AL5809 datasheet(8/16 Pages) DIODES

8 / 16 page

AL5809

Document number: DS36625 Rev. 5 - 2

8 of 16

www.diodes.com

December 2016

AL5809

Application Information (cont.)

Power Dissipation

The maximum ambient temperature range of the AL5809 is determined by its power dissipation and thermal impedance of the PCB onto which it

is mounted. Its junction temperature must be kept equal to or less than +125°C.

The power dissipated is determined by the LED current version that has been selected (15, 20, 25 30, 40, 50, 60, 90, 100, 120 or 150mA) and

the difference between the input voltage and LED chain voltage.

VINOUT * ILED

In a typical 12V system, the input voltage can vary between 11.4V and 12.6V. The recommended minimum VINOUT voltage of 2.5V enables the

AL5809 to drive 2 LED in series from the 12V rail (assuming VLED <3.25V).

The AL5809’s power dissipation under minimum input voltage conditions will be:

VINOUT * ILED = (11.4-6.5) * ILED

So for the 20mA AL5809-20PI under these conditions this equals:

4.9V * 20mA = 98mW

Under maximum input conditions (12.6V) the AL5809’s power dissipation will be:

VINOUT * ILED = (12.6-6.5) * ILED

So for the 20mA AL5809-20QPI this equals:

6.1V * 20mA = 122mW

So there is a large difference in power dissipation of the Linear LED driver between minimum and maximum battery voltages. And care must be

taken to calculate expected power dissipations and then determine the suitable PCB material and layout. See Figures 9, 10 and 11 for graphs

showing power dissipation and maximum VINOUT, for different currents and PCB material. Maximizing the area and mass of the ground plane and

additional vias between the pad of the OUT pin will improve the thermal impedance (JA) of the AL5809.

0.00

0.25

0.50

0.75

1.00

1.25

1.50

100

125

Ambient Temperature (°C)

Power Dissipation [W] vs. Ambient Temperature [⁰C]

for

PDI-123,SOD-123 on FR4, and PDI-123 on Getek

SOD on FR4 (

max Tj=145)

PDI on Getek (

max Tj=145)

PDI on FR4 (

max Tj=145)

Figure 9 Power Dissipation vs. Ambient Temperature

@TJ = +145°C

100

125

150

Ambient Temperature [°C]

Maximum VInOut [V] vs. Ambient Temperature [

⁰C]forPDIonFR4

Current Options (referred by Apps board FR4, 2-layer, 0.69"x0.76")

PDI on FR4 15mA

PDI on FR4 20mA

PDI on FR4 25mA

PDI on FR4 50mA

PDI on FR4 90mA

PDI on FR4 150mA

PDI on FR4 40mA

PDI on FR4 60mA

Figure 10 Maximum VInOut vs. Temperature

100

125

Ambient Temperature [°C]

Maximum VInOut [V] vs. Ambient Temperature [

⁰C]forCurrentOptiononPDI

FR4 Board (referred by Apps board FR4, 2-layer, 0.69"x0.76" size)

SOD on FR4 15mA

SOD on FR4 20mA

SOD on FR4 30mA

SOD on FR4 40mA

SOD on FR4 50mA

Figure 11 Maximum VInOut vs. Temperature

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