Datasheet AL3353 (Diodes) - 11

AL3353 Application Information
(Cont.) the maximum peak inductor current at the lowest input voltage and full load output. The OCP resistor (R3 in Figure 4) sets the desired OCP current with the Equation 12. 𝑅𝑅𝑂𝑂𝑂𝑂𝑃𝑃 = 500𝑚𝑚𝑚𝑚 …………………………………………………………… (12) 𝐼𝐼 𝑂𝑂𝑂𝑂𝑃𝑃
OVP Setting
AL3353 shuts down the output if the output voltage is higher than the preset output threshold voltage. The threshold voltage must be set higher
T
than the nominal output voltage with at least 20% margin to avoid mis-trigger. The threshold voltage is set by a resistor divider (R4 and R5 in Figure 4) with Equation 13.
UC
𝑉𝑉𝑂𝑂𝐷𝐷𝐷𝐷_𝑂𝑂𝑚𝑚𝑃𝑃 = 𝑅𝑅4+𝑅𝑅5 ∙ 2𝑉𝑉 …………………………………………………………… (13) 𝑅𝑅 5
OD R
The output threshold voltage cannot be set to a much too high voltage, since the AL3353 monitors the OVP pin voltage during start up and shuts the system if OVP pin voltage is lower than 150mV.
P W MOSFET Selection
To select a proper power MOSFET, a few key parameters are considered, such as the drain source breakdown voltage (BV
NE
DSS), on-resistance (RDS(ON)) and package type (i.e. TO-220, TO-252, etc.). The MOSFET BVDSS should be higher than the OVP voltage with 20% margin, since the MOSFET stress voltage when OVP is exactly the output voltage. The package of a power MOSFET greatly affects its thermal performance, a TO- 252 (DPAK) package is recommended for typical applications. A low RDS(ON) MOSFET is beneficial for both system efficiency and the MOSFET thermal performance, but lower RDS(ON) usually means higher cost. Taking the peak inductor current and RMS current into account, a trade-off of system efficiency, thermal performance and cost is needed for selection.
Diode Selection
To maximize converter efficiency, the use of a Schottky diode is strongly recommended. In a boost converter, the average forward current of the rectifier diode IF is equal to the output current IOUT. 𝐼𝐼𝐹𝐹 = 𝐼𝐼𝑂𝑂𝐷𝐷𝐷𝐷…………………………………………………………… (14) In practical application diode selection, the forward current rating should be at least 3 times the LED current. The peak repetitive reverse voltage (VRRM) of the diode should be higher than the OVP voltage with at least 20% margin, since the stress voltage when OVP is exactly the output voltage. A low forward drop diode is preferable for its efficiency advantages and low power dissipation of the diode.
Constant Voltage Application
Besides boost converter for LED driver applications, the AL3353 can also be applied in constant output voltage boost applications. Figure 5 shows the typical circuit schematic for constant voltage applications. To make the output voltage constant, a resistor divider (R6 and R7) is added and the FB pin is connected to sense the divided output voltage. The output voltage VOUT is set with Equation 15. 𝑉𝑉𝑂𝑂𝐷𝐷𝐷𝐷 = 𝑅𝑅6+𝑅𝑅7 ∙ 0.4𝑉𝑉…………………………………………………………… (15) 𝑅𝑅 7 L1 F1 VIN VOUT D2 D1 EN R4 R6 C3 PWM VIN PWM R1 C1 GATE OVP GND
3353
GND CMP
AL
Q1 C2 R5 R7 CS FB R2 R3 AL3353 11 of 15 February 2019 Document number: DS41487 Rev. 2 - 2
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