LED series and parallel drive circuit characteristics

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LED series and parallel drive circuit characteristics

 

1. LED series configuration

In a series configuration, the number of LEDs is limited by the maximum voltage of the driver, if the maximum voltage is 40V. In the series configuration, according to the forward voltage of the white LED, this maximum voltage can drive up to 10 to 13 white LEDs, and the driving current ranges from 10 to 350 mA in a continuous state. The advantage of this configuration is that a series of white LEDs can carry current with a single line. The disadvantage is that current density on the copper wire is a problem when the pcb space is limited (especially at high power), and if a white LED fails in series mode, all white LEDs will go out. However, from a design point of view, if there are n white LEDs, the battery voltage should be raised to n×VF, so a boost structure must be used, and the current slope can be accurately monitored by the inductive component, thereby limiting the uncontrolled transient current. The resulting EMI, typical boost topology is shown in Figure 1.
 

 

 

 

2. LED parallel configuration

In a parallel configuration, the number of white LEDs in a particular array is limited by the driver package level and the number of connector pins. In addition, when white LEDs are connected in parallel, each white LED must be current controlled to ensure that each white LED is between The match is great for a specific application. In fact, the inconsistency of the two white LED currents exceeds 10%, which will affect the quality of the display image of the color LCD (the white LED is used as the backlight of the LCD). In addition, the parallel configuration enables the transfer of energy from the battery to the white LED array using two ceramic capacitors using charge pump technology A charge pump-based LED driver block diagram is shown in Figure 2. A charge pump based on battery and dedicated current source for energy conversion and regulation allows the white LED current to be unaffected by forward voltage and input power after optimized current source design. influences.

3. LED series and parallel drive circuit comparison

The LED driver circuit topology is available in either a boost converter or a charge pump topology. The choice is to consider all the specific factors of the two solutions. An important parameter of the white LED driver circuit based on the charge pump is the noise generated by the LED driver. Because the capacitor is charged and discharged, the charge pump is the source of high current glitch. To reduce this effect, you must set up a high-performance input filter circuit. White LED drivers based on inductive boost converters can cause electromagnetic interference (EMI) due to the presence of inductance. Normally, changing the switching frequency reduces interference, but the frequency value depends on the operating conditions of the converter.

The TPS60230 charge pump is used to drive the white LED. The typical circuit is shown in Figure 3. The TPS60230 is directly powered by a lithium-ion battery. The typical input voltage range is 3.0~4.2V, which can supply power for up to 5 white LEDs at the same time. The LED current is 20mA.

A typical circuit for driving a white LED using the TPS61062 boost converter is shown in Figure 4. The boost converter shown in Figure 4 is one of the latest developments in IC technology. As a fully integrated synchronous boost converter, the smallest size solution can be achieved without the need for an external Schottky diode. least.

(1) Comparison of charge pump and boost converter efficiency

The white LED solution shown in Figure 3 and Figure 4, it is hard to say which solution is an efficient solution, because the overall efficiency depends on the white LED forward voltage, lithium-ion battery discharge characteristics and white LED Specific application parameters such as current. A typical efficiency curve for a charge pump based solution is shown in Figure 5. When the converter is operating in 1x mode, the gain is 1, the input voltage range is reduced from 4.2V to 3.6V, and the efficiency level is higher than 75%. In the 1x mode, the charge pump acts like an LDO , and the input voltage is regulated to the white LED forward voltage, typically 3.1 to 3.5V. Another advantage of the LDO mode is that the switching device does not operate in the switching state, thus avoiding EMI problems.

However, depending on the LED forward voltage and the voltage drop inside the driver IC, when the driver is switched from the "LDO mode" to the boost mode (boost mode) and the gain is 1.5 times, the efficiency is greatly reduced. In boost mode, the switching device operates in a switching state with an output voltage 1.5 times the input voltage, which needs to be adjusted to reduce the level of forward voltage required by the white LED, which reduces efficiency. Therefore, the longer the driver operates in LDO mode, the higher the charge pump efficiency.

Unlike the charge pump solution, the typical efficiency curve for the boost converter TPS61062 solution is shown in Figure 6. The efficiency is 75% to 80% over the entire input voltage range of a lithium-ion battery. Some boost converter solutions are typically as efficient as 85% with externally calibrated diodes. If the TPS61042 is used to drive less than 5 white LEDs, the efficiency will increase because the input-to-output voltage conversion is low. In general, boost converters are slightly more efficient than charge pump solutions, especially when driving more than four white LEDs.

(2) Comparison of charge pump and boost converter footprint

In the past, charge pump solutions had significant advantages in terms of footprint, mainly due to the larger inductors and external Schottky diodes used in boost converters With the development of the latest technology and higher integration, the boost converter solution size is also roughly equivalent to the charge pump solution. Since the number of pins required for the charge pump driver is large, the device package is correspondingly large, requiring two external pump capacitors. In this case, the charge pump solution has a larger footprint than the boost converter. Still bigger. If the switching frequency of the boost converter is raised to as high as 1MHz, a small inductor and a small-capacity output and input capacitor can be used. For example, the TPS61062 device can control the inductor current from an internal control loop that is less than the maximum switching current during normal operation. This allows for a smaller inductor with a maximum current rating just above the inductor's maximum peak current. For example, when powering four white LEDs, an inductor with a saturation current of 200 mA is sufficient. Without a specific internal loop design, the inductor saturation current must be rated at 400mA, which requires a larger inductor and results in a larger footprint.

(3) Height comparison between charge pump and boost converter components

When the component height is less than 1mm, the inductor-based boost converter will lose its advantage, so the charge pump solution is a better choice when the component height must be less than 1mm.

(4) Comparison of charge pump and boost converter EMI

When considering EMI issues, the boost converter inductor should be analyzed for EMI issues. Generally speaking, the possible electromagnetic radiation will not be a big problem, because the inductor around the RF sensitive area is shielded. The cause of the EMI problem caused by the inductive boost converter is the conducted interference caused by the insufficient filtering of the input and output voltages. Electromagnetic interference caused by unreasonable layout or wiring of printed circuit boards (PCBs).

In a lithium-ion battery-powered electronic device, a white LED driver with a pulsating input current has an input directly connected to the battery electrode terminal. Since the RF portion is also powered by the battery, the switching noise at the input of the white LED driver also exists. The battery connection also exists at the input of the RF circuit, which can cause serious interference. To determine which white LED driver solution performs better in conducting EMI, the input voltage ripple of the boost converter and charge pump solution should be compared.

One way to evaluate the solution is to check the input with a spectrum analyzer. If the device is operating at a fixed switching frequency, the spectrum will show the switching frequency of the fundamental and its harmonics.

In order to minimize the interference in the RF section, the fundamental frequency and its harmonics should be as high as possible and the amplitude should be kept low. This is because the switching frequency of the converter is mixed with the carrier frequency of the transmitter, so that the sideband also has a carrier frequency. The sidebands appear in the output band of the transmitter, just one bit higher or lower than the transmitter frequency. The lower the switching frequency, the closer the sideband is to the carrier frequency, which reduces the signal-to-noise ratio of the transmitter. The higher the switching frequency, the farther the sideband is from the carrier frequency and the greater the signal-to-noise ratio of the transmitter. Of course, the lower the amplitude of the fundamental frequency of the converter switching frequency, the higher the signal-to-noise ratio. Because of this, a fixed converter switching frequency equal to or higher than 1 MHz is usually suitable for most applications.

At the same setting, the input ripple voltage of the charge pump solution is twice that of the boost converter solution. This is because the charge pump operates in 1.5 times the voltage mode and produces an almost square wave input current. As an input filter , the charge pump has only input capacitance. The boost converter has an inductor and an input capacitor to better perform the input filter operation, resulting in lower input voltage ripple. To further reduce input voltage ripple, the most efficient way to use a boost converter and a charge pump solution is to increase the value of the input capacitor. For very sensitive applications, consider adding additional LC input filters or using smaller ferrite beads.

It can be clearly seen that the charge pump solution does not satisfy all applications, as does the boost converter solution. When choosing a solution, consider the specific final application requirements and key parameters. In addition, charge pump solutions are not superior to boost converter solutions in terms of EMI.



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