Datasheet AL3353 (Diodes) - 10

AL3353 Application Information
The AL3353 is a highly integrated and cost effective LED controller optimized for LCD monitor and LCD TV backlighting application. It provides a high performance LED backlight solution with low BOM cost. To build a boost converter with AL3353, several configurations are in need. The LED current, OCP and OVP should be set with corresponding resistors, and proper inductor, power MOSFET and Schottky diode should be selected. L1 F1 VIN

D2 D1
T
EN R4 C3
UC
PWM VIN PWM R1 C1 GATE OVP
OD
GND
3353 R
GND CMP
AL
Q1
P
C2 R5 CS FB
W
R2
NE
R3 R6 Figure 4. Typical Circuit Schematic of AL3353 LED Driver Application
LED Current Setting
The LED current is controlled by the feedback resistor (R6 in Figure 4). LED current accuracy is determined by the regulator's feedback threshold accuracy and is independent of the LED's forward voltage variation. So, precision resistors are preferred. 𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿 = 400𝑚𝑚𝑚𝑚 (mA) 𝑅𝑅 …………………………………………………………… (7) 𝐹𝐹𝐹𝐹
Inductor Design
An inductor is the energy storage component in a converter. Designing an inductor means specifying its size, structure, material, inductance, saturation level, dc-resistance (DCR), and core loss. In a boost converter, inductance has a direct impact on the inductor ripple current. The peak- to-peak inductor ripple current can be calculated as: 𝐼𝐼𝑃𝑃−𝑃𝑃 = 𝑚𝑚𝐼𝐼𝐼𝐼∙(𝑚𝑚𝑂𝑂𝑂𝑂𝑂𝑂−𝑚𝑚𝐼𝐼𝐼𝐼) 𝑚𝑚 …………………………………………………………… (8) 𝑂𝑂𝑂𝑂𝑂𝑂∙𝐿𝐿∙𝑓𝑓 where VIN is the input voltage, VOUT is the output voltage, f is the switching frequency, and L is the inductance. The boost converter average inductor current can be calculated with Equation 9, in which η is the converter efficiency. And the inductor current ripple rate is defined with Equation 10. 𝐼𝐼𝐿𝐿 = 𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿∙𝑚𝑚𝑂𝑂𝑂𝑂𝑂𝑂…………………………………………………………… (9) 𝑚𝑚𝐼𝐼𝐼𝐼∙𝜂𝜂 𝛾𝛾 = 𝐼𝐼𝑃𝑃−𝑃𝑃…………………………………………………………… (10) 𝐼𝐼𝐿𝐿 Higher inductance means lower ripple current, lower RMS (Root Mean Square) current, lower voltage ripple on both input and output, and higher efficiency unless the resistive loss of the inductor dominates the overall loss. However, higher inductance also means a bigger inductor size and a slower response to transients. For optimal practical design, the inductor current ripple rate γ is set 0.3 to 0.5, The saturation level is another important parameter in designing inductor. The maximum peak inductor current can be calculated with Equation 11: 𝐼𝐼𝑃𝑃𝑃𝑃 = 𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿∙𝑚𝑚𝑂𝑂𝑂𝑂𝑂𝑂 + 𝐼𝐼𝑃𝑃−𝑃𝑃…………………………………………………………… (11) 𝑚𝑚𝐼𝐼𝐼𝐼∙𝜂𝜂 2 The inductor saturation current should be higher than the inductor peak current with a reasonable margin. Considering the system efficiency, cost requirements and the peak inductor current limitations, an inductor is then designed accordingly.
OCP Setting
To protect the power MOSFET from over current and other anomalies, AL3353 integrates an over current protection (OCP) function. AL3353 senses the CS pin voltage and enters the OCP protection when the threshold voltage is touched. The OCP current should be set 30% higher than AL3353 10 of 15 February 2019 Document number: DS41487 Rev. 2 - 2
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