“Do you know the design of the negative charge pump white LED driver? What are its characteristics? Many portable consumer Electronic products, such as mobile phones, PDAs, MP3 players, notebooks, etc., have displays, although different applications have different effects on the Display. The type and size will be different, but for the majority of designers, it is necessary to design a backlight circuit for it. White LEDs are considered to be the ideal backlight source for color displays of small handheld devices.
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Do you know the design of the negative charge pump white LED driver? What are its characteristics? Many portable consumer electronic products, such as mobile phones, PDAs, MP3 players, notebooks, etc., have displays, although different applications have different effects on the display. The type and size will be different, but for the majority of designers, it is necessary to design a backlight circuit for it. White LEDs are considered to be the ideal backlight source for color displays of small handheld devices.
The easiest way to drive a white LED is to use a voltage source to drive the LED through a ballast resistor (as shown in Figure 1). The advantage of this driving method is that there is a lot of room for selecting a voltage source, and only one connection terminal is needed between the regulator and the LED. But the shortcomings are also obvious: First, the low efficiency, which is mainly caused by the loss of the ballast resistor; Second, the LED current has poor current stabilization ability, inaccurate control, and LED caused by temperature drift and LED mismatch. The change of the forward voltage will make the final LED current have a big change, thereby affecting the control of the backlight brightness.
Therefore, the ideal white light LED driving method is to use constant current driving, which can avoid the current fluctuation of the white light LED due to temperature drift, or the uneven brightness caused by the mismatch of the LED, and can generate a controllable LED forward current. At this time, the driver does not need to output a stable voltage, and only needs to control the current flowing through the LED to be constant to achieve controllable brightness control.
Comparison of common topologies
The luminous intensity of the LED is related to the current flowing through the LED. The greater the current, the higher the light intensity. Common digital cameras and cellular phones generally require 2 to 3 LEDs as backlights, while PDAs generally require 3 to 6 LED backlights. The LEDs can be driven in parallel or in series. These two methods have their own advantages and disadvantages: the LED current in the series scheme is the same, and the circuit control is simple, but requires a higher driving voltage; the parallel scheme has a simpler circuit and the required driving voltage It is also low, but when the number of LEDs is large, multiple control channels are required, and the consistency of the current is also poor.
LED drivers are divided into topological structure, which can be mainly divided into inductance-based DC/DC drivers and capacitor-based charge pump drivers. Of course, there are a few LED drivers that use linear regulators. Since inductance-based drivers can provide a relatively wide range of output voltages and have high efficiency, inductance-based driver structures are used in many designs to drive multiple LEDs in series. The capacitor-based charge pump driver eliminates the need for external inductance, has the characteristics of small size, simple design, and low cost, and is also more popular. Since the charge pump-based LED driver can only generate multiples of the input voltage (such as 1.5 times, 2 times), the limited driving voltage makes the charge pump-based LED driver often used to drive multiple LEDs in parallel. As for the LED drive architecture using linear regulators, due to its low efficiency and can only work under reduced voltage conditions, its application range is relatively limited and cannot be used for handheld devices powered by a single-cell Li+ battery. This article mainly discusses the two common topologies of Inductor-based DC/DC drivers and capacitor-based charge pump drivers.
In order to adapt to the application needs of portable products, MAXIM provides LED drivers with a variety of topologies, including inductance-based LED drivers, represented by MAX1553-MAX1554, and MAX1561, MAX1582 and other devices; and capacitor-based charge pump drivers. MAX1570 is representative, and other products include MAX1575, MAX1576, etc.
The MAX1553-MAX1554 is a high-efficiency, 40V boost converter that can be used to drive 2-10 white LEDs in series to provide high-efficiency backlight displays for cellular phones, PDAs and other handheld devices. The boost converter has a built-in 40V, low RDSON N-channel MOSFET switch, which greatly improves the conversion efficiency and effectively extends the battery life. The device has two methods of brightness adjustment in analog/PWM modes, and an independent enable input can also be used for on/off control. The soft start function can effectively suppress the inrush current during the start-up process. The device also has an adjustable overvoltage protection circuit, when the output overvoltage is detected, the internal MOSFET can be turned off, thereby reducing the output voltage. Figure 2 shows a typical operating circuit of the MAX1553.
The MAX1570 fractional charge pump can drive up to 5 white LEDs with a constant current to obtain uniform brightness. MAX1570 utilizes 1x/1.5x fractional charge pump and low dropout current regulator to maintain the highest efficiency in the entire Li+ battery supply voltage range. MAX1570 works at a fixed frequency of 1MHz, allowing the selection of small external components. The optimized current regulation structure ensures low EMI and low input ripple. The device can use an external resistor to set the full-scale LED current, two digital inputs to control on/off or select one of the three levels of brightness. The device can also use pulse width modulation (PWM) signals to adjust the brightness of the LEDs. The typical working circuit of MAX1570 is shown in Figure 3.
From Figure 2 and Figure 3, it can be seen that the circuit structure of the inductor-based LED driver is more complicated than that of the capacitor-based charge pump LED driver; Said it is a difficult point; in addition, the volume of the inductor is also relatively large, which takes up more space on the circuit board. Capacitor-based charge pump LED drivers require only a few capacitors, the design is relatively simple, and the circuit board space is saved. However, inductance-based LED drivers have obvious advantages in efficiency compared with charge pump LED drivers. MAX1553 can basically maintain an efficiency of about 80% within the LED operating current range (see Figure 4a), and the efficiency varies with current The variation fluctuation is small, and the efficiency of the MAX1570 charge pump LED driver fluctuates greatly within the operating current range of the LED, and the efficiency at light load will be less than 80% (see Figure 4b).
It can be seen that although the capacitor-based charge pump LED driver has the advantages of simple design and saving circuit board space, its relatively low efficiency often limits the application of the device, especially for efficiency-sensitive applications, such as mobile phones in handheld devices, For products such as PDA, people often hope that the battery has a long enough power supply time. In response to this demand, MAXIM has introduced a new negative charge pump LED driver. Compared with the traditional positive charge pump LED driver, the efficiency of the device has been increased by 12%, which greatly reduces the power consumption of the drive scheme.
High-efficiency new negative charge pump LED driver
The MAX8647 can drive 6 white LEDs or 2 groups of RBG LEDs with a constant current, and is suitable for applications such as display backlighting or entertainment lighting. Through a negative charge pump and an adaptive ultra-low dropout current regulator, these devices can maintain extremely high efficiency in the entire input voltage range of a Li+ battery, even when there is a large mismatch in the LED forward voltage. Figure 5 shows the MAX8647 typical application circuit diagram and internal principle block diagram.
The charge pump of the traditional positive charge pump type LED driver is located between the input power supply (usually a battery) and all LEDs. When the input power drops to a certain value, resulting in insufficient forward voltage drop of any LED, the positive charge pump is turned on. Those LEDs with lower VF will consume more power. Take LED5 and LED6 in Figure 3 as an example, suppose that the forward voltage drop of LED5 is VF5> VF6. When VIN drops below VF5 + 0.15V (the forward voltage drop of the current regulator for normal steady flow), the entire charge pump will switch to 1.5 times mode, increasing VOUT to 1.5 times VIN to ensure that LED5 is fully turned on. However, since the charge pump of the traditional positive charge pump architecture is connected in series between VIN and LED, it is impossible to dynamically switch the output of each LED. The regulator loop corresponding to the LED with lower VF will consume additional power consumption (such as LED6, The other way is the same), thereby reducing the efficiency of the entire drive.
MAX8647, a new type of negative charge pump, eliminates the line impedance between the input power supply and the LED. The adaptive switching technology of the device can dynamically switch each LED, and realize independent power supply, dimming and current stabilization for each LED. . When the forward voltage drop of a certain LED is not enough, the internal negative charge pump of the device starts to stabilize the voltage of NEG to a voltage not exceeding VIN 5V, and at the same time independently switches the current loop of the LED from GND to NEG, and It is not that the current loops of all LEDs are switched over at the same time.
For example, LED5 and LED6 in Figure 5 also assume that VF5> VF6 of LED5. When VIN is high, the negative charge pump is turned off. With the consumption of energy, when VIN is reduced to insufficient forward voltage drop on LED5, the device starts the negative charge pump. LED5 is the first to switch the current loop of LED to NEG, while LED6 remains in the original state, thus enabling the entire LED driver The efficiency is improved. The result of this unique topology can significantly improve battery life and increase efficiency by approximately 12%. Figure 6 shows the comparison between the efficiency of the MAX8647 and the traditional charge pump LED driver.
In addition, the MAX8647 also has an I2C serial interface, which can independently turn on/off the backlight of the main screen or the sub-screen and control the brightness; the current can be set in 32 levels in a pseudo-logarithmic form within the range of 24mA to 0.1mA; with temperature drop This function guarantees safety when set to 24mA full-scale output current. When the ambient temperature is higher than +60℃, the device will reduce the current by 2.5%/℃ to protect the LED. At the same time, the device also provides a thermal shutdown function (when the IC temperature exceeds 160 ℃ to turn off the IC) and open circuit and short circuit protection.
Concluding remarks
The MAX8647 negative charge pump white LED driver has the common advantages of small size and simple design of the charge pump LED driver. Compared with the positive charge pump LED driver, the efficiency is increased by 12%. It is suitable for various efficiency-sensitive LED drivers. Handheld devices, including cellular phones, smart phones, and media players. The above is the design analysis of the negative charge pump white light LED driver, I hope it can give some help to the designer in the study.
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