White LED driver solution for LCD backlight

LCD currently uses CCFL as the backlight source more commonly. However, due to the complex CCFL backlight driving circuit, high driving voltage and color rendering capabilities are required, and the backlight source is the part that consumes the most power in the system, so with power restrictions becoming increasingly stringent, the industry is gradually discussing the use of LED as a replacement.

In order to meet the needs of energy conservation and environmental protection, various new energy consumption standards have been issued by many governments and energy agencies around the world for different applications and different power consumption ranges. At the same time, more stringent regulations are also being formulated. Reducing energy consumption has become an important issue that cannot be avoided, so higher requirements have been put forward for power management.

LED forward current control scheme

LEDs are current-driven devices whose brightness is proportional to the forward current. There are two ways to control the forward current. The first method is to use the LED V-I curve, which generally uses a voltage source and a rectifier resistor to determine the voltage required to provide the LED to produce the expected forward current. However, this method has some disadvantages, such as any change in the LED forward voltage will result in a change in the LED current.

Assuming a fixed voltage of 3.6V and a current of 20mA, when the voltage changes to 4.0V, the current may drop to 14mA due to a specific voltage change caused by temperature or manufacturing variations. Therefore, when the forward voltage changes greatly, it will lead to a greater change in the forward current. In addition, the voltage drop and power consumption will waste power and reduce the battery life. The second method is to use a fixed current to drive the LED. The fixed current can eliminate the current change caused by the change in the forward voltage, so a fixed LED brightness can be generated. Using a fixed current only requires adjusting the voltage through the current detection resistor, without adjusting the output voltage of the power supply.

The power supply voltage and the current sensing resistor value determine the LED current. When driving multiple LEDs, a fixed current can be achieved in each LED by simply connecting them in series. When driving parallel LEDs, a rectifier resistor must be placed in each LED string, but this will result in reduced efficiency and current mismatch.

Since the battery life is the key to the overall application in portable applications, the LED driver must be highly efficient. However, the efficiency measurement of LED drivers is somewhat different from that of typical power supplies. The definition of typical power supply efficiency measurement is output power divided by input power. For LED drivers, output power is not a relevant parameter, but the input power value required to achieve the expected LED brightness is the key. At this point, the answer can be obtained by dividing the LED power by the input power.

Overvoltage protection

In the fixed current mode, the LED driver component must provide overvoltage protection. Regardless of the load, a fixed current can be generated. However, if the load resistance increases, the output voltage of the corresponding power supply must also increase accordingly. When the power supply detects 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 driver. For example, a Zener diode can be connected in parallel with the LED, which can limit the output voltage to the Zener diode breakdown voltage and the power supply. When an overvoltage occurs, the output voltage will increase to the Zener diode breakdown point, pass through the Zener diode, and then to the grounded current detection resistor, so when the Zener diode is connected in parallel with the LED, the output current can be stably provided.

Alternatively, the output voltage can be monitored to shut down the power supply before an overvoltage condition occurs. When an overvoltage condition occurs, the LED driver can reduce power consumption and extend battery life.

PWM dimming

Many portable LCD backlight applications require limited brightness adjustment. There are two dimming methods that can be used in this part, namely analog or PWM. With analog dimming, as everyone is familiar with, increasing the current on the LED by 50% can increase the brightness by 50%. However, this method has disadvantages, that is, LED color shift will occur and analog control signals are required. Therefore, this mode is generally not used much. The key to using PWM to adjust the brightness is that in order to ensure that the user's eyes cannot see the PWM pulse phenomenon, the frequency of the PWM signal must be higher than 100Hz, and the maximum PWM frequency depends on the power activation and response time.

Load disconnect

Load disconnection is an often overlooked function in LED driver power supplies because it can be used to disconnect the LED from the power supply when the power fails. This function is very important in the following two situations, that is, power failure and PWM dimming. For example, during the power failure of the boost converter, the load is still connected to the input voltage through the inductor and the diode. Because the input voltage is still connected to the LED, the total power supply has failed and a small current will continue to be generated. When leakage current occurs for a long time, it will shorten the battery life. In addition, load disconnection is also very important when PWM is used for brightness control. Because during the non-operation of PWM, the power supply fails, but the output capacitor is still connected to the LED.

If there is no load disconnection, the output capacitor will still provide LED power until the PWM turns on the power again. Because the capacitor will discharge at the beginning of each PWM cycle, the primary power supply must charge the output capacitor at the beginning of each PWM cycle, so a surge pulse will be generated when each PWM cycle occurs. The inrush current will cause the system efficiency to decrease and a transient voltage will appear on the input bus. If there is a load disconnection function, the LED will be disconnected from the power supply, so that when the power fails, there will be no leakage current, and the output capacitor will be fully charged between the cycles of PWM brightness adjustment.

Currently, major global companies are actively developing white LED driver circuits with more complete structures and higher backlight efficiency. As mobile phones continue to develop in the direction of multifunctional intelligence, the demand for LED drivers is expected to continue to increase. For example, ordinary mobile phones currently only use 2 to 4 LED drivers, but dual-screen camera phones with more functions require 7 to 9 LED drivers to meet lighting requirements.

Supertex second generation high voltage LED driver chip HV9911

Supertex's second generation high voltage LED driver chip HV9911 provides engineers with applications in backlight design for products such as LCD backlight, automotive lighting and battery-driven LED lights. HV9911 is a closed loop switch mode LED driver IC design for LED current and good transient response to pulse width adjustment dimming. When multiple LED drivers are required, they can be synchronized to prevent system subharmonic vibration.

The HV9911 also provides slope compensation to allow 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's (NS) small white LED driver model LM2751 can provide a fixed-frequency switched capacitor charge pump with 4.5V and 5.0V regulated outputs within an input voltage range of 2.8V to 5.5V, and an output current of 150mA (operating at 4.5V) or 80mA (operating at 5.0V), without the need for an additional inductor, but requires four additional ceramic capacitors.

LM2751 is suitable for white LED display and keyboard backlight system, and general boost or buck/boost regulator and many other applications. LM2751 chip can use fixed frequency for pre-regulation, the accuracy of regulated output is very high, the error will not exceed plus or minus 3%, the peak efficiency is more than 90%, ensuring 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 light LED driver chip LT3486

Linear Technology's dual-channel boost LT3486 drives 16 white LEDs (8 serial LEDs per channel) with constant current. In addition to providing PWM dimming, it can also keep the LEDs at a fixed luminous color. It is suitable for portable products and automotive display screen backlight applications.

The LT3486 dims by controlling the duty cycle of the PWM driver, achieving a 1000:1 dimming range, and uses current mode and fixed frequency to maintain uniform LED brightness. The two independent converters of the LT3486 can drive an asymmetric LED array from an input voltage of 2.5V to 24V with an efficiency of 85%. The converter switching frequency can be set between 200kHz and 2MHz using a single resistor. Other features include internal soft activation, surge current limiting, and open-circuit LED protection. The LT3486EDHC uses DFN-16 packaging technology, while the LT3486EFE uses TSSOP-16E packaging.

Peiheng parallel white light LED driver chip AIC1848

Peihen Semiconductor has launched the parallel white LED driver chip AIC1848 to meet the small size and high-efficiency power requirements of portable electronic products such as mobile phones, PDAs, and digital cameras. AIC1848 uses a 2X Charge-Pump to provide a stable 5V output voltage and drives multiple white LEDs in parallel at a constant voltage, with an operating frequency of up to 1.8MHz.

AIC1848 only needs three ceramic capacitors and no inductor, saving more design space. In addition, AIC1848 has built-in over-current (OCP) and thermal shutdown functions, making the white LED drive circuit more reliable and more stable, providing portable system designers with the most competitive power supply solution for parallel white LEDs.

Toshiba white light LED driver chip TB62752AFUG

Toshiba's high-efficiency, high-precision, high-voltage, low-power LED driver chip TB62752AFUG can drive up to 8 white light LEDs. TB62752AFUG adopts BiCD process technology and has a built-in overvoltage protection function (OVP). The maximum voltage resistance is 40V. In the event of overvoltage, the internal circuit is immediately cut off to protect the chip, white light LED and application products. The overall power conversion efficiency is as high as 80%, and the current error value is about 5%.

TB62752AFUG adopts a highly interchangeable PIN angle design. It can provide built-in LCD panel backlight driving capabilities for portable video games, mobile phones, digital cameras, etc. TB62752AFUG has been mass-produced at the end of 2005, using SOT23-6 packaging technology.

BCDSemi white light LED driver chip AP3008

BCD series boost white LED driver chip AP3008 can meet the requirements of LCD backlight uniformity, low cost, high performance, constant current, etc., and can meet customers' low-cost requirements in terms of price. AP3008 is a current mode boost DC-DC converter, which consists of 1.25V reference, error amplifier, 1.2MHZ oscillator and ramp generator, current sampling amplifier circuit, PWM, R/S trigger, output drive and power switch tube, overvoltage protection (OVP), etc.

AP3008 has a built-in UVLO circuit and a wide operating voltage range. The minimum operating voltage can be as low as 2.5V and the maximum operating voltage can be as high as 15V. The 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 is activated and the PWM switch circuit is shut down. To make the circuit work again, a signal from low to high can be applied to the Shutdownpin or Vinpin.

The reference voltage of AP3008 is not 1.25V directly, but 95mV after resistor voltage division, which is a measure to reduce loss. In addition, due to the high-frequency 36VBipoler process adopted by AP3008, the withstand voltage is much higher than some boost DC/DC converters of CMOS process. When the input of AP3008 is 3.2V, it can drive 4 white light LEDs with a maximum output current of 20mA; when the input is 5V, it can drive 6 white light LEDs with a maximum output current of 20mA. AP3008 adopts SOT23-5 and TSOT23-5 packages, which can make the height of IC less than 1.1mm and 0.9mm.

Austriamicrosystems white light LED driver chip AS3691

austriamicrosystems' white light LED driver chip AS3691 allows designers to set the operating current of each LED channel only through external resistors, and the brightness of the LED can be controlled by four independent pulse width modulation input components. Each AS3691 chip integrates four independently operating current components, so it can choose to drive four white light LEDs at the same time, each with an input current of 400mA, or only drive one white light LED with an input current of 1.6 amps. The linear driver of AS3691 can avoid various problems derived from inductive boosters, such as electromagnetic interference or image flickering on LCD screens. In order to improve the power efficiency of the application, each channel of the component contains a feedback output terminal, which can simply adjust one or more external power supplies to minimize the overall power consumption.

The AS3691 current accuracy is 0.5%, which is very suitable for precise color management applications such as LCD screens with RGB backlight. In addition, the AS3691 can be used with various color LED settings, such as RGGB or RGBA. Its current input component supports a 15V voltage range, allowing extremely high LED power supply voltages, and the maximum voltage value is only limited by the maximum power consumption of the entire product. The AS3691 uses a small QFN4? package to keep the overall power consumption within a minimum range.

Catalyst Semiconductor high-efficiency white light LED driver chip CAT3691

Catalyst Semiconductor has expanded its solid-state lighting product line 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 replaces the inductive boost circuit required for traditional high-brightness backlights, simplifying system design. Due to its high output current, the CAT3608 can also be used to drive a main and secondary screen mobile phone display, or a main display plus a low-power camera flash. Independent control of each LED enables PWM dimming and standby modes for the 4 LEDs on the main display and the 2 LEDs on the secondary display. To reduce battery consumption, the component also features a complete system shutdown function that reduces power consumption to less than 1μA. The component also has a flexible digital interface for use with a variety of brightness control circuits. Its low-noise, 1MHz fixed-frequency control scheme makes it possible to use small external capacitors, thereby reducing cost and board space. The CAT3691 is optimized for lithium battery systems to maximize power transfer with efficiency up to 90%. The component also has the efficiency, low noise and dual-display capabilities of the CAT3691, making it ideal for white light LED drivers in mobile phones, smart phones, PDAs and digital cameras.

Vishay high efficiency white light LED driver chip SiP12401

For high-efficiency white LED driver applications, Vishay has launched two white LED driver chips, SiP12401, which can provide efficient controllable brightness in applications such as white LED backlight for LCD displays in portable electronic devices such as mobile phones and digital cameras, with an efficiency of 80%.

The SiP12401 boost controller chip uses dual-core NiMH or alkaline and Li-ion batteries to drive series-connected white LEDs without the need for ballast resistors. 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, resulting in a smaller and lighter end product design. To extend battery life, the SiP12401 and PWM modes with a switching frequency of 600kHz can achieve high typical efficiencies of 80% and 75%, respectively. The LED current can be adjusted externally to achieve analog control of intensity. The logic-controlled standby current is only 1 microamp.

Maxim 480mA white light LED voltage doubler chip MAX1576

Maxim's MAX1576 white LED voltage doubler chip can provide 480mA white LED 1x/1.5x/2x charge pump for backlight and camera flash white LED driving.

The MAX1576 charge pump drives up to 8 white LEDs with constant current regulation for uniform light intensity. Each main group (LED1-LED4) of LEDs can be driven at 30mA for backlighting. The flash group LEDs (LED5-LED8) are individually controlled and can be driven at 100mA each (or 400mA total). By using adaptive 1x/1.5x/2x charge pump modes and an ultra-low dropout current regulator, the MAX1576 is able to achieve high efficiency over the entire voltage range of a 1-cell lithium battery. The 1MHz fixed-frequency switching requires very small external components. The MAX1576 uses two external resistors to set the maximum (100%) current for the main and flash LEDs. Four control pins are used for LED brightness control via serial control or 2-bit logic control per group. ENM1 and ENM2 set the current of the main LED to 10%, 30%, or 100% of the maximum current. ENF1 and ENF2 can set the flash LED current to 20%, 40%, or 100% of the maximum current. In addition, connecting each pair of control pins together allows 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

ON Semiconductor's high-brightness white LED driver chip NCP5603 is targeted at lighting or flash applications in mobile phones, cameras and other portable consumer electronics. NCP5603 uses a charging pump architecture with two external ceramic capacitors to provide power conversion functions and eliminates the need for large and bulky inductors during power conversion. It is supplied in DFN-10 package, which is only 3mm? mm? . 9mm in size. Its footprint is quite limited, making it very suitable for thin portable consumer electronics applications. The new LED driver can provide a stable voltage output to the load through a 2.7V to 5.5V battery voltage input. It has 1X, 1.5X and 2X automatic operation modes, allowing the operating efficiency to reach 90%, making it particularly suitable for high-current LED driving in low-cost, low-power applications that require longer battery life. Since this component can tolerate high output current pulses up to 350mA, it can be used to drive high-brightness LEDs with a power of up to 1W, bringing higher camera flash efficiency and better image quality. The component also has a PWM/EN input pin, which is suitable for lighting control purposes. In addition, since the NCP5603 does not require an external inductor to store energy, it can also be used as a low-cost, small step-up DC-DC converter to provide 4.5V or 5V fixed voltage outputs for space-constrained applications such as mobile phones, MP3 players or digital cameras.

Arques white LED driver chip supports two LCD backlights and FlashLED

Arques Technology's AQ913X series and AQ914X series white LED drivers can use dual-channel, dual-gain charge pumps to regulate current, and are suitable for LED backlight applications in small-size LCD displays. These components can drive parallel white LEDs with programmable output currents up to 120mA; they can operate in a lithium-ion battery input voltage range of 2.7V to 6.0V, and have an undervoltage lockout function. 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, and drive the Flash LED in two modes: continuous lighting and instantaneous highlight. The drive for the Flash LED can provide a pre-flash mode and a highlight exposure mode. In the pre-flash mode, the main screen of the phone can maintain brightness.

AQ913X and AQ914X series have CurrentRegulationMode and adaptive automatic switching modes. When the power supply voltage is sufficient, the IC maintains in LDO (1x) mode to reduce input current to improve efficiency. When the LED current is insufficient, it automatically switches to 1.5x (Chargepump) mode to improve battery efficiency. The output can support up to 6 WLEDs in two channels, and the maximum allowable current of each LED pin can reach 50mA.