LED driver explore

Today, a large number of portable electronic products require backlight LED driver solution, which has the following characteristics: DC control, high efficiency, PWM dimming, overvoltage protection, load disconnect, small size and ease of use. This article will explore the characteristics of each LED driver as well as the methods to achieve these characteristics, suggests that the LED High Bay Light availability of each characteristic typical circuit. DC control The LED is driven by the current device, its brightness and the forward current is proportional relationship. There are two ways to control the forward current. The first method is the use of the LED VI curve determine produce the desired forward current to the voltage applied to the LED required.

Its implementations generally use a voltage supply and a ballast resistor. Figure 1 illustrates this approach. As described below, this method has a number of inadequacies. LED forward voltage of any changes will result in the change of the LED current. Rated forward voltage of 3.6V, Figure 1, the LED current of 20 mA. If the voltage becomes 4.0V, which is a specific pressure change caused by temperature or manufacturing variations, the forward current is reduced to 14mA. The forward voltage variation of 11% would lead to greater changes in forward current, which was 30%. In addition, according to the available input voltage, the voltage drop and power consumption of the ballast resistor would be a waste of power and reduce battery life. Figure 1: with ballast resistor voltage power efficiency and forward current mismatch The second method is also preferred that the LED current adjustment method is the use of a constant current source to drive the LEDs. The constant current power supply may eliminate the current change caused by the forward voltage variations. Therefore, the LED may produce a constant brightness, regardless of forward current changes.

 Generating a constant current power supply is very easy. Only need to adjust the voltage through the current sense resistor, without having to adjust the output voltage of the power supply. Figure 2 illustrates this approach. Supply reference voltage and current sense resistor value determines the LED current. Driving a plurality of LED, simply put them in series to achieve a constant current in each LED.LED High Bay Lights IP65 Drive parallel LED ballast that is placed in each LED string, which results in reduced efficiency and current mismatch. Figure 2: drive LED constant current power supply High efficiency The battery life in portable applications is essential. LED driver, if practical, we must offer efficient. LED drive efficiency measurements with typical power efficiency measurements. Definition of the measurement of the efficiency of a typical power supply is the output power divided by input power. For the LED drive, the output power is not a relevant parameter. Is important to produce the required input power value of the expected brightness of the LED. This can be simply determined by the LED power divided by the input power to. Please note: If this is the definition of efficiency, the power dissipation in the current sense resistor power Power Dissipation. By the formula shown in Figure 3, we can see that the smaller current sensing voltage will produce a higher efficiency of the LED drive.

 Figure 4 illustrates the selection of the reference voltage of 0.25V power compared with the selection of the reference voltage of 1V power efficiency of both. Lower current sensing voltage power more effective, regardless of the input voltage or the LED current, as long as the other conditions are the same, the lower reference voltage can improve efficiency and extend battery life. Figure 3: LED driver efficiency shows the importance of the loss of the current sense resistor Figure 4: a low current sensing voltage is more effective PWM dimming Many portable LED applications require brightness adjustment. In applications such as LCD backlighting, dimming function provides brightness and contrast adjustment. LED Bulb Light There are two dimming method: Analog and PWM. Analog dimming, brightness of 50% can be achieved by 50% of the maximum current is applied to the LED. The disadvantage of this method will appear LED color shift and requires the use of an analog control signal, and therefore is generally not high utilization rate. The full current is applied to the LED PWM dimming to less busy. Up to 50% of the 50% duty cycle applied full current brightness.

 To ensure that the human eye to see less than the PWM pulse, the frequency of the PWM signal must be higher than 100Hz. The maximum PWM frequency depends on the power-up response time. In order to provide maximum flexibility and ease of integration, LED drivers should be able to accept up to 50kHz PWM frequency. Overvoltage protection Operating in the constant current mode power supply overvoltage protection function. No matter how much load, constant current power supply can produce a constant output current. If the load resistance increases, the output voltage of the power supply must also increases. This is the power to maintain constant current output. If power is detected to be excessively large load resistance, or the load is disconnected, the output voltage can be increased to exceed the rated voltage range of the IC or other discrete circuit element.

 Constant current LED driver can use a variety of over-voltage protection. One way is to make the zener diode in parallel with the LED. This approach can limit the output voltage in attendance satisfied breakdown voltage and power supply reference voltage. Overvoltage condition, the output voltage will increase the zener breakdown point and start conducting. Output current through the Zener diode, and current sense resistor to ground. Zener diode limits the maximum output of the power continuously produce a constant output current. Overvoltage protection is better to monitor the output voltage and to turn off the power when the pressure cut-off point. If a fault occurs, turn off the power in an overvoltage condition can reduce power consumption and extend battery life. Load disconnect LED drive power of a function that is oftenETL T5 LED Tube overlooked is the load is disconnected. Load disconnect feature LED is disconnected from the power supply when the power failure. This function is critical in the following two situations, namely power and PWM dimming. Shown in Figure 2, in the boost converter during a power outage, the load is still connected through the inductor and catch diode and input voltage. Input voltage is still connected to the LED, even when the power has failed, it will continue to generate a small current. Even a small leakage current will in the long idle period greatly shorten battery life. Load is disconnected when PWM dimming is also very important. PWM idle periods, the power supply has failed, but the output capacitor is still connected to the LED.

 If there is no load disconnect function, the output capacitor will discharge through the LED to turn on the power again until the PWM pulse. Partial discharge the capacitor at the beginning of each PWM cycle, once the power must be at the beginning of each PWM cycle to the output capacitor charging. In each PWM cycle inrush current pulse is generated. Inrush current will reduce the efficiency of the system and the instantaneous voltage on the input bus. If you have load disconnect function, the LED will be disconnected from the circuit, so that a power failure would not exist leakage current, and output capacitors are filled with the cycle of the PWM dimming. Implementation of the load off the circuit is best placed between the LED and current sensing resistor a MOSFET. Placed MOSFET current sensing resistor and ground will produce an additional pressure drop in the output current set point will appear as an error. Easy to use Ease of use is a relative term. In assessing the ease of use of the circuit, not only must consider the complexity of the initial design, but also must be considered in the future and quickly modify the circuit for different input or output requirements of the program needs to be done. In short, the lag controller is very easy to use. Lag controller eliminates the traditional power design necessary the complex frequency compensation function. Frequency compensation for experienced power supply designers is a piece of cake, but for the novice is not so easy. Due optimum compensation varies with the input and output conditions of the type, the conventional power supply design can not be achieved quickly modify for different operating conditions. The hysteretic controller having inherent stability and thus without changing the change in the input / output conditions. Small size The small size is an important characteristic of the portable circuit. The size of the circuit elements by a variety of factors. One factor is the switching frequency.

 The high switching frequency allows the use of small passive components. The modern LED driver for portable applications should be able to switching frequency up to 1MHz. Since the switching frequency does not significantly reduce the circuit size, and higher switching losses will reduce the efficiency and reduce the battery life, it is recommended that the switching frequency is generally not more than 1 MHz. The various functions are integrated into the control IC is one of the most important factors to achieve a compact drive solutions. If all of the above features are achieved through separate components, board space they need beyond the space occupied by the power supply itself. Them integrated into the control IC can greatly reduce the size of the whole drive. The second equally important advantage of functional integration can reduce the total cost of the solution. If the step-by-step implementation of all the expected features, LED drivers will lead to an increase of $ 0.60 to 0.70 per additional individual cost. When integrated into the control IC, these features will only increase IC costs 0.10 to $ 0.15. Practical solutions TPS61042 is an excellent example of modern LED driver control IC. Figure 5 illustrates a block diagram of the TPS61042.

 The block diagram shows a highly integrated control IC. Q1 is a low resistance integrated power FET. This member is a low-resistance helps to achieve high efficiency. LED Candle Lamp0.25V reference voltage can reduce the loss in the current sensing resistor. This IC can be easily applied to the CTRL pin PWM signal up to 50kHz frequency case, the PWM dimming. Q2 integrated load disconnect circuit. As has been integrated, load disconnect circuit can be perfectly synchronized with the PWM dimming frequency. Overvoltage protection function has also been integrated into the IC. Most experienced power supply designers will be seen that the error amplifier compensation circuit is omitted. This function has been the error comparator replaced.

The IC uses lag the control feedback topology, so no compensation and has inherent stability. IC physical dimensions are not shown in the block diagram. All circuits and functions are integrated into QFN package 3mm'3mm the. Figure 6 illustrates a typical LED driver, driving four LED forward current of 20mA, the input voltage range of 1.8V to 6.0V. The entire circuit is determined by the control IC, a small ceramic cap, one inductor, a diode and a current sensing resistor. This compact, highly integrated circuits today's LED driver can achieve a high level of integration. Use control IC and 6 small surface mount passive components to achieve the main power supply and secondary functions, such as: load disconnect, overvoltage protection, PWM dimming.