Design and Research of Constant Current LDO White LED Driver Chip
Abstract: A design of a white LED constant current driver chip with very low dropout voltage (LDO) is completed. A 0.75V bandgap reference is internally integrated with a temperature-compensated and quadratic proportional resistor divider to provide a constant drive current of 350mA over a 2.7V to 7.0V operating voltage range. When the ambient temperature changes from -10 °C to 100 °C, the drive current changes less than 5.06%; when the power supply voltage has ±10% jump, the drive current changes less than ±0.8%; the minimum dropout voltage can reach 120mV; control circuit work The consumption is less than 1.75mW, and the conversion efficiency of the whole circuit can reach 75%.
At present, the driving methods of the semiconductor light source are two types of constant voltage driving and constant current driving. Since the forward voltage drop of the semiconductor light source exhibits a certain difference due to the process dispersion, the constant voltage drive easily causes the brightness difference between the individual light sources, and the current flowing through the light source easily exceeds the nominal value of the safety application. Additional current limiting resistors are often required for overcurrent protection and brightness adjustment in applications. The constant current driving method can well avoid the difference of forward voltage drop caused by the process dispersion of the semiconductor light source, so that the current of each semiconductor light source is uniform (the brightness is uniform - the constant driving current can also make the semiconductor light source work at the nominal current To extend its service life. Constant current drive peripheral circuit is simple # In most cases, no additional peripheral components are required, which reduces design and manufacturing costs.
1. Circuit structure and working principle
1.1 overall structure
The structure of the circuit is shown in Figure 1. The whole circuit can be divided into three parts, a bandgap reference (control and protection circuit and output adjustment tube. To reduce the power consumption of the modulation control circuit), an integrated output voltage of 0.75 is integrated. V, band gap reference with good power supply rejection ratio and temperature characteristics - The control circuit clamps the voltage across the sampling resistor to ensure that the current flowing through the LED during normal operation is constant. To prevent damage to the circuit caused by short circuit, The internal circuit also integrates a short circuit and overheat protection circuit. When the temperature exceeds the set value, the internal circuit turns off the output current to protect the drive circuit.
Figure 1 circuit structure block diagram
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