LCD currently uses CCFL as the backlight source. However, due to the complex CCFL backlight drive line, high driving voltage and color rendering ability, plus the backlight source is the largest part of the system, so the power limit is increasing. Under severe conditions, LEDs have been gradually replaced by industry discussions.
In order to meet the needs of energy conservation and environmental protection, various new energy consumption standards of many governments and energy agencies around the world have also been released for different applications and different power consumption ranges. At the same time, more stringent specifications are being developed. Reducing energy consumption has become an important issue that cannot be avoided, so higher requirements have been placed on power management.
LED controlled forward current scheme
LEDs are current-driven devices whose brightness is proportional to forward current. There are two ways to control the forward current. The first method uses an LEDV-I curve, typically using a voltage supply and a rectifying resistor to determine the voltage that is required to be supplied to the LED to produce the desired forward current. However, this method has some disadvantages, such as any change in the LED forward voltage will cause a change in the LED current.
Assuming a fixed voltage of 3.6V and a current of 20mA, when the voltage becomes 4. 0V, the temperature or manufacturing changes will cause a specific pressure change, then the current will be reduced to 14mA. Therefore, when there is a large change in the forward voltage, it will cause a larger change in forward current. In addition, voltage drop and power consumption will also waste power and reduce battery life. The second method is to use a fixed current to drive the LED. The fixed current eliminates the change in current caused by the forward voltage change and, therefore, produces a fixed LED brightness. With a fixed current, it is only necessary to adjust the voltage across the current sense resistor without adjusting the output voltage of the power supply.
The supply voltage and current sense resistor values ​​determine the LED current. When driving multiple LEDs, a fixed current can be achieved in each LED by simply connecting in series. When driving parallel LEDs, a rectifying resistor must be placed in each LED string, but this will result in reduced efficiency and current mismatch.
Due to the portable application, the battery life is the key to the overall application. So LED drivers must be efficient. However, the efficiency measurement of an LED driver is somewhat different from the efficiency measurement of a typical power supply. A typical power efficiency measurement is defined as the output power divided by the input power. For LED drivers, the output power is not a relevant parameter, but the input power value required for LED brightness is the focus. At this point you can use the LED power divided by the input power to get the answer.
Overvoltage protection
In fixed current mode, the LED driver assembly must provide overvoltage protection. A fixed current can be generated regardless of the load. However, if the load resistance increases, the output voltage of the opposing power supply must also increase. When the power supply detects an excessive load resistance, or the load is disconnected, the output voltage may exceed the maximum operating voltage range of the IC or other components. Therefore, overvoltage protection must be provided in the drive. For example, a Zener diode can be used in parallel with the LED in such a way that the output voltage is limited to the Zener diode breakdown voltage and the power supply. When an overvoltage occurs, the output voltage is raised to the Zener diode breakdown point, and through the Zener diode, and then to the grounded current sense resistor, so the Zener diode and the LED in parallel can provide the output current steadily.
Alternatively, you can use the monitor output voltage to turn off the power before reaching an overvoltage. When overvoltage conditions occur, LED drivers can reduce power consumption and extend battery life.
PWM dimming
Many portable LCD backlight applications require limited brightness adjustment. In this part, two dimming methods can be used, which are analog or PWM methods. With analog dimming, as you are familiar with, you can increase the current by 50% on the LED, which can increase the brightness by 50%. However, this method has the disadvantage that LED color shift occurs and analog control signals are required. Therefore, this mode is generally not used much. The key to using PWM to adjust the brightness is to ensure that the user's eyes can not see the PWM pulse phenomenon, the frequency of the PWM signal must be higher than 100Hz, the maximum PWM frequency depends on the power activation and response time.
Load disconnection
Load disconnection is a frequently overlooked feature in LED drive power because the load can be disconnected to disconnect the LED from the power supply in the event of a power failure. This feature is quite important in the following two situations: power down and PWM dimming. For example, during the power-down of the boost converter, the load is still connected to the input voltage through the inductor and diode. Because the input voltage is still connected to the LED, the total power supply has failed, and still will continue to generate a small current. When the leakage current occurs for a long time, the battery life will be shortened. In addition, the load disconnection is also very important when the PWM performs brightness control. Because the power supply is in a failure condition while the PWM is not operating, the output capacitor is still connected to the LED.
If no load is disconnected, the output capacitor will still provide LED power until the PWM is turned back on. Because the capacitor will discharge at the beginning of each PWM cycle, the primary supply must charge the output capacitor at the beginning of each PWM cycle, so a glitch will be generated each time the PWM cycle occurs. Inrush current can cause system efficiency to drop and transient voltages appear on the input bus. If there is a load disconnect function, the LED will be disconnected from the power supply, so that when the power supply fails, there will be no leakage current, and the output capacitor is full during the cycle of PWM brightness adjustment.
At present, the world's major players are actively developing white LED driver circuits with more complete structure and higher backlight efficiency. Therefore, as mobile phones continue to move toward multi-functional intelligence, the demand for LED drivers is expected to continue to increase. For example, at present, ordinary mobile phones generally only use 2 to 4 LED drivers, but a more feature-rich dual-screen camera phone requires 7 to 9 LED drivers to meet the lighting requirements.
Supertex second generation high voltage LED driver chip HV9911
Supertex's second-generation high-voltage LED driver chip, the HV9911, provides engineers with backlighting designs for backlights for LCDs, automotive lighting, and battery-powered LEDs. The HV9911 is a closed-loop, switch-mode LED driver IC designed for good transient response to LED current and pulse width adjustment dimming. When multiple LED drivers are required, the system can be synchronized to prevent harmonic vibration.
The HV9911 also provides slope compensation to allow for a wider operating range in fixed frequency mode and an internal regulator for low voltage and high voltage applications.
NS small white LED driver chip LM2751
National Semiconductor (NS) model LM2751 small white LED driver that provides fixed frequency switching capacitors with regulated output of 4.5 volts and 5.0 volts over an input voltage range of 2.8 volts to 5.5 volts The charge pump, and the output current of 150mA (operating at 4.5V) or 80mA (operating at 5.0V), does not require an inductor, but requires an additional four ceramic capacitors.
The LM2751 is suitable for a wide range of applications including white LED displays and keyboard backlight systems, as well as general purpose boost or buck/boost regulators. The LM2751 chip can be pre-regulated with a fixed frequency. The accuracy of the regulated output is extremely high. The error does not exceed 3% or more, and the peak efficiency is over 90%. This ensures that the noise mixed into the system with the input current can be minimized. And its switching frequency can be predicted. And you can choose to set the switching frequency of 725kHz, 300kHz, 37kHz or 9. 5kHz.
Linear white LED driver chip LT3486
LinearTechnology's dual-channel boost LT3486, with constant current driving 16 white LEDs (8 serial LEDs per channel), in addition to providing PWM dimming, also keeps the LEDs in a fixed illumination color. Suitable for portable products and automotive display screen backlight applications.
The LT3486 is dimmed by the duty cycle control of the PWM driver to achieve a 1000:1 dimming range and maintain uniform LED brightness with current mode and fixed frequency. The LT3486's two independent converters can drive an asymmetric LED array with an input voltage of 2.5V to 24V, with an efficiency of 85%. The converter switching frequency can be set between 200kHz and 2MHz with a single resistor. Other features include internal soft activation, surge current limiting, and open LED protection. The LT3486EDHC is available in DFN-16 package technology and the LT3486EFE is available in TSSOP-16E package.
Pei Heng Parallel White LED Driver Chip AIC1848
Peiheng Semiconductor has introduced the AIC1848, a parallel white LED driver chip, for the small-volume and high-efficiency power supply requirements of portable electronic products such as mobile phones, PDAs and digital cameras. The AIC1848 uses a double charge pump (2XCharge-Pump) to provide a stable 5V output voltage, and drives multiple white LEDs in parallel at a constant voltage, operating at up to 1.8MHz.
The AIC1848 requires only three ceramic capacitors, eliminating the need for inductors and saving more design space. In addition, the AIC1848 features overcurrent (OCP) and thermal shutdown (ThermalShutdown), which enables the white LED driver circuit to have a more stable and stable working system, providing portable system designers with the most competitive power solution for parallel white LEDs. Program.
Toshiba white LED driver chip TB62752AFUG
Toshiba can drive up to eight white LEDs with high efficiency, high precision, high withstand voltage and low power consumption. LED driver chip TB62752AFUG, TB62752AFUG uses BiCD process technology, built-in overvoltage protection function (OVP), maximum withstand voltage The voltage is 40V. In the case of excessive voltage, the internal circuit is cut off immediately to protect the safety of the chip, white LED and application products. The overall power conversion efficiency is over 80%, and the current error is about 5%.
The TB62752AFUG features a highly interchangeable PIN angle design. It can provide backlighting drive capability for built-in LCD panels such as portable video games, mobile phones, and digital cameras. The TB62752AFUG was mass-produced at the end of 2005 and uses SOT23-6 packaging technology.
BCDSemi white LED driver chip AP3008
BCD series boost type white LED driver chip AP3008 can meet the requirements of LCD backlight uniformity, low cost, high performance, constant current, etc., and can meet the low cost requirements of customers from the price. AP3008 is a current mode boost DC-DC converter with 1.25V reference, error amplifier, 1.2MHz oscillator and ramp generator, current sampling amplifier circuit, PWM, R/S flip-flop, output drive And power switch tube, over voltage protection (OVP) and other components.
The AP3008 has a built-in UVLO circuit with a wide operating voltage range. The minimum operating voltage is up to 2.5V and the maximum operating voltage is up to 15V. An overvoltage protection (OVP) circuit detects the peak voltage of the PWM pulse output. When the peak voltage of the PWM pulse is higher than the internally set threshold voltage, the overvoltage protection (OVP) circuit operates and the PWM switching circuit is Shutdown. If the circuit is re-operated, a low to high signal can be applied to the Shutdownpin or Vinpin.
The reference voltage of the AP3008 is not directly used for 1.25V, but it is 95mV after being divided by resistors. This is a measure to reduce losses. In addition, due to the high frequency 36VBipoler process adopted by the AP3008, the withstand voltage is much higher than that of some CMOS process boost DC/DC converters. When the AP3008 inputs 3. 2V, it can drive 4 white LEDs with an output current of up to 20mA. When inputting 5V, it can drive 6 white LEDs with an output current of up to 20mA. The AP3008 is available in SOT23-5 and TSOT23-5 packages, allowing IC heights to be less than 1.1mm and 0.9mm.
Austriamicrosystems white LED driver chip AS3691
The austriamicrosystems white LED driver chip AS3691 allows designers to set the operating current of each LED channel with only an external resistor, while the brightness of the LED can be controlled by four independent pulse width adjustment input components. Each AS3691 chip integrates four independently operating current components, so it is possible to drive four white LEDs simultaneously, each capable of inputting 400mA of current, or driving only one white LED input with a current of 1.6 amps. The linear driver of the AS3691 avoids problems caused by inductive boosters, such as electromagnetic interference or image flickering of LCD screen quality. To increase the power efficiency of the application, the component contains a feedback output for each channel, allowing easy adjustment of one or more external power supplies to minimize overall power consumption.
With a current accuracy of 0.5%, the AS3691 is ideal for sophisticated color management applications such as LCD screens with RGB backlights. In addition, the AS3691 can be paired with a variety of color LED settings such as RGGB or RGBA. Its current input component supports a 15V voltage range, allowing extremely high LED supply voltages, and the highest voltage value is limited only by the highest power consumption of the entire product. The AS3691 is housed in a small QFN4TM package that minimizes overall power consumption.
CatalystSemiconductor high efficiency white LED driver chip CAT3691
CatalystSemiconductor expands its range of solid state lighting products with a new high efficiency white LED driver. The CAT3691 regulated charge pump provides consistent, flicker-free backlighting for large flat panel, dual LCD displays.
The CAT3691 simplifies system design by replacing the inductor boost circuit required for traditional high-brightness backlights. Due to the large output current, the CAT3608 can also be used to drive the main and secondary screen phone displays, or the main display plus low power camera flash. Independent control of each LED enables PWM dimming and standby mode of the 4 LEDs of the primary display and the 2 LEDs of the secondary display. To reduce battery consumption, the unit also features a full system shutdown feature that reduces power consumption to less than 1μA. For use with a variety of brightness control circuits, the unit also features a flexible digital interface. Its low-noise, 1MHz fixed-frequency control scheme provides the possibility to use small external capacitors, reducing cost and board footprint. The CAT3691 is optimized for lithium battery systems to maximize power transfer with up to 90% efficiency. The CAT3691's efficiency, low noise and dual display capabilities make it ideal for white LED drivers in cell phones, smart phones, PDAs and digital cameras.
Vishay High Efficiency White LED Driver Chip SiP12401
For high-efficiency white LED driver applications, Vishay has introduced two white LED driver ICs, the SiP12401, which provides efficient, controllable brightness in applications such as LCD backlights for portable electronic devices such as cell phones and digital cameras. %.
The SiP12401 boost controller chip uses a dual-core NiMH or alkaline and Li-Ion battery to drive a series of white LEDs without the need for a ballast resistor. The component supports input voltages ranging from 1.8V to 5.0V. A voltage-mode PWM design with internal frequency compensation is used to help reduce the overall component count for a smaller, lighter end product design. To extend battery life, the SiP12401 and PWM mode with a 600kHz conversion frequency achieve high typical efficiencies of 80% and 75%, respectively. The LED current can be externally adjusted to achieve analog control of the intensity. The logic controlled standby current is only 1 microamp.
Maxim480mA white LED double voltage chip MAX1576
Maxim's MAX1576 white LED doubling chip provides a 480mA white LED1x/1. 5x/2x charge pump for backlighting and camera flash white LED driving.
The MAX1576 charge pump can drive up to eight white LEDs with constant current regulation for uniform light intensity. Each main group (LED1-LED4) LED can be driven at a current of 30 mA for backlighting. The flash group LEDs (LED5-LED8) are individually controlled and can drive each LED (or a total of 400mA) at 100mA. By using an adaptive 1x/1. 5x/2x charge pump mode and an ultra low dropout current regulator, the MAX1576 is capable of achieving high efficiency over the entire voltage range of a 1-cell lithium battery. A 1MHz fixed frequency switch requires only very small external components. The MAX1576 uses two external resistors to set the maximum (100%) current of the main and flash LEDs. Four control pins are used for LED brightness control via serial control or each set of 2-bit logic control. ENM1 and ENM2 set the current of the main LED to 10%, 30% or 100% of the maximum current. ENF1 and ENF2 set the flash LED current to 20%, 40% or 100% of the maximum current. In addition, each pair of control pins are connected together for single-wire, serial pulse brightness control. The MAX1576 is available in a 24-pin thin QFN package.
ON Semiconductor high brightness white LED driver chip NCP5603
The high-brightness white LED driver chip NCP5603 from ON Semiconductor is aimed at lighting or flash applications in cell phones, cameras and other portable consumer electronics. The NCP5603 is powered by a charge pump architecture with two external ceramic capacitors to provide power conversion and eliminates the need for large bulky inductors for power conversion. It is available in a DFN-10 package and measures only 3mm?mm?. 9mm, its footprint is quite limited, it is very suitable for thin consumer electronics applications. The new LED driver can provide a stable voltage output with a 2. 7V to 5. 5V battery voltage input. It has 1X, 1.5X and 2X automatic operation modes, which can achieve 90% operational efficiency, making it especially suitable for needs. High-current LED driver for low-cost, low-power applications with long battery life, because this component can tolerate high output current pulses up to 350mA, it can be used to drive high-brightness LEDs up to 1W, resulting in higher camera flash Efficiency and better picture quality, the module also has a PWM / EN input pin, suitable for lighting control purposes. In addition, because the NCP5603 does not require external inductors to store energy, it can also be used as a low-cost small boost DC-DC converter for fixed-space applications such as mobile phones, MP3 players or digital cameras. 4. 5V or 5V fixed Voltage output.
Arques white LED driver chip supports two LCD backlights and FlashLED
ArquesTechnology's AQ913X series and AQ914X series of white LED drivers utilize dual-channel, dual-gain chargepump to regulate current for LED backlight applications in small LCD displays. These components drive parallel white LEDs with programmable output currents up to 120mA; they operate from a 2. 7V to 6.0V Li-Ion battery input voltage range with undervoltage lockout. The new components also have a fixed frequency of 650kHz. In a typical folding camera phone application, the AQ9133B can drive the LCD backlight, including the main screen and the sub-screen, while driving the Flash LED in both continuous and instant highlight modes. The flash LED driver can provide a pre-flash mode and a high-brightness exposure mode. In the pre-flash mode, the main screen of the mobile phone can maintain brightness.
The AQ913X and AQ914X series have CurrentRegulationMode and adaptive auto-switching mode. When the power supply voltage is sufficient, the IC maintains the LDO (1x) mode to reduce the input current to improve efficiency. When the LED current is insufficient, it automatically switches to the 1.5 times (Chargepump) mode. Improve battery efficiency. The output can support up to 6 WLEDs on two channels, each with a maximum allowable current of 50mA.
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