Today, many portable electronic products require a backlight LED driver solution with the following features: DC control, high efficiency, PWM dimming, overvoltage protection, load disconnection, small size, and ease of use. This article explores each feature and how to implement it, and finally shows a typical circuit with each feature.
DC control
LEDs are current-driven devices whose brightness is proportional to forward current. There are two ways to control the forward current. The first method is to use the LEDV-I curve to determine the voltage that is required to be applied to the LED to produce the desired forward current. The implementation method generally uses a voltage supply and a ballast resistor. Figure 1 illustrates this approach. As described below, this method has several shortcomings. Any change in the LED forward voltage will result in a change in the LED current. If the rated forward voltage is 3.6V, the current of the LED in Figure 1 is 20mA. If the voltage becomes 4.0V, which is a specific pressure change caused by temperature or manufacturing changes, the forward current is reduced to 14 mA. A 11% change in forward voltage results in a larger forward current change of up to 30%. In addition, depending on the available input voltage, the ballast resistor's voltage drop and power dissipation can waste power and reduce battery life.
The second method, which is also the preferred method of LED current regulation, uses a constant current source to drive the LEDs. The constant current supply eliminates current changes caused by forward voltage changes. This produces a constant LED brightness regardless of the forward current. It is easy to generate a constant current power supply. It is only necessary to adjust the voltage across the current sense resistor without adjusting the output voltage of the power supply. Figure 2 illustrates this approach. The power reference voltage and current sense resistor values ​​determine the LED current. When driving multiple LEDs, it is only necessary to connect them in series to achieve a constant current in each LED. Driving a parallel LED requires placing a ballast resistor in each LED string, which can result in reduced efficiency and current mismatch.
high efficiency
Battery life is critical in portable applications. LED drivers must be efficient if they are practical. The efficiency measurement of an LED driver is different from the efficiency measurement of a typical power supply. A typical power efficiency measurement is defined as the output power divided by the input power. For LED drivers, the output power is not a relevant parameter. What is important is the input power value required to produce the desired LED brightness. This can be determined simply by dividing the LED power by the input power. Please note: If efficiency is defined in this way, the power dissipation in the current sense resistor will cause the power supply to dissipate. Using the formula shown in Figure 3, we can see that a smaller current sense voltage produces a higher efficiency LED driver. Figure 4 illustrates the efficiency improvement of a power supply with a 0.25V reference voltage compared to a power supply with a 1V reference voltage. Lower current sense voltage supplies are more efficient, regardless of input voltage or LED current, as long as other conditions are the same, lower reference voltages can increase efficiency and extend battery life.
PWM dimming
Photometric adjustments are required for many portable LED applications. In applications such as LCD backlights, the dimming function provides brightness and contrast adjustment. We can use two dimming methods: analog and PWM. With analog dimming, 50% brightness can be achieved by applying 50% of the maximum current to the LED. The disadvantage of this method is that there is an LED color shift and an analog control signal is required, so the usage rate is generally not high. PWM dimming can be achieved by applying full current to the LED with lower busyness. 50% brightness can be achieved by applying full current at 50% busy. To ensure that the human eye does not see the PWM pulse, the frequency of the PWM signal must be higher than 100 Hz. The maximum PWM frequency depends on the power supply startup and response time. For maximum flexibility and ease of integration, LED drivers should be able to accept PWM frequencies up to 50kHz.
Overvoltage protection
Operating the power supply in constant current mode requires overvoltage protection. The constant current source produces a constant output current regardless of the load. If the load resistance increases, the output voltage of the power supply must also increase. This is how the power supply maintains a constant current output. If the power supply detects an excessive load resistance, or if the load is disconnected, the output voltage can be increased beyond the rated voltage range of the IC or other discrete circuit components. Constant current LED drivers are available in a variety of overvoltage protection methods. One way is to have the Zener diode in parallel with the LED.
This method limits the output voltage to the Zener breakdown voltage and the reference voltage of the power supply. Under overvoltage conditions, the output voltage rises to the Zener breakdown point and begins to conduct. The output current is passed through a Zener diode and then grounded through a current sense resistor. The power supply continuously produces a constant output current when the Zener diode limits the maximum output. A better method of overvoltage protection is to monitor the output voltage and turn off the power when the overvoltage cut-off point is reached. In the event of a fault, turning off the power supply under overvoltage conditions can reduce power consumption and extend battery life.
Load disconnection
One often overlooked feature of LED drive power is the load disconnection. The load disconnect function disconnects the LED from the power supply when the power fails. This feature is critical in two situations, power down and PWM dimming. As shown in Figure 2, during the power-down of the boost converter, the load is still connected to the input voltage through the inductor and the capture diode. Since the input voltage is still connected to the LED, even if the power supply has failed, a small current will continue to be generated. Even small leakage currents can significantly shorten battery life during long idle periods. Load disconnection is also important during PWM dimming. During PWM idle, 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 until the PWM pulse turns the power back on. Since the capacitor is partially discharged at the beginning of each PWM cycle, the primary supply must charge the output capacitor at the beginning of each PWM cycle. Therefore, an inrush current pulse is generated in each PWM cycle. Inrush current reduces system efficiency and produces transient voltages on the input bus. If there is a load disconnect function, the LED will be disconnected from the circuit, so that there will be no leakage current in the event of a power failure, and the output capacitor will be full between PWM dimming cycles. It is best to place a MOSFET between the LED and the current sense resistor when implementing the load disconnect circuit. Placing a MOSFET between the current sense resistor and ground creates an additional voltage drop that shows itself as an error at the output current set point.
Easy to use
Easy to use is relatively speaking. When evaluating the ease of use of a circuit, you must consider not only the complexity of the initial design, but also the work you need to do to make rapid modifications in the future and use the circuit for other programs that have different input or output requirements. In short, the lag controller is very easy to use. The lag controller eliminates the complex frequency compensation features necessary in traditional power supply designs. Although frequency compensation is a piece of cake for experienced power supply designers, it is not so easy for novices. Since the best compensation varies with input and output conditions, conventional power supply designs cannot achieve rapid modification for different operating conditions. The hysteresis controller has inherent stability so that there is no need to change when the output/output conditions change.
Small size
Small size is an important feature of portable circuits. The size of circuit components is affected by a number of factors. One of the factors is the switching frequency. The high switching frequency allows the use of small passive components. Modern LED drivers for portable applications should be able to switch at frequencies up to 1 MHz. Since the switching frequency does not significantly reduce the circuit size, and higher switching loss reduces efficiency and shortens battery life, it is recommended that the switching frequency generally does not exceed 1 MHz. Integrating various functions into the control IC is one of the most important factors in implementing a small drive solution. If all of the above functions are implemented by separate components, they will require more board space than the power supply itself. Integrating them into the control IC can greatly reduce the overall drive size. The second equally important advantage of functional integration is the ability to reduce the total cost of the solution. If implemented step by step, all expected functions in the LED driver will result in an additional $0.60 to $0.70 per additional individual cost. When integrated into the control IC, these features only increase the IC cost by $0.10 to $0.15.
Practical solution
The TPS61042 is an excellent example of modern LED driver control ICs. 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. The low resistance of this part helps achieve extremely high efficiency. The 0.25V reference voltage reduces losses in the current sense resistor. This IC makes it easy to implement PWM dimming by applying a PWM signal to the CTRL pin at frequencies up to 50kHz. Q2 implements an integrated load disconnect circuit. Since it is already integrated, the load disconnect circuit can be perfectly synchronized with the PWM dimming frequency. Overvoltage protection has also been integrated into the IC. Most experienced power supply designers will see that the error amplifier and associated compensation circuitry are omitted. This function has been replaced by an error comparator. The IC operates with a hysteretic control feedback topology, so no compensation is required and inherent stability. The physical size of the IC is not shown in the block diagram. All circuits and functions are integrated into a 3mm x 3mm QFN package.
Figure 6 illustrates a typical LED driver application that drives four LEDs with a forward current of 20mA and an input voltage range of 1.8V to 6.0V. The entire circuit consists of a control IC, two small ceramic caps, an inductor, a diode, and a current sense resistor. This compact, highly integrated circuit illustrates the high level of integration that can be achieved with today's LED drivers. Main power and auxiliary functions such as load disconnection, overvoltage protection, PWM dimming, etc. can be realized with the control IC and 6 small surface mount passive components.
1200 - 2000 Puffs (included)
Shenzhen Zpal Technology Co.,Ltd , https://www.zpal-vape.com