Datasheet AL5822 (Diodes) - 6

AL5822 Functional Descriptions General
The AL5822 is a 100Hz/120Hz LED current ripple suppressor for LED lighting applications. Combined with a pre-stage APFC LED driver, the AL5822 circuitry can efficiently suppress the low frequency (100Hz/120Hz) LED current ripple from the APFC output, thus eliminate the low frequency flickering for various LED lamps.
Operation Principle
The AL5822 drives an external MOSFET, the LED ripple voltage will be applied to the MOSFET drain to source voltage, thus the LED chain’s voltage will be maintain constant without the voltage ripple, in this way, the LED current ripple will be removed. AL5822 will automatically adjust the LED chain cathode voltage to minimum to improve the system efficiency.
T Ripple Suppressor C
The AL5822 senses the LED current through the RS resistor, CC capacitor is a loop compensation capacitor on VC pin. The AL5822 transfers the voltage on CC capacitor to an internal reference voltage; the current control loop adjusts R
U
S voltage to be equal to reference voltage. The relationship between CC capacitor voltage and R
D
S is shown below:
O
VRs= ILED*RS=VVC/10
R P
In order to remove the LED ripple current, the CC capacitance should be big enough, however, too big capacitance will lead to poor system dynamic response.
W E Adaptive Adjustment N
The AL5822 monitors the external MOSFET status to dynamically adjust the voltage on CC capacitor. When the MOSFET is working on the saturation region, the system efficiency will be low, the AL5822 detects this and charge CC capacitor, and then increase VVC and ILED, the gate driver voltage (VG) will decrease accordingly, thus the Drain to Source voltage of the MOSFET will decrease. On the contrary, if the MOSFET is working on the linear region, the LED current control will be an open loop control. The AL5822 will discharge the CC capacitor to decrease VVC and ILED, then the VG voltage will increase, accordingly, the Drain to Source voltage of the MOSFET will increase. When in balance, the AL5822 controls the Drain to Source voltage to a certain level to suppress the LED current ripple in an efficient way. ILED Linear Region Saturation Region Open Loop Control Close Loop Control CC Discharge CC Charge VLED Figure 7. AL5822 Operating Control
MOSFET Drain Voltage Limitation
If the pre-stage output voltage ripple is very large, the external MOSFET Drain to Source voltage will be possibly large accordingly. LEDS R VLMT LMT1 AL5822 Q1 VG RLMT2 Rs Figure 8. Pin VLMT AL5822 6 of 10 January 2019 Document number: DS41708 Rev. 1 - 2
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