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DETAILED DESCRIPTION

Input under voltage protection BLL2770 provides an input voltage up to 40V and operates from an input voltage range of 7V to 40V. If VIN drops below 6V, the UVLO circuit inhibits switching. Once VIN rises above 7V, the UVLO clears, and the chip activates. Constant current output BLL2770 presets the IFB voltage to 150mV. An internal compensated loop will provide enough gain and stability to ensure this voltage under the right conditions. When the internal loop works normally, the external output capacitor must be parallel to the LED load. The Output Current can be set by an external resistor using the following equation: IOUT = 150mV RISEN Note that this equation only stands when the junction temperature is lower than the threshold value of thermal regulation, for which please read the following section.

Thermal regulation The junction temperature of the IC is monitored internally. If the junction temperature exceeds the threshold value (typically 140°C), the converter reduces the voltage of IFB according to the temperature rise, thus to reduce the output current accordingly (i.e. when the junction temperature reaches to 145°C, the current will be drop to 75% of the original current; when reaches to 150°C, the current will be 50%). For a 30% current drop will cause a 51% heat drop generated by MOSFETs' RDS(ON) due to P=I2R, and since the DC power loss makes the most contribution to heat generation, normally, this method can stabilize the temperature at between 140°C and 150°C without turning off the output when the power dissipation condition is insufficient (i.e. when the ventilation fan stops). But if external temperature continues to rise, the chip will completely shut down. This is non-latch protection. Once the junction temperature drops, the IFB voltage will rise again and the chip will resume to work.