AL5802LP datasheet(6/10 Pages) DIODES

6 / 10 page

AL5802LP

Document number: DS37549 Rev. 4 - 2

6 of 10

www.diodes.com

March 2016

AL5802LP

Typical Performance Characteristics (cont.)

VBIAS (V)

Figure 11 Output Current vs. VBIAS

Application Information

The AL5802LP is designed for driving low current LEDs with typical LED current range of 10mA to 100mA. It provides a cost-effective way for

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 a 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 expressed as follows:

ILED = VBE(Q1) / REXT

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

REXT = VBE(Q1) / ILED

Two or more AL5802LP devices 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 AL5802LP'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 AL5802LP increases, it is necessary to provide appropriate thermal relief to the device. The power dissipation supported

by the device is dependent upon the properties of the PCB board material, the copper pad areas and the ambient temperature. The maximum

dissipation the device can handle is given as follows:

PD = ( TJ(MAX) - TA) /RθJA

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