rev. 1.0, 2007-03-20