AL5802LP4 datasheet(7/10 Pages) DIODES

7 / 10 page

AL5802LP4

Document number: DS37441 Rev. 4 - 2

7 of 10

www.diodes.com

February 2016

AL5802LP4

Application Information

The AL5802LP4 is designed for driving low current LEDs with typical LED currents of 10mA to 100mA. It provides a cost effective way of driving

low current LEDs compared with more complex switching regulator solutions. Furthermore, it reduces the PCB board area of the solution as there

is no need for external components like inductors, capacitors and switching diodes.

Figure 12 shows a typical application circuit diagram for driving an LED or string of LEDs. The NPN transistor Q1 measures the LED current by

sensing the voltage across an external resistor REXT. Q1 uses its VBE as reference to set the voltage across REXT and controls the base current into

Q2. Q2 operates in linear mode to regulate the LED current. The LED current is:

ILED = VBE(Q1) / REXT

From this, for any required LED current the necessary external resistor REXT can be calculated from:

REXT = VBE(Q1) / ILED

Two or more AL5802LP4 can be connected in parallel to construct higher current LED strings as shown in Figure 13.

Consideration of the expected linear mode power dissipation must be factored into the design, with respect to the AL5802LP4's thermal resistance.

The maximum voltage across the device can be calculated by taking the maximum supply voltage less the voltage across the LED string.

VCE(Q2) = VCC – VLED – VBE(Q1)

PD = VCE(Q2) * ILED + ( VCC – VBE(Q2) – VBE(Q1))

2 / R1

As the output current of AL5802LP4 increases, it is necessary to provide appropriate thermal relief to the device. The power dissipation supported

by the device is dependent upon the PCB board material, the copper area and the ambient temperature. The maximum dissipation the device can

handle is given by:

PD = (TJ(MAX) - TA) /RθJA

Figure 12 Typical Application Circuit for

Linear Mode Current Sink LED Driver

Figure 13 Application Circuit for Increasing LED Current

AL5802LP4 Datasheet(PDF) 7 Page - Diodes Incorporated