A low voltage WLED driving scheme using low dropout LDO

White light emitting diodes ( WLEDs ) have been widely used, mainly because WLEDs can be used to provide backlighting for portable electronic product displays. It is generally believed that a single WLED requires a 4-volt driving voltage. Since lithium batteries provide a voltage of 3.6 volts, the industry generally believes that a step-up converter is required to power WLEDs with a single lithium battery. Therefore, many ICs can be used to drive WLEDs, most of which require external inductors or flying capacitors to boost the battery power to a sufficient voltage.

As WLED technology matures, the demand for forward voltage is gradually decreasing. Currently, many LEDs have a general forward voltage (VF) range of 3.2 to 3.5 volts, with a maximum range of 3.7 to 4 volts. Data sheets usually specify this voltage for LED currents of about 15 to 25 milliamperes. This article explores lower current applications and how these applications affect the forward voltage of WLEDs. The article also uses the new LED driver TPS75105 from Texas Instruments (TI) as an example to illustrate how to effectively drive these lower voltage LEDs in a smaller size and at a lower cost .

Identifying the forward voltage of WLED applications through IV curves

WLEDs are similar to other standard pn junction diodes and must have sufficient forward voltage to conduct electricity. When the voltage exceeds a critical value, the forward current increases by the forward voltage of the WLED. Figure 1 shows the general IV curves of two WLEDs.

Figure 1 General WLED IV curve

Interpreting this graph is fairly easy. On a typical diode IV curve, when the voltage exceeds a critical value, the current increases dramatically with the voltage. Figure 1a shows that the typical forward voltage of the device is specified at 3.2 volts, the forward current is 20 mA, and the maximum value reaches 3.7 volts during processing and temperature changes. It can be seen that the application requires a boost DC-DC converter to drive the WLED with the 3-4.2 volt output of a single lithium battery. However, this is not the case in reality. For example, for a 5 mA WLED current application, the curve in Figure 1a shows that the forward voltage required to drive 5 mA is about 2.9 volts, which is much lower than the typical voltage required to drive 20 mA shown in the data sheet. Therefore, only a 3.6 volt lithium battery can be used to drive the 2.9 volt output voltage, without the need for a boost converter.

The specifications for WLEDs cover both typical and maximum values ​​for batch-to-batch process and manufacturing variations. The IV curves provided in the datasheet are generally for components that meet the typical specifications. Although the shape of the curve is valid for each part manufactured, the curve may deviate to the right or left depending on the different test conditions of the individual device. If a different LED with the same part number as in the previous example is used, the forward voltage measured under typical test conditions (20 mA forward current) is 3.7 volts (rated upper limit), which is 0.5 volts higher than the typical device, indicating that a maximum forward voltage of 3.4 volts (2.9 volts plus 0.5 volts) is required to drive this  WLED at 5 mA . Depending on the cutoff voltage of the application, this particular WLED can be driven at 5 mA without the use of a boost converter. This technique can be used to determine the maximum forward voltage for any application. Temperature Changes Affect LED Characteristics

Some applications require WLEDs to operate in harsh environments with extreme temperatures. Temperature changes can affect LED characteristics, but the impact on low and high currents is not very strong. The graph in Figure 2 from a general WLED data sheet shows the relationship between forward voltage and temperature.

Figure 2 Relationship between forward voltage and temperature (Nichia NSSW100CT)

This graph shows that the temperature dependency becomes more pronounced as current and forward voltage increase. In addition, as temperature increases, the forward voltage decreases. The 5 mA curve shows that the forward voltage decreases by approximately 0.1 volt when changing from room temperature (25°C) to the rated upper temperature limit (85°C). This should be taken into account when determining the required forward voltage, but the impact is not significant. If a specific application requires driving the LED in an extremely cold environment , the brightness of the low input voltage will be reduced when the forward voltage increases.

Realizing an extremely small LED driver solution

The general approach to driving multiple WLEDs is to connect them in series and then drive the series string with an inductive boost converter or charge pump, which is an excellent approach for higher WLED currents that require a higher forward voltage. However, as mentioned earlier, not all WLED driver applications require a boost converter, and a simpler and lower-cost driver for low-current WLED applications is the very small TPS75105 LED driver IC.

The TPS75105 is a linear power supply with an extremely low dropout voltage of 28 millivolts, suitable for driving four parallel WLEDs divided into two separate groups. This device provides four 2% matching current paths in the two groups that are individually activated. It uses an extremely small 9-ball 1.5 square millimeter wafer-level square size package (WCSP) and can use the preset current output without any external components, so the volume is reduced to 1.5 square millimeters. Figure 3 shows the application circuit of the TPS75105.

Figure 3 TPS75105 application circuit

At first glance, it seems impractical to use low dropout linear circuits because linear regulators (LDOs) have always had poor performance issues. In fact, there is a general misunderstanding about LDOs. LDO efficiency is entirely based on the input/output voltage ratio, so the efficiency of driving WLEDs is quite high. For example, driving a 3V WLED with an input power of a 3.6V lithium battery can achieve an LED efficiency of 83%. Figure 4 shows the performance of the TPS75105 for various WLED forward voltages within the lithium battery range. The LED efficiency of the TPS75105 is comparable to or even better than other WLED driver solutions.

Figure 4 TPS75105 LED efficiency

Figure 5 shows the TPS7510x LED efficiency on the lithium battery discharge curve. In all three curves, the average efficiency is over 80% over the entire discharge range and reaches 90% at 3.3 volts for the WLED. Since this article focuses on low current applications, the TPS7510x is able to drive 25 mA per LED as long as the input voltage is sufficient. These applications have the advantage of extremely small size.

Figure 5 TPS7510x LED efficiency on lithium battery discharge curve

When evaluating an LED driver application, special consideration must be given to how much current the application requires. If the required current is much lower than the application WLED forward voltage (VF) specification, the WLED datasheet IV curve must be checked to determine the actual VF for the application. This application can use a low dropout linear power supply such as the TPS75105 to reduce size and cost without sacrificing the efficiency of a switching boost converter.