High power LED driver circuit without external switch

With the new generation of LED to achieve a high power and efficiency, the application of these devices gradually expanded to new areas, such as flashlights or vehicle applications. High power LED and incandescent bulbs and fluorescent tubes are used in environmental lighting. Current source is the best way to power LED. Because most of the energy, including batteries, generators and industrial power supply, more and more like a voltage source rather than like a current source, LED needs to be inserted between the power supply and some electronic circuits. This circuit can be as simple as a series resistor. However, considering the energy efficiency and other factors, the best is the high voltage feed in current source. For the current greater than 0.35A LED, the inductive switching regulator is usually the best choice.

The design example provides a series of switching circuits based on single power supply integrated circuit, mainly in order to improve efficiency and reduce the volume. The circuit designers in order to achieve this goal, to minimize the use of larger components, such as external power transistor, switch, capacitor, current sense resistor, and the density of light continued as far as possible extended illumination range to maintain the normal operation of the circuit.

The circuit in Figure 1, 2, and is suitable for the use of the power supply of three or four alkaline batteries, nickel metal hydride battery (NiMH) or cadmium nickel battery (NiCd). The circuit in Figure 4 and figure 5 can be used for a vehicle whose nominal voltage of the distribution system is 12V, 24V or 42V. The circuit in Figure 4 and 5 can also be used for industrial systems and emergency subsystems and telecommunications applications that include 24V distribution lines for control.

These circuits are designed using the same concept: a fully integrated single core IC switching regulator and Micropower Operational amplifier. Operational amplifier driver 1.25V feedback terminal on IC. Although the node is aimed at the topology of a standard voltage regulator, the op amp matches it with a much smaller current detection voltage and a slightly different current regulator topology. These circuits do not require the use of an external power switch. Since it does not need to smooth the high frequency ripple in the LED current, this design avoids the larger filter capacitor used in the switching regulator. The common feature of all circuits is the ability to select the dimming function by introducing a bias that can be adjusted by the resistor and potentiometer at the input of the operational amplifier. According to the IC, the resistor and potentiometer can be supplied by VD or CVL terminal of the internal voltage regulator.

A high frequency switching regulator is used as the basic LED regulator circuit (Figure 1). It has an input voltage of 3.6V to 6.5V and drives a single LED with a current of up to 1A, and uses a current sense resistor to control the current regulation loop. The circuit in Figure 2 is similar, but it uses the parasitic resistance of the inductor instead of the current sense resistor. Similar to the circuit in Figure 1, it operates with a 3.6V to 6.5V input voltage and 1A current drive LED.

For a single LED circuit in Figure 3, the starting voltage of the MAX1685 defines the minimum input voltage range of 2.7V. The maximum current of the circuit in Figure 1 and Figure 2 are 0.5A and 1A, respectively. Maximum operating voltage or 6.5V. Once the circuit is started, the power supply to the LED input voltage as low as 1.7V. The applications of circuits in Figures 1, 2, and 3 include headlights, flashlights, and any other portable lamps powered by either a three or four alkaline primary battery, a Ni MH / Ni Cd battery, or a single lithium secondary battery powered by a two cell.

The circuits in Figure 4 and 5 operate at voltages up to 8V 50V. If there is a 12V system in which all components are identified, since the IC input power supply terminal VIN has an absolute maximum rated voltage of 76V, this circuit can withstand load unloading. The maximum available current is 1A, as long as the lower limit of the working voltage is increased to 11.5V, the circuit can drive three series connected LED. The two circuit is very similar, but the circuit in Figure 5 uses the inductive resistance as the current sensor. Because the resistivity of copper has a large temperature coefficient, the disadvantage of using inductive resistor is that the output current depends on the temperature. The inductor winding is made of copper, and its DC resistance has a first order temperature coefficient of 3.9parts/1000/ C. As a result, the operating temperature range, the temperature increased by 10 degrees C, the current will be reduced by 4%.

Source: EDN China