Most small-size color LCD displays today use white LEDs for backlighting. These systems usually involve handheld devices with an LED drive circuit powered by a battery whose output voltage varies over time. Therefore, an optimum LED driver design requires a system approach covering:
Battery type
LCD characteristics
System power requirements and efficiency
LED driver IC and its external components
PCB layout and component placement
Possible noise generated by the LED driver
RF immunity in cell-phone applications
The most widely used battery nowadays is Li-ion. These batteries start at 4.2 V when fully charged, but drop down to 3.2 V when discharged. Therefore, the driver circuit must operate properly over this input voltage range.
Power requirements affect LED brightness and efficiency. An LED’s light output is proportional to its current, so uniform brightness requires dedicated drive circuitry that controls a constant current for every member of the LED array. This must remain true for exposure to cold temperature or when the battery is low.
In most backlight designs, white LEDs are spaced evenly along one side of the LCD. The number of LEDs is proportional to the dimensions of the LCD. Some LCDs have integrated LEDs already connected either in series or in parallel. Typically, larger-size LCDs require serial topologies, sometimes with multiple strings in parallel. A serial configuration has the benefit of ensuring that all LEDs in a string have the same current; thus, they will exhibit similar brightness throughout the panel. Serial topologies have the advantage of minimizing the number of connections to the LCD. This is usually accomplished with a flex PCB that results in smaller size and lower cost.
LED Configuration and Driver IC Small, white LEDs for backlighting typically exhibit forward voltages of 3.4 V at 20 mA. These LEDs may require a higher voltage than that available from the battery, so their drive voltage must get a boost. There are two ways to increase LED voltage in LCD designs: use a capacitive charge-pump topology or an inductive boost converter. Series topologies often employ inductive boost LED drivers, while parallel LED topologies typically go with fractional charge-pump ICs. The LCD configuration and overall system requirements usually dictate the choice of charge pump or inductive boost converter approach. A charge pump is usually easier to implement and guarantees lower noise performance, whereas the inductive boost converter generally exhibits higher efficiency.
Figure 1 shows a typical circuit for backlighting a two-inch cell-phone LCD incorporating a fractional charge pump. The charge pump is attractive because it requires a small number of external components and minimal pc-board real estate. Meeting this goal today are 3-mm by 3-mm driver ICs and 0402-size capacitors.
The 1X/1.5X fractional charge pump supports two modes of operation with automatic mode selection, depending on the battery input voltage as well as the LED forward voltage. Typically, when the battery voltage exceeds 3.6 V, the driver operates in "1X mode" where the supply is directly connected to the output via a pass transistor. This linear mode exhibits the highest efficiency and lowest noise.
When the battery voltage drops below 3.6 V, the driver transitions from 1X mode to 1.5X mode, boosting the output to 1.5 times the battery input voltage. It uses a switching scheme with two flying capacitors (C1 and C2 in Figure 1) that transfer energy to the load. During the transition, the input current steps up 1.5 times, causing the battery to discharge faster. For example, with three LEDs at 20 mA, the supply current is about 60 mA in 1X mode, and goes up to about 90 mA in 1.5X mode.
Charge-Pump Efficiency Charge-pump efficiency is the ratio of the output-to-input power:
Efficiency = PLED/ PIN = PLED / (VIN x IIN)
where PLED= LED power and PIN = input power. Therefore, for a given LED current and VIN, an input current increase reduces circuit efficiency.
Under normal conditions, most white LEDs have a forward voltage (VF) between 3 and 3.6 V. Lower VF LEDs allow the driver to operate longer in 1X mode, which has better efficiency. Ambient temperature also affects the forward voltage with a coefficient of about -7 mV/°C, causing VF to increase at a lower temperature.
The 1.5X mode guarantees a regulated and accurate current in the LEDs. Any switching noise reflects back onto the supply, but is mostly filtered by the input capacitor. Charge-pump solutions are desirable when low-noise performance is critical, such as in RF communication devices.
There are two methods for adjusting LED current. One uses an external resistor RSET, as shown in Figure 1. The other approach is to control the driver with pulse-width modulation (PWM) that controls LED current and brightness.
Component placement is critical. Therefore, all four capacitors (Fig. 1, again) should be placed close to the LED driver IC. When the driver operates in 1.5X mode, switching current flows through the flying capacitors (C1 and C2). Therefore, the loop areas should be as small as possible to avoid any interference. Because of their low equivalent series resistances (ESRs), use ceramic capacitors (X5R or X7R dielectric material) rather than tantalum types. You can locate these LEDs remotely with no degradation to the LED current, which is dc (making EMI a non-issue).
The driver will amplify noise in the RSET resistor, resulting in LED current noise that flows back into the supply, which leads to potential RF interference in cell-phone applications. The cell phone’s RF section should not be placed close to the RSET resistor. The RSET resistor should be tied directly to the ground plane with a minimum loop area and avoid any current path.
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