SM2253EKD 线性恒流产品系列

应用电路图原理

Function Description

The SM2253EKD is a three-segment high power factor LED linear constant current control chip that operates in a segmented  automatic switching mode. The chip integrates constant power and over-temperature protection functions to improve system application  reliability.

Output current

The SM2253EKD has three current drive ports, and the output current of each port is adjusted by an external resistor R. The output  currents of each switch are turned on as follows: I OUT1  0.36/R , I OUT2  0.42/R , I OUT3  0.5/R , The system output current is  equal to the effective value after the current is superimposed on each port.

Input line voltage compensation function

When the system works normally, when the LED of the OUT3 port is turned on, the voltage of the OUT3 port starts to rise, and the  voltage of the port connected to the VT through the RVT also rises. The chip modulates the output current by detecting the voltage level  of the VT terminal, and the system enters the constant power modulation. The modulation amplitude of the output current is set by the  external VT to the RVT resistor of OUT3. The relationship is as follows:

RVT: line voltage compensation resistor.

Output LED lamp bead drop and lamp bead ratio design

It is recommended that the SM2253EKD OUT1~OUT3 port lamp bead voltage drop ratio is 8:3:3 (using 18V lamp bead, 220Vac  input system as an example), which can make the system get lower THD, better light efficiency and higher power factor. 

System efficiency calculation

The system operating efficiency of the SM2253EKD is calculated as follows:

D1, D2, and D3 are duty cycles of ON1, OUT2, and OUT3, respectively, during the single-wire period. ILED1, ILED2, and ILED3 are constant current output currents when OUT1, OUT2, and OUT3 are turned on, respectively. VLED1, VLED2, and VLED3 are the lamp voltages when OUT1, OUT2, and OUT3 are turned on, respectively.

Heat dissipation measures

The SM2253EKD has internal temperature compensation circuit, to avoid lower current under high temperature, the system uses  excellent heat dissipation process. It guarantees the chip operates in proper temperature range, common heat dissipation measures are  shown below: 

1）The system uses aluminum substrate.  

2）Increase the copper covered area of SM2396EK substrate; 

3）Enlarge heat dissipation base of the lamps.

The SM2253EKD supports chip parallel applications. If the system output power is too high and the chip temperature is high,  multiple SM2253EKD chips can be used in parallel.

Over temperature adjustment

When the interior temperature of the LED lamp is over high, there will be strong light failure and the life span of the LED will be  decreased. The SM2253EKD integrates temperature compensation, when the interior of the chip exceeds 130ºC, the output current will  be decreased automatically to lower down the interior temperature of the LED and improve system reliability.

Typical Application

SM2253EKD projection lamp application（30W）

1. The LED string voltage is recommended to be controlled between 240V and 260V, and the system works optimally;  

2. Adjust operating output current through adjusting R1, R2 and R3; 

3. R4, R5 and R6 are system VT detection resistor. The recommended value is 560K, depending on the constant power effect  of the scheme; 

4. Suggest to keep RV1, FR1 and FR2, to improve system reliability;

5. Suggest to keep C1, D1 and D2, to improve system reliability of resistant high voltage.

SM2253EKD fractional harmonic authentication scheme（30W）

1． The LED string voltage is recommended to be controlled between 240V and 260V, and the system works optimally;  

2． R1 and R2 are recommended to take 750Ω, and adjust according to the actual situation to optimize THD; 

3． Adjust operating output current through adjusting R4, R5 and R6; 

4． Suggest to keep RV1, FR1 and FR2, to improve system reliability;

5． Suggest to keep C1, D1 and D2, to improve system reliability of resistant high voltage.

PCB layout Attention

（1） IC substrate and PCB use solder paste process, to guarantee better touch of IC substrate and PCB. Red glue process is  prohibited on IC substrate. 

（2） Actual system output power is related to heat dissipation of PCB board and lamp shell, actual application power needs to match  heat dissipation condition. 

（3） Laying copper on IC substrate for heat dissipation and improve reliability. Copper laying is shown above, suggested substrate  bonding pad size is 2.5mm*1.8mm. 

（4） Leakage of copper from IC substrate pad must keep at least 0.6mm away from the OUT port.

引脚定义及功能详解

脚位说明