Linear matched independent current source and traditional white light LED driver solution

1 Introduction

Early mobile phones used inexpensive color light-emitting diodes (LEDs) for keypad lighting and monochrome liquid crystal display (LCD) backlights. As LED technology has evolved, blue and white LEDs have been used for mobile phone keypad lighting. White LEDs (WLEDs) are blue LEDs with special phosphors added to produce white light. Since WLEDs can enable LCD displays to display a full color spectrum, WLEDs are the dominant backlight source in mobile phones today. In addition to full-color LCD backlights, WLEDs can also be used for keyboard, trackball and control button lighting, camera flashes, and flashlights.

The first generation of WLEDs required high forward voltages (>4.2 V) and forward currents (>20 mA) to achieve the necessary luminosity or brightness for mobile phone applications. These voltages are usually higher than the battery voltage and require a driver IC to boost the LED supply voltage. Since WLEDs require high currents, they are often the main power consumer in mobile phones. To reduce power consumption and increase battery life, more advanced WLED process technologies have been used to reduce production costs and produce cheaper WLEDs that require less current to achieve the desired brightness. Since the forward current of this type of WLED is already small (<10 mA), the forward voltage required for operation is also low.

2 TPS7510x Linear Matching Independent Current Source

The inherent LDO linear regulator topology of the TPS7510x (Figure 1) is superior to traditional constant mode, mixed mode charge pump and inductive boost converter topologies. In many applications, the TPS7510x can replace the above design solutions. Among all the available design solutions, the TPS7510x can reduce or eliminate many external components, significantly reducing costs. This not only limits the material cost, but also reduces other manufacturing costs caused by additional components.

Another benefit of reducing the number of components is that it reduces the size of the solution. Since the TPS7510x does not require external components, the overall solution size is reduced to a size similar to the size of the IC, that is, 1.44 mm2 (WCSP package). The third advantage of the TPS7510x is that almost all of the input current (99%) can drive the LED; no current is consumed by the charge pump capacitor or boost inductor. This power-saving architecture can increase the average efficiency during the battery discharge cycle to 87%. When the battery voltage is rated at 3.6 V, the efficiency of this solution is typically greater than 99%.
The biggest disadvantage of the LDO topology is that the forward voltage of the LED is limited to the input voltage minus the voltage drop (typical value 30 mV, maximum value 100 mV). Since multiple white light LEDs are available today, their forward voltage is usually equal to or less than 3 V and the forward current is equal to or less than the LED current in mobile solutions (3 mA to 10 mA), so this limitation is no longer a major drawback. The second proven limitation of linear solutions is that they can only be used in parallel LED configurations. Standard single-cell lithium-ion battery applications require a large forward voltage when using a series configuration. TPS7510x solutions are limited to parallel LED configurations.

3 Fixed Boost Charge Pumps

In applications that require a fixed boost charge pump (Figure 2), the output voltage is boosted to a fixed value and the LED current is adjusted via individual resistors. This solution is relatively inexpensive due to the use of a low-cost charge pump, but has poor LED current matching and efficiency (average efficiency is 43% over the battery discharge cycle). A key benefit of the charge pump is that it reduces the dependence of the LED forward voltage on the supply voltage (which is usually slightly less than the supply voltage). However, since the charge pump generates a voltage large enough to drive multiple LEDs in series, the efficiency is low and the cost is high, so this solution is limited to parallel configurations.

The TPS7510x provides an efficient step-up solution for these applications, reducing component count and cost, but only when the forward voltage is lower than the supply voltage.

4 Fixed-Mode Charge Pumps

In mixed-mode charge pump applications (Figure 3), the output voltage is regulated to keep the current through each LED constant. The LED current sources of these ICs are well matched, which is determined by the topology of the solution. However, the actual matching is not ideal because the forward voltages are not matched. The efficiency of these circuits is moderate (average efficiency over the battery discharge cycle is 70%), allowing forward voltages greater than the supply voltage.

In these applications, the TPS7510x can significantly reduce cost and improve efficiency. Charge pump circuits usually require one or two switched capacitors and input and output capacitors for stable operation. As mentioned above, using the TPS7510x can reduce component count, solution size, and cost. The disadvantage of linear solutions relative to mixed-mode charge pump solutions is that the LED forward voltage (headroom voltage) is limited. However, the mixed-mode charge pump solution works well together when the LED voltage is the same; the TPS7510x matches well regardless of the LED forward voltage.

5 Inductive boost converters

In the inductive boost converter application (most commonly used to drive series LEDs), as shown in Figure 4, the current flowing through each series LED is equal (ideal current matching). For the charge pump solution, it is allowed to use LEDs with a forward voltage greater than the supply voltage. In some applications, such as the backlight LCD module of a flip phone, there is only one LCD drive line, and the inductive boost converter is the best (sometimes even the only viable) solution.

In some applications, inductive switches can generate electromagnetic interference (EMI), causing new problems. At this time, the efficient, ultra-low noise linear TPS7510x can be used. Inductors, output capacitors or feedback resistors can also be omitted, reducing costs and being cost-effective.

6 Conclusion

With the widespread use of white light LEDs in mobile phones, driving these LEDs is no longer limited to a single high-voltage, high-current solution. The wide application of devices has also promoted the diversification of solutions. The recent advent of brighter and more efficient white light LEDs only requires a smaller current to drive, thus developing a new solution using a linear current source. This solution has the lowest cost, the fewest components, and the smallest size, making it very suitable for mobile phone applications.