AL5817

Document number: DS40391 Rev. 3 - 2

8 of 13

www.diodes.com

August 2018

© Diodes Incorporated

AL5817

Application Information

AL5817

VCC

REF

FB

VFAULT

OUT

VSET

GND

ENB

C1

R2

R3

D1

R4

R5

R1

Q1

AL5817

VCC

REF

FB

VFAULT

OUT

VSET

GND

ENB

C1

R2

R3

D1

R4

R5

R1

Q1

Figure 3. Typical Application

Output Drive

Figure 3 shows the typical output drive configuration. The feedback loop regulates the current through the external LEDs. The voltage across the

the external transistor.

The output current can be set as following:

The power in the sense resistor is calculated as:

transistor

’s power dissipation also must be considered to prevent thermal damage to the transistor, which can further damage the LED controller

IC.

Power Consideration of the NMOS or BJT Device

The power rating of the transistor (either BJT or NMOS) used in the typical application circuit is important. A correctly mounted transistor used in a

typical application can dissipate power of up to 2W. To calculate power dissipation, first calculate the voltage drop across the transistor as follows:

Then calculate the power dissipation requirement:

If power dissipation is higher than the transistor package and layout can dissipate then a higher power dissipation transistor must be selected

and/or use a better PCB layout.

Multiple LED Strings in Parallel

AL5817 can drive more than one channel of LED strings. As shown in Figure 4

, the sense voltage of two channels’ (or more) output current can be

implemented by connecting the voltage of one sense resistor to the FB pin.

By utilizing the same type transistors, sense resistors and series base resistors, the currents in all channels will match.