The W LED solution integrates the advantages of a white LED process with innovative energy-saving solutions (eg ambient light-controlled lighting) to enable a wider range of applications and greater efficiencies than ever before. The vast majority of existing W LED driver solutions still work with voltage boost driver solutions; however, with the diversity of WLED applications and technologies, the choice of WLED driver solutions is growing. A linear matching current source is such a new solution. The low-dropout (LDO) linear WLED driver TPS7510x from Texas Instruments can be used to drive WLEDs. This application note compares this new technology to existing boost solutions.
1 background
Early handsets had cheaper color light-emitting diodes (LEDs) for keyboard illumination and black-and-white liquid crystal display (LCD) backlighting. At the turn of the century, advances in LED technology enabled blue-and-white LED lighting for mobile phone keypads. A white LED (WLED) is simply a blue LED with a special coating that produces a white light wavelength. Since WLED can emit a full-color spectrum on an LCD display, WLED is now the dominant lighting color in mobile phones. In addition to full-color LCD backlighting, WLEDs can also be used for keyboard, trackball and control button illumination, camera flash and flash.
The first generation of WLEDs required higher forward voltage (>4.2V) and current (>20mA) to achieve the luminosity or brightness required for mobile applications. These voltages are generally higher than the Battery Power Supply and require a driver IC to boost the LED supply voltage. Because of the high current required by WLEDs, they typically consume most of the battery power in mobile handsets. In order to reduce power consumption and extend battery life, advances in WLED process technology and lower production costs have resulted in cheaper WLEDs that require less current to produce the desired luminosity. At lower currents (<10 mA), these WLEDs can be implemented with lower forward voltages.
2 TPS7510x: A Linearly Matched Independent Current Source
The TPS7510x's LDO linear regulator topology (shown in Figure 1) offers additional advantages over traditional fixed-mode and mixed-mode charge pump and inductive boost converter topologies. For most applications, the TPS7510x is an efficient alternative to the above solutions. The TPS7510x offers even more cost savings than any other alternative solution due to the reduction or elimination of many external components. This not only reduces bill of materials costs, but also reduces the additional production costs required to place more components.
Figure 1 Linear Matching Current Source (TPS7510x)
Another advantage of the reduced number of components is the reduction in solution size. Since the TPS7510x does not require external components, the overall solution size is just down to the size of the IC, which is 1.44mm2 for the IC size in the WCSP package. A third advantage of the TPS7510x is that nearly all of the input current (99%) will be used to drive the LED; current will not be lost on the charge pump capacitor or boost inductor. This energy-efficient architecture increases the average efficiency of battery discharge life by more than 87%. For a battery rated at 3.6V, the solution is typically more than 99% efficient.
One of the biggest disadvantages of the LDO topology is that the forward voltage of the LED is limited to the difference between the input voltage and the voltage drop (typically 30mV, with a maximum of 100mV). Since there are many white LEDs available today, the LED current level (3mA–10mA) used in mobile solutions typically produces forward voltages of typically 3V or less, so this limitation is no longer the solution. A major drawback. Another well-proven shortcoming is the limitations of linear solutions, which can only be applied to parallel LED structures. The series configuration can cause the standard single-cell Li-Ion battery application to impose too high a requirement on the forward voltage. Therefore, the TPS7510x solution can only be used in parallel LED structures.
3 fixed boost charge pump
For applications using a fixed boost charge pump (see Figure 2), the output will be boosted to a fixed voltage while the LED current is regulated to flow through each resistor. Due to the lower cost of the charge pump device, the cost of this method is relatively low, but LED current matching and efficiency may be low (the average efficiency of battery discharge is 43%). An important advantage of the charge pump is that it reduces the dependence of the LED forward voltage on the supply voltage (within a few volt supply rails). Although the charge pump can generate a high enough voltage to drive multiple LEDs in series, it is very inefficient and costly, so this solution is generally limited to parallel LED structures.
Figure 2 Fixed Boost Charge Pump (REG710)
In this type of application, the TPS7510x not only achieves efficiency gains, but also achieves cost savings by reducing the overall number of components, but is limited to forward voltages below the supply voltage.
4 mixed mode charge pump
For applications that use a mixed mode charge pump (shown in Figure 3), the output voltage is regulated to allow a constant current to flow across each LED. Due to the topology used in this type of solution, the LED current sources of these ICs are well matched. However, due to the mismatch of forward voltages, the actual matching degree will be reduced. These circuits are very efficient (average efficiency of 70% when the battery is discharged) and allow the forward voltage to be higher than the input voltage.
Figure 3 Mixed mode charge pump (TPS60231)
In these applications, the TPS7510x has been greatly improved in terms of cost and efficiency. Charge pump circuits typically require one or two switched capacitors, as well as input and output capacitors for stability. As mentioned earlier, the TPS7510x reduces the number of components, which reduces the size of the solution while reducing costs. The disadvantage of a linear solution over the mixed mode charge pump solution is the limitation of the LED forward voltage (headroom voltage). However, the hybrid mode charge pump solution can be well matched to the same LED voltage; the TPS7510x is perfectly matched regardless of the LED forward voltage.
5 inductor boost
For applications that use an inductive boost converter (shown in Figure 4) (usually for driving a series LED string), the current flowing through each LED on the LED string is the same size (ideal current matching) ). In the case of a charge pump solution, an LED with a forward voltage higher than the supply voltage can be used. In applications where only one LED driver line is available (such as a backlit LCD module for a flip phone), an inductive boost converter is generally (and sometimes only optional) the best solution.
Figure 4 Inductor Boost (TPS61061)
Problems can arise in other applications where electromagnetic interference (EMI) is caused by inductive switching. The linear TPS7510x is an efficient and extremely low noise solution. In addition, the cost savings achieved by removing inductors, output capacitors or feedback resistors is more practical.
With the wider use of white LEDs in mobile handsets, driving these LEDs is no longer limited to a high voltage, high current solution. Today, these solutions not only grow as these device applications grow, but also evolve with these applications. The recently introduced higher brightness and higher efficiency LEDs can be driven with lower currents, which opens up new solutions using linear current sources. With the lowest cost, the smallest component count, and the smallest form factor, the solution is ideal for mobile handheld applications. (small soup)
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Non-Rechargeable Battery:
BA-3030/U(LR20/FR20): 1.5V alkaline battery (and lithium iron disulfide);
BA-3042/U(LR14/FR14): 1.5V alkaline battery (and lithium iron disulfide);
BA-3058/U(LR6/FR6): 1.5V alkaline battery (and lithium iron disulfide);
BA-200/U: 6V primary zinc chloride battery;
BA-3200/U: 6V alkaline battery;
BA-5372/U: 6V/500mAh lithium manganese dioxide battery;
BA-5800/U: 6V/7.5Ah lithium sulfur dioxide battery;
BA-5390/U: 15V/30V,10Ah/20Ah lithium manganese dioxide battery;
BA-5590/U: 14V/28V,6.4Ah/12.8Ah lithium sulfur dioxide battery;
BA-3590/U: 15V/30V,7Ah/14Ah alkaline battery;
BA-3791/U: 15V,16Ah alkaline battery;
BA-3386/U: 15V,15Ah alkaline battery
Rechargeable battery:
BB-590/U: 12V/24V,2.4Ah/4.8Ah Ni-Cd battery;
BB-390/U: 12V/24V,4.5Ah/9Ah Ni-MMH battery;
BB-390B/U: 12V/24V,4.5Ah/9Ah Ni-MMH battery, with [LCD" display ;
BB-2590/U: 14.4V/28.8V,7.5Ah/15Ah, lithium ion battery, with [LCD" display;
TLI-9380E: 14.4V/15Ah, lithium ion battery;
BB-2590/U: 14.4V/28.8V,7.5Ah/15Ah, lithium ion battery, with [LCD" display and SMBUS ;
BB-2791: 14.4V/15Ah, lithium ion battery, with [LCD" display and SMBUS ;
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