Datasheet AL5814Q (Diodes) - 8

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AL5814Q Application Information
VIN VIN
AL5814Q
D1
AL5814Q
D1 VCC VFAULT VCC VFAULT REF OUT R3 REF OUT R3
T
R2 Q1 R2 Q1 R5 VSET SFAULT R5 VSET SFAULT
UC
C1 C1 GND FB GND FB R6 R1 R6 R1
OD
R4 R4
R P Figure 3. Typical Application Circuits W E Output Drive N
Figure 3 shows the typical output drive configuration. The feedback loop regulates the current through the external LEDs. The voltage across the external sense resistor (R1) is fed to the FB pin for sensing. When the voltage exceeds VSET voltage the OUT goes lower, decreasing the drive to the external transistor. The output current can be set as: 𝑉 𝐼 𝑉𝑆𝐸𝑇 𝐿𝐸𝐷 = 𝑅 1 Where I
LED
is the desired LED current, VVSET is determined by R5 and R6 resistor divider, and R
1
is the sense resistor. The power in the sense resistor is calculated as: 𝑃𝑜𝑤𝑒𝑟 = 𝑉𝑉𝑆𝐸𝑇 ∗ 𝐼𝐿𝐸𝐷 Where VVSET = VSET voltage, and ILED is the desired LED string current. For most cases, a standard 1/4W resister works. Similarly, the external transistor’s power dissipation must also 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: 𝑉𝐷𝑆 = 𝑉𝐶𝐶 − 𝑉𝐿𝐸𝐷 − 𝑉𝑉𝑆𝐸𝑇 Then calculate the power dissipation requirement: 𝑃𝑜𝑤𝑒𝑟 = 𝑉𝐷𝑆 ∗ 𝐼𝐿𝐸𝐷 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
The AL5814Q 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 match. AL5814Q 8 of 14 April 2019 Document number: DS41792 Rev. 1 - 2
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© Diodes Incorporated Document Outline Pin Assignments Description Applications Features Package Outline Dimensions Suggested Pad Layout