Discussion on the LED Driver

DC control


led by the current-driven devices, the brightness is proportional to forward current. There are two ways to control the forward current. The first method is to use LED VI curve to determine the current required to produce the desired positive voltage applied to the LED. The implementations generally use a voltage source and a ballast resistor. Figure 1 illustrates this approach. As described below, this method has several shortcomings. LED forward voltage will result in any change in LED current changes. If the rated forward voltage of 3.6V, the LED current in Figure 1 is 20mA. If the voltage becomes 4.0V, which is caused by temperature changes or create a specific pressure change, then the forward current is reduced to 14mA. Forward Voltage 11% change would result in greater changes in forward current, up to 30%. Also, according to the available input voltage, the ballast resistor voltage drop and power consumption will waste power and reduce battery life.

The second method is the preferred method is to adjust the LED current constant current power supply to drive the LED. Constant current power supply to eliminate the forward voltage changes caused by current changes. Therefore, the LED can produce a constant brightness, regardless of changes in forward current. Generate constant current power supply is easy. Only need to adjust the current sense resistor by the voltage, instead of adjusting the output voltage. Figure 2 illustrates this approach. Power supply reference voltage and current sense resistor value determines the LED current. In the drive multiple LED, just put them in series to achieve in the constant current of each LED. Driver Parallel LED in each LED string need to place a ballast resistor, which results in lower efficiency and the current mismatch.

High efficiency


Battery life in portable applications is essential. LED drive if practical, we must have efficient. Measuring the efficiency LED driver power supply with a typical efficiency measurement are different. Typical power efficiency measurement is defined as output power divided by input power. As for the LED driver, the output power is not the relevant parameters. Important to produce the desired LED brightness value of the required input power. This can be simply divided by the LED power to determine the input power. Please note: If this definition of efficiency, then the current sense resistor in the power consumption will lead to power dissipation. By the formula shown in Figure 3, we can see a smaller current sense voltage will produce high efficiency LED driver. Figure 4 illustrates the selection of 0.25V reference voltage of 1V reference voltage power supply and power use compared to the situation of both efficiency. Low voltage power supply current sensing is more effective, regardless of how the input voltage or LED current, as long as other things being equal, a lower reference voltage can improve efficiency and extend battery life.

PWM dimming


Many portable applications require LED brightness adjustment. In applications such as LCD backlighting, dimming feature provides brightness and contrast adjustment. We can use two dimming methods: analog and PWM. Use analog dimming, 50% imposed by the maximum LED current can achieve 50% brightness. This approach is that there will be LED color shift and the need for analog control signal, so usage is generally not high. Busy degree of a lower full current applied to the LED PWM dimming can be achieved. Degree in applied busy 50% of full current up to 50% brightness. To ensure that the human eye can not see the PWM pulse, PWM signal frequency must be higher than 100Hz. Maximum PWM frequency depends on the power supply startup and response time. To provide maximum flexibility and ease of integration, LED drivers should be able to accept the PWM frequency up to 50kHz.


Overvoltage protection


In constant current mode operation power supply need to adopt over-voltage protection function. No matter how much load, constant current power supply can generate a constant output current. If the load resistance increases, the power supply output voltage must be greater. This is the power source to maintain constant current output. If the power detected by the load resistance is too large, or if the load is disconnected, the output voltage can be increased to beyond the IC or discrete circuit elements rated voltage range. Constant current LED driver can be a variety of over-voltage protection method. One way is to make the zener diode in parallel with the LED. This method can limit the output voltage to breakdown voltage and power in attendance satisfied the reference voltage. In the over-voltage conditions, the output voltage will increase in attendance and began transmission breakdown point is satisfied. Output current through the zener diode will then grounded through the current sense resistor. Zener diode limits the maximum output power can be continuously produced under constant output current. Better approach is to monitor the output voltage over-voltage protection and over-voltage cut-off point to turn off the power. If a fault occurs in the over-voltage conditions, turn off the power to reduce power consumption and extend battery life.


Load Disconnect


LED drive power in an often overlooked feature is the load off. Disconnect the power failure, the load function can be disconnected from the power LED. This capability is critical in the following two situations, namely, power, and PWM dimming. Shown in Figure 2, the boost converter during power outages, the load is still through the inductor and catch diode and input voltage connection. Since the input voltage is still connected with the LED, even when the power has failed and will continue to produce a small current. Even a small leakage current will be very long idle periods in the great battery life. Load disconnect the PWM dimming is also important. In PWM idle period, the power has failed, but the output capacitor and the LED is still connected. If you do not load disconnect function, the output capacitor will discharge through the LED, turn on the power again until the PWM pulse. As the capacitor start of each PWM cycle are part of the discharge, the primary power source must be the beginning of each PWM cycle to the output capacitor charging. Therefore, each PWM cycle will be generated in the inrush current pulse. Inrush current will reduce system efficiency and transient generated in the input bus voltage. If a load disconnect feature, LED will be disconnected from the circuit, so that the power failure will not leak current, and PWM dimming in the loop is filled between the output capacitor. Implementation of the load off the best in the LED circuit and current sensing resistor placed between a MOSFET. In the current sense resistor placed between the MOSFET and the ground will produce an additional pressure drop, and its output current set point will itself appears as a mistake.


Easy to use


Ease of use is a relative term. Ease of use in assessing the nature of the circuit, not only must consider the complexity of the initial design, but also must take into account rapid changes in the future, and the circuit used for other input or output requirements of different program needs to be done. In short, the delay controller is very easy to use. Hysteresis controller can eliminate the traditional complex power supply design in the required frequency compensation. Although the frequency compensation for an experienced power supply designer is a piece of cake, but it is not so easy for a novice. As compensation for the best conditions with the input and output varies, the traditional power supply design can not be achieved rapidly for different operating conditions change. The delay inherent stability of the controller to the input / output without changing the conditions change.


Small size


Small size is an important feature of portable circuit. The size of circuit elements is affected by many factors. One factor is the switching frequency. High switching frequency allows the use of smaller passive components. Modern LED for portable applications should be able to drive up to 1MHz frequency switching. Since the switching frequency does not significantly reduce circuit size, and higher switching losses will reduce efficiency and shorten battery life, it is recommended that no more than switching frequency of 1MHz. To control the various functions into a small driver IC solutions for the realization of one of the most important factor. If all the above features are achieved through the separation of the components, then they need the board space will exceed the space occupied by its own power. To integrate them into the control IC can significantly reduce the overall drive size. Functional integration of the second equally important advantage is to reduce total solution cost. If sub-step, then, LED drivers can cause all the expected functions of the individual costs for each additional 0.60 to 0.70 dollars. When integrated into the control IC, these capabilities will only increase the cost of IC 0.10 to 0.15 dollars.


Practical Solutions


TPS61042 LED driver control of modern IC's excellent example. Figure 5 illustrates a block diagram of TPS61042. Block diagram shows a highly integrated control IC.Q1 is a low resistance integrated power FET. This component helps to achieve low resistance and high efficiency. 0.25V reference voltage can reduce the current sensing resistor in the loss. Up to 50kHz frequency by case PWM signal applied to the CTRL pin, the IC PWM dimming can be easily implemented. Q2 to achieve the integrated load disconnect circuit. As has been integrated, load disconnect circuit with PWM dimming frequency of perfect synchronization. Overvoltage protection has also been integrated into the IC. Most of the experienced staff will see the power supply design is omitted and the associated error amplifier compensation circuit. This feature has been replaced by the error comparator. The IC control using delayed feedback topology work, so no compensation and have the inner stability. IC does not appear in the block diagram of the physical size. All circuits and functions are integrated into 3mm'3mm the QFN package. Figure 6 shows a typical LED driver applications, driven by four LED, forward current of 20mA, the input voltage range of 1.8V ~ 6.0V. The entire circuit is controlled by the IC, 2 small ceramic cap, an inductor, a diode and a current sense resistors. This compact, highly integrated circuit illustrates the use of today's LED driver can achieve a high level of integration. Using the control IC and 6 small surface mount passive components can achieve the main power supply functions and auxiliary functions, such as: load disconnect, overvoltage protection, PWM dimming.