One of the most pervasive challenges in the world today is increasing energy efficiency. The consumer electronics industry is evolving towards higher efficiency due to newer and stricter energy standards as well as consumer awareness. The demand for higher efficiency drives innovative companies to develop technology with smarter power management. One of the fastest growing areas is in display backlighting. Whether it is in mobile phones, MP3 players, portable gaming consoles or GPS systems, the light source behind LCD screens helps bring the colors to life. Powering these screens, like so many engineering challenges, comes in various solutions depending on the specific application. In the portable display backlighting market, a newer and smarter solution will revolutionize the way LCD screens are lit.
Boost WLED Driver
In the portable market, the lithium ion battery is one of the popular power sources used to provide energy. A typical lithium ion battery is fully charged at around 4.2V. It will discharge as current is drawn, but about 80 percent of the battery life is found within the 3.9V to 3.5V range. All it takes is around 3.3V to forward bias a LED so that it generates light.
A boost LED driver can power multiple LEDs simultaneously by boosting a lower input voltage (from the lithium ion battery) to a higher output voltage (across the LEDs). The higher output voltage is used to forward bias a string of LEDs in series. Depending on how much voltage the boost can handle, it can drive multiple LEDs at the same time. For example, if the switching transistor is rated up to 24V, then the boost LED driver can easily forward bias six LEDs in the string (6 × 3.3VLED ≈ 20V). Figure 1 shows a typical boost LED driver with six LEDs in series.
The typical boost LED driver circuit requires an input capacitor (CIN), an output capacitor (COUT), an inductor (L) and a set resistor (RSET) for a total of four external components. As shown in Figure 1, the LED current is set by the voltage at FB divided by the resistor (RSET). The boost LED driver is basically a modified boost regulator with a lower feedback voltage to reduce the power lost through the RSET resistor. Similar to a boost regulator, this topology uses the inductor and the internal power transistor to transfer and deliver energy to the output (OUT). As a result, it also inherits the disadvantages of a boost regulator, such as low efficiency at light loads, switching noise and sometimes audible noise problems caused by the Piezoelectric Effect (noise generated by high voltage alternating across ceramic capacitors). Figure 2 shows a plot of a typical boost LED driver’s efficiency.
As shown in Figure 2, a typical boost LED driver used to drive six LEDs has around 80 percent peak efficiency. This number can vary depending on the inductor used in the application. A larger sized inductor generally offers better efficiency but at the expense of solution size and cost.
At lower LED currents (during dimming), the efficiency diminishes due to switching losses and is often the tradeoff for using a boost LED driver. The low efficiency is potentially wasting power depending upon the application and certainly leaves room for improvements. If a boost LED driver is chosen as the solution, but the application is used in the lower current region (below 20mA) for a majority of the time, then the system will be inefficient. Assuming the system designer is using the boost LED driver at full brightness for a majority of the time, then the efficiency can sustain 80 percent for most of the lithium ion’s battery life, as shown in Figure 2. Finding the right LED driver for a given application can be challenging, but detrimental to precious battery life in a portable system if all parameters are not well considered.
Charge Pump WLED Driver
Another LED driver currently on the market is the charge pump LED driver. The circuit is shown in Figure 3.
The charge pump LED driver drives each individual LED in parallel. A typical charge pump requires an input capacitor (C1), an output capacitor (C2), two charge pump capacitors (Cx, Cy) and a set resistor (RSET), for a total of five external components. One of the benefits of the charge pump over the boost regulator is that it eliminates the large external inductor while using two charge pump capacitors instead.
The other benefit is the efficiency at light loads. Without switching losses at light loads, the charge pump can maintain higher efficiency across various load ranges. When the lithium ion battery voltage is high, the charge pump is simply in bypass mode. In this mode, the input (VIN) connects to the output (OUT) through the charge pumps internal transistors. When the battery voltage is below the forward voltage needed to forward bias the LEDs, the charge pump activates. By charging the capacitors (Cx and Cy) in series and then connecting them in parallel to deliver energy, the voltage seen at the output is increased by 50 percent.
This method of charge pumping allows the LEDs to be fully biased even when the battery voltage is lower than the LED forward voltage. However, it is accomplished by alternating multiple internal switches and non-ideal switches create power loss. For example, for a majority of the time, the charge pump is in bypass mode, which connects the input to the output through the internal switches. As long as the LEDs are on, there will be power loss in the switches, even when the charge pump is not active.
When the charge pump is pumping, it is extremely inefficient due to energy loss in charging and discharging capacitors and in the switches. Since a majority of the lithium ion’s battery life is from 3.9V to 3.5V, pumping the voltage when battery life is almost depleted is futile. The inherent problem with charge pump LED drivers is that they waste power in bypass mode and they are inefficient when they are finally charge pumping. The inefficiencies of one solution in a given application can give rise to another without such flaws.
Linear WLED Driver
The Micrel Linear LED Driver family is designed specifically to drive LEDs in the portable display backlighting market. Figure 4 shows a typical linear LED driver circuit. As shown in Figure 4, the MIC2844A drives the LEDs in parallel straight from the lithium ion battery (VIN) and requires two external components: the input capacitor (C1) and the set resistor (RSET). The D pins (D1 to D6) are low dropout linear drivers that are designed to sink current equal to the value set by RSET. Since the brightness of the LED is dependent upon the LED current, the current matching from pin to pin is designed to be less than 1.5 percent under normal operating conditions. This ensures uniform brightness across the LCD panel.
With the LEDs driven directly from the lithium ion battery, having low dropout at the D pins is essential in extending battery life. As the lithium ion battery discharges, the voltage drops. In order to ensure that the LEDs are fully biased for the longest amount of time, the dropout voltage at each D pin is designed to be under 40mV at 20mA. For example, if the LED forward voltage is 3.3V, then the lithium ion battery can be as low as 3.34V and still fully bias the LED. The efficiency of the linear LED driver can be calculated by the equation:
Efficiency = (VLED X ILED) / (VBATTERY X (ILED + ISUPPLY))
The supply bias current (ISUPPLY = 1.4mA) would be the only energy loss in the system, but it’s necessary in order to bias the internal circuitry. Substituting values into the LED forward voltage (VLED = 3.3V), the LED current (ILED= 20mA) and the lithium ion battery voltage (VBATTERY = 3.34V) and the resulting efficiency of the linear LED driver can top 92%. Figure 5 shows the efficiency of a linear LED driver. Since the majority of the lithium ion’s battery life is from 3.9V to 3.5V, an LED with a forward voltage of 3.3V at 20mA would yield 85 to 92 percent efficiency for the linear LED driver. This is greater efficiency than what a boost LED driver can offer, as was shown in Figure 2. Since the linear LED driver also uses fewer external components, it is an obvious choice given the current circumstances. Both the charge pump and the linear LED driver drive the LEDs in parallel and their efficiencies might seem similar. However, without the losses in internal switching transistors, the linear LED driver is more efficient than the charge pump LED driver, whether it is pumping or not.
If the lithium ion battery is ever lower than 3.3V, most portable devices will have a power saving mode, which lowers the LED current. This, in turn, lowers the LED forward voltage and allows the LEDs to be fully driven, even at lower battery voltages. This mechanism renders the charge pump useless. Since most portable systems are designed to shut down when the battery voltage drops any lower than 3.3V, it is not necessary to charge pump the battery voltage at this stage. The considerations of how most portable electronics operate allow the linear LED driver to rise above all other current solutions in the market.
Technology developments are constantly pushing the envelope in the portable electronics industry. Whenever there is room for improvement, there will be a new solution to replace the old. There are various solutions that will work in the LCD backlighting market. The Micrel linear WLED driver family was designed specifically to target portable applications where efficiency, size and cost are most critical. The linear drivers maintain the highest efficiency (topping 92 percent), the smallest solution size (2mm x 2mm package) and the lowest solution cost, requiring only one resistor and capacitor. And while the linear LED driver wins in those three critical categories, it does not sacrifice flexibility or ease of use.
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