Datasheet AL3353 (Diodes) - 8

AL3353 Function Description
(Cont.) In Figure 2, ΔI(n), ΔI(n+1) and ΔI(n+2) represent the inductor current increments when there is disturbance in the inductor current. And the relationship of the inductor current increment can be calculated with Equation 2. Δ𝐼𝐼(𝑛𝑛+1) = 𝑆𝑆𝑓𝑓−𝑆𝑆𝑒𝑒 ∙ Δ𝐼𝐼 𝑆𝑆 (𝑛𝑛) ……………………………………………………… (2) 𝑛𝑛+𝑆𝑆𝑒𝑒 Where Sn is the slope of the inductor current when rising, Sf is the slope of the inductor current when falling and Se is the slope of control signal. If the control signal has no compensation, the inductor current ramps up till slope reach the control signal, and then ramps down. If the PWM duty is
T
higher than 50%, the distribution will not be converged from duty to duty, and system runs into sub-harmonic oscillation. To avoid sub-harmonic oscillation, a control compensation is needed and (Sf - Se) / (Sn + Se) should always be smaller than 1. For boost applications, the slopes Se, Sf
UC
and Sn can be calculated with the equations below.
OD
S𝑒𝑒 = V𝑆𝑆𝐿𝐿 ∙ 𝑓𝑓𝑠𝑠 …………………………………………………………… (3)
R
S𝑓𝑓 = R𝑠𝑠𝑒𝑒𝑛𝑛 ∙ (𝑉𝑉𝑂𝑂𝐷𝐷𝐷𝐷 − 𝑉𝑉𝐼𝐼𝐼𝐼)/𝐿𝐿 …………………………………………………………… (4)
P
S𝑛𝑛 = 𝑅𝑅𝑠𝑠𝑒𝑒𝑛𝑛 ∙ 𝑉𝑉𝐼𝐼𝐼𝐼/𝐿𝐿 …………………………………………………………… (5)
W
Where Rsen is the current sense resistance and typical value of VSL is 100mV.
NE
From the equations, the factor (Sf - Se) / (Sn + Se) will decrease if Se increases. When the duty cycle is greater than 50%, lower inductance can also result in the factor increase. And for more flexibility, slope compensation amplitude can be increased by adding one external resistor, RSL, in the CS path. Figure 3 shows the circuit setup. The externally generated slope compensation is then added to the internal slope compensation of the AL3353. When using external slope compensation, the equation for Se becomes: S𝑒𝑒 = (V𝑆𝑆𝐿𝐿 + 𝐼𝐼𝑆𝑆 ∙ R𝑆𝑆𝐿𝐿) ∙ 𝑓𝑓𝑠𝑠 …………………………………………………………… (6) Where Is typical value is 50µA. GATE
AL3353
RSL CS RSen Figure 3. Adding External Slope Compensation It is a good design practice to only add as much slope compensation as needed to avoid sub-harmonic oscillation. Additional slope compensation minimizes the influence of the sensed current in the control loop. With very large slope compensation, the control loop characteristics are similar to a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current.
LED Open Protection
AL3353 shuts down the output if the output voltage is higher than the preset threshold level. When LED failure happens, the LED current and the resulted voltage drop on sensing resistor is nearly zero; the converter will run at maximum duty cycle that boosts the output voltage to a very high level. To prevent this from happening, the output voltage is monitored through the OVP pin and the converter is shut down if the output voltage exceeds the preset level. This protection is latched and AL3353 can be reset from VIN or PWM.
LED Cathode Short to GND Protection
When the LED cathode is short to GND, the output voltage will increase to the OVP voltage and IC will turn off the GATE signal. However, in some conditions, if the output power is limited by the OCP protection before the output voltage reaches the OVP voltage, the GATE will never be turned off. In this case, large current goes through both the power MOSFET and Schottky diode, and leads to damages. To enhance safety, the AL3353 monitors the voltage on the COMP pin and latches the GATE signal if the COMP voltage is continuously greater than 3.2V for 37ms.
Diode & Inductor Short Protection
When the Schottky diode or the inductor is short, the current of the power MOSFET increases significantly, which would cause the MOSFET damage if the IC continues working. AL3353 monitors the voltage on the CS pin cycle by cycle. If the CS pin voltage is greater than 1.2V (typical) for continuous 17 cycles, the IC will latch and shut down the GATE output to prevent the MOSFET from permanent damage. AL3353 8 of 15 February 2019 Document number: DS41487 Rev. 2 - 2
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