With the wide adoption of color display in portable markets (such as mobile phones, PDAs and ultra small PCs), a white backlight or side light is required for a monochrome LCD lighting. Compared with the common CCFL (cold cathode fluorescent lamp) backlight, led seems to be a good choice for backlight applications because it requires lower power consumption and smaller space. The typical forward voltage of white LED is between 3V and 5V. Because the best choice of power supply for white LED is to select a constant current power supply, and the input voltage range of lithium-ion battery is lower than or equal to the led forward voltage, a new power supply solution is needed.
The main power requirements include high efficiency, small solution size and the possibility of adjusting LED brightness. For portable systems with wireless functions, acceptable EMI performance has become another focus of our attention. When high efficiency is the most concerned standard for us to select power supply, boost converter is an attractive solution, while other common solutions are charging pump converter. In this paper, we discuss two solutions for driving white LED, and discuss their relationship with the main power requirements. Another important design consideration is the control method of adjusting LED brightness, which will not only affect the efficiency of the whole converter, but also lead to the chromaticity conversion of white LED. The following will introduce a simple solution that uses a PWM signal to control its brightness. Another advantage of this solution over other standard solutions is its higher efficiency.
Once the power supply is selected for the white LED, the main requirements for a portable system are efficiency, overall solution size, solution cost and the last but very important EMI (electromagnetic interference) performance. According to different portable systems, the emphasis on these requirements is also different. Efficiency is usually the most important or secondary consideration in key design parameters, so this factor should be carefully considered when selecting power supply. Fig. 1 shows the basic circuit of white LED power supply.
The lithium ion battery has a voltage range of 2.7V ~ 4.2V. The main task of the power supply is to provide a constant current and a typical 3.5V forward voltage for white LED.
Compared with the charging pump solution, the boost converter can achieve higher efficiency
Generally speaking, there are two power topologies for driving white LEDs: charging pump or switched capacitor solution and boost converter. Both solutions provide higher output and input voltages. The main difference between the two is that the conversion gain M = Vout / VIN, which will directly affect the efficiency; Generally speaking, the conversion gain of charging pump solution is fixed. A simple charging pump solution with a fixed conversion gain of 2 usually produces a much higher voltage than the led forward voltage, as shown in equation (1). It will result in an efficiency of only 47%, as shown in equation (2).
Where vchrgpump is the voltage generated inside the charging pump IC, and Vbat is the typical battery voltage of lithium ion battery. The charging pump needs to provide a constant current and an output voltage equivalent to the typical forward voltage of led3.5v. Generally, a charging pump with a fixed conversion gain of 2 will internally generate a higher voltage (1), which will lead to an internal voltage drop (2) that reduces the overall system efficiency. The more advanced charging pump solution overcomes this disadvantage by switching between 1.5 and 1 conversion gain. In this way, the operation between 90% ~ 95% efficiency level can be realized when the battery voltage is slightly higher than the LED voltage, so as to allow the conversion gain with gain value of 1. Equations (3) and (4) show this performance improvement.
When the battery voltage is further reduced, the charging pump needs to be converted to 1.5 gain, resulting in a reduction in efficiency to 60% ~ 70%, as shown in examples (5) and (6).
Figure 2 shows the theoretical and actual efficiency curves of the charging pump solution under different conversion gain M.
The real voltage doubling charging pump with conversion gain of 2 has very low efficiency (as low as 40%) and is not very attractive to portable devices; The charging pump with combined conversion gain (gain of 1.0 and 1.5) shows better effect. The next problem with such a charging pump is the conversion point from gain M = 1.0 to M = 1.5, because the efficiency will drop to 60% after gain conversion. When the efficiency reduction (conversion) occurs where the battery can operate normally most of the time, the overall efficiency will be reduced. Therefore, high efficiency can be achieved when conversion occurs at a low battery voltage close to 3.5V.
However, the conversion point depends on the led forward voltage, LED current, charging pump I2R loss and the voltage drop required by the current sensing circuit. These parameters will move the switching point to a higher battery voltage. Therefore, in the specific system, such a charging pump must be carefully evaluated in order to achieve high efficiency.
The calculated efficiency value shows the best theoretical value of the charging pump solution. In real life, more losses will occur according to different current control methods, which has a great impact on efficiency. In addition to I2R loss, the switching loss and static loss in the device will further reduce the efficiency of the charging pump solution.
These shortcomings can be overcome by using an inductive boost converter with a variable conversion gain m, as shown in equation (7) and figure 3.
The duty cycle D of the boost converter can vary between 0% and about 85%, as shown in Fig. 3.
The variable conversion gain can achieve a voltage just matching the led forward voltage, thus avoiding the internal voltage drop and achieving an efficiency of up to 85%.
Standard boost converter capable of driving 4 white LEDs
The boost converter in Fig. 4 is configured as a current source that can drive 4 white LEDs. The device adjusts the voltage at both ends of the detection resistor rs to 1.233v to obtain a defined LED current.
The boost converter used in this structure will have a voltage drop at both ends of the 1.233v current detection resistor, and the power consumption of the detection resistor will reduce the efficiency of the solution. Therefore, the voltage drop for detecting and adjusting the LED current must be reduced. In addition, the possibility of adjusting LED current and LED brightness is also necessary for many applications. The circuit in Figure 5 implements these two requirements.
In Figure 5, an optional zener diode is added to the circuit to clamp the output voltage to prevent an LED from being disconnected or high impedance. A PWM signal with 3.3V amplitude is applied to the feedback circuit of the converter, and a low-pass filter RF and CF are used to filter the DC part of the PWM signal and establish an analog voltage (vadj) at R2. The analog voltage is increased or decreased by changing the duty cycle of the applied PWM signal, so as to adjust the feedback voltage of the converter, which will increase or decrease the LED current of the converter. By applying an analog voltage higher than the converter feedback voltage (1.233v) at R2, a lower induced voltage can be achieved at both ends of the detection resistor. For a 20maled current, the induced voltage drops from 1.233v to 0.98v (even to 0.49v for a 10maled current).
When a PWM signal with 3.3V amplitude is used, the duty cycle range for controlling LED brightness must be adjusted from 50% to 100% to obtain an analog voltage that is usually higher than 1.233v feedback voltage. At 50% duty cycle, the analog voltage will be 1.65v, resulting in an induced voltage of 20mA and 0.98v. Limiting the duty cycle range to 70% ~ 100% will further reduce the induced voltage. The resulting efficiency curve is shown in Figure 6.
The efficiency also depends on the selected inductance. In this application, a small inductance with a size of 1210 can achieve an efficiency of up to 83%, so that the overall solution size is comparable to a solution requiring two flying capacitor charging pumps with a size of 0603.
Figure 7 shows the LED current as a linear function of PWM duty cycle controlling LED brightness.
The above solution shows the structure of a standard boost converter for driving white LEDs and the possibility of improving efficiency by limiting the PWM duty cycle range and selecting a different current control feedback network. Logically, we’ll talk about a solution that integrates all these features.
Dedicated LED drivers reduce the number of external components
Figure 8 shows a device that integrates the characteristics described above. The LED current can be controlled by directly applying a PWM signal to the CTRL pin.
The current induced voltage is reduced to 250Mv, and the overvoltage protection function is integrated into a small 3mm × In 3mmqfn package. The efficiency curve is shown in Figure 9 and figure 10.
Figure 10 shows that more than 80% efficiency can be achieved in the whole voltage range of lithium ion battery (2.7V ~ 4.2V). In this application, an inductor with a height of only 1.2mm is used (Sumida cmd4d11-4r7, 3.5mm * 5.3mm * 1.2mm).
As can be seen from the efficiency curve in Figure 10, the boost converter can achieve higher efficiency than the charging pump solution in most applications. However, EMI needs to be considered when using boost converter or charging pump in wireless applications.
Since both solutions are switching converters operating at conversion frequencies up to 1MHz and can rise and fall rapidly, special care must be taken no matter which solution (charging pump or boost converter) is used. If the charging pump solution is used, there is no need to use inductance, so there is no problem that the magnetic field will cause EMI. However, the flying capacitor of the charging pump solution continuously charges and discharges by turning on and off the switch at high frequency. This will cause the current peak and extremely fast rise, and interfere with other circuits. Therefore, the flying capacitor should be as close to the IC connection as possible and the trace should be very short to minimize EMI emission. A low ESR input capacitor must be used to minimize high current peaks (especially at the input).
If a boost converter is used, the shielded inductor will have a more limited magnetic field to achieve better EMI performance. The conversion frequency of the converter shall be selected to minimize all interference to the wireless part of the system. PCB layout will have a significant impact on EMI, especially keep the trace carrying switch or AC current as small as possible to minimize EMI emission, as shown in Figure 11.
Thick traces should be routed first and a star ground or ground plane must be used to minimize noise. The input and output capacitors shall be low ESR ceramic capacitors to minimize input and output voltage ripple.
In most applications, the boost converter shows higher efficiency than the charging pump. The use of a boost converter with the same inductance size as the 1210 housing reduces the advantage of the charging pump in the overall solution size. At a minimum, efficiency needs to be evaluated based on the size of the overall solution. In terms of EMI performance, more factors and more relevant knowledge need to be considered in the design of boost converter.
In conclusion, for many systems, especially when the device has a flexible conversion gain from 1.0 to 1.5, the charging pump solution will be a good solution. Such a solution will achieve excellent efficiency when the conversion gain from 1.0 to 1.5 occurs slightly higher than the led forward voltage. When selecting a boost converter or charging pump solution for each application, the key requirements of portable systems need to be fully considered. If efficiency is the key requirement, the boost converter will be a more suitable solution.
Source; International LED network