AL3353
Function Description
The AL3353 is a LED driver controller designed for boost converters in a constant frequency mode. It implements a peak current mode control
scheme and an internal trans-conductance amplifier to accurately control the output current over a wide input and load conditions.
The AL3353 has comprehensive protection features such as power MOSFET over current protection (OCP), output over voltage protection (OVP),
Diode & Inductor short protection, LED cathode short to GND protection, IC power supply under voltage lockout (UVLO), LED open protection,
VOUT short protection and over temperature protection (OTP).
Startup
The AL3353 is enabled by applying a voltage of greater than 2.5V to the PWM pin. The PWM pin is pulled down to GND with an on-chip 220kΩ
resistor. When the first rising edge is applied to the PWM pin, the AL3353 will power up immediately, and remains powered up until the PWM input
is lower than 1.3V for at least 20ms (typical), at this time the IC will enter standby mode. At standby mode, the current consumption of the AL3353
will be lower than 130µA (typical). Once AL3353 is enabled, the internal 5V regulator will be activated and consumes less than 4mA.
When PWM is active high, the AL3353 checks the topology connection first. The IC monitors the OVP pin to see if the Schottky diode is connected
or the boost output is short to GND. If the voltage at OVP pin is lower than 150mV, the output will be disabled. The AL3353 will also check other
faults (UVLO, CS high, CMP high, FB high, OCP and OTP), and the boost converter will boost up the output with the internal soft start if no fault
conditions.
UVLO
AL3353 contains an under voltage lock out (UVLO) protection. AL3353 is not turned ON until the power supply VIN has reached 8.3V (typical).
Whenever the input voltage falls below approximately 7.5V (typical), the device is turned off. The UVLO circuit has a hysteresis of 800mV.
LED Current Regulation
The AL3353 senses the FB pin voltage to control the LED current. The error between the sensed voltage and the internal FB reference voltage is
amplified and compared to the CS pin sensing current signal plus the slope compensation to determine the power MOSFET on-time. The error
amplifier sources or sinks current to the COMP pin to adjust the required inductor current responding to the load changes. The slope compensation
signal is added to the current-sense signal to guarantee system stability at high duty cycle.
The error amplifier reference is set by internal reference voltage and the PWM duty cycle at PWM pin. And the average LED current is
approximated by Equation 1:
ꢀꢁꢂꢃ[mA] = 400ꢄꢅ∙ꢃꢆꢇꢈ ……………………………………………………… (1)
ꢉꢊꢋ[Ω]
AL3353 enters the pulse skip mode at light load, to improve efficiency and prevent over charging the output capacitor. AL3353 turns the GATE
signal high for a minimum on-time (typical 600ns), and remains low until another pulse is needed to maintain the boost converter output voltage.
Dimming Control
The LED current is controlled by the external PWM signal with different duty cycles. The AL3353 can support PWM signals with frequency ranging
from 5kHz to 50kHz, ant the PWM signal shall be higher than 2.5V for high logic and be lower than 1.3V for low logic. An internal square wave with
duty cycle same as the external PWM signal is filtered to provide reference voltage of EA input, which determines the FB reference.
Slope Compensation
The AL3353 adopts a peak current mode control scheme. The main advantages of current mode are inherent cycle-by-cycle current limit of the
power MOSFET and simple control loop characteristics. However, current mode control could cause the sub-harmonic oscillation for duty cycles
greater than 50%, leading to system instability. The AL3353 has a build-in slope compensation to avoid the sub-harmonic oscillation.
Input of PWM Comparator Waveform
Se
Sf
Sn
ΔI(n+2)
ΔI(n)
Vsense
ΔI(n+1)
Time
Figure 2 Sub-Harmonic Oscillation at Duty>50% and Compensation Slope to Avoid Sub-Harmonic Oscillation
7 of 15
February 2019
© Diodes Incorporated
AL3353
www.diodes.com
Document number: DS41487 Rev. 2 - 2
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