The new design of the bookmark0 has passed the circuit structure, working principle and technical characteristics of the typical device of Yuqinggan. * With the trend of color LCDs gradually replacing monochrome displays, white LEDs as a miniature "broad spectrum" light-emitting device are also in the field of LCD backlights, especially in mobile phones, handheld computers and handheld devices. Applications have begun to be found in small electronic devices such as meters.
In the past, the light-emitting devices used for LCD backlights mainly include cold cathode fluorescent lamps (CCFLs) and electroluminescent devices (ELs, also known as "field-lights"), both of which are powered by an alternating current source, and the required voltage is from a hundred volts to Hundreds of volts, the matching inverter power supply operating frequency is often in the D-phase of 0.5~40kHz range, and the white LED is a low-voltage DC device, which does not generate electromagnetic interference itself; that is, it uses the inverter boost mode to supply power. The driving circuit also requires only a small number of peripheral components, and the operating frequency is selected in a wide range without affecting the illuminating performance of the LED. Therefore, one can save the printing plate area and facilitate the miniaturization of the entire device, and the second is easy to avoid between the components. Electromagnetic interference facilitates the overall layout of the device.
There are many integrated devices, which are roughly classified into three types: constant current source and charge pump switching power supply. Feng Wen will introduce some typical driving devices in a classification, and a brief analysis and discussion on the circuit structure and working conditions of EHtiail:asian丨nfoonrine.sh.cn.
Most LCD backlights are equipped with a brightness controller so that the user can adjust accordingly according to the intensity of the ambient light. Since the photon emission of the light emitting diode is derived from the recombination of electron-hole pairs, the emission rate and the luminous intensity are both proportional to the forward current of the diode. From this point of view, the controllable constant current source should be the simplest scheme for the actual brightness control; unfortunately, the "color temperature" of the white LED is also directly related to the working current. This kind of control scheme will make the light source adjust the range of the new technology in the whole brightness design. The color consistency inside is very poor, so it can only be used in places where the color fidelity is very low, such as button illumination. LCD backlights mostly use pulse width modulation (PWM) power to power the LEDs to avoid this. A "color shift" problem.
From a practical point of view, for a vehicle with a high power supply voltage, since multiple LEDs can be used in series, it is simple to use an open collector (0C) or current mirror output stage with a pulse width modulator. The LED driver is one of the application examples. In the figure, the digitally controlled potentiometer IC/XgClOdOOkXicor) is used as a brightness adjuster, the dual one-shot trigger IC2 is used as a pulse width modulator, and the three transistors constitute a current mirror switch output circuit. It is not difficult to see that the pulse period of the entire driver depends on the time constant RA, and the pulse width depends on the time constant (RI(:, +R2)C2, and the ratio of the latter to the former is the pulse duty factor.
In applications where the supply voltage is limited, multiple LEDs can only operate in parallel. At this time, according to the traditional design method, it is usually necessary to equip each LED with an independent current limiting resistor, so as to overcome the disadvantage that the LED forward voltage drop consistency is so poor that it cannot be directly connected in parallel; however, the integrated constant current has been used instead. The source is powered by the LED, which eliminates the adverse effects of power supply voltage variations; the MAX1916 (Maxim) and the LX1190 (Microsemi) are two of the more common constant current source integrated circuits.
The device is a three-way current mirror output with a mirror current ratio of 230. The output current of the device or the brightness of the LED can be adjusted in two ways. One is the optional resistor Rskt to set the reference current of the current mirror; the other is The ENABLE terminal applies a pulse width modulated signal with a different duty cycle. The setting range of the reference current is 5260|jlA, which is roughly equivalent to 1~60mA of output per channel; the enable terminal is actually the power input terminal of the on-chip undervoltage lockout, voltage reference and error amplifier, but its power consumption Only for lfxA. The device can operate from 2.55.5V power supply with on-chip thermal protection circuit. When the device is in shutdown state, the whole device consumes less power than lpA. LX1990 has the same function as MAX1916, but it only has two outputs. The circuit is 0.1~30mA, the maximum working voltage is 12V, and the saturation voltage drop is 0.4V. The current setting resistor is powered by the on-chip voltage reference, and the mirror current ratio is 100; the brightness adjustment range of the pulse width modulation signal can reach 500:1. The static and off-state currents are respectively 2 charge pump white LEDs can have a forward voltage drop of up to 4V, but currently a large number of popular handheld electronic devices are powered by a single lithium battery, which is no longer directly driven by the device power supply. LEDs must rely on various types of power conversion devices...
There are many types of power conversion devices that can be used as white LED driver, and the voltage output type charge pump circuit should be the simplest. For typical applications of such devices, the LM3354 switched capacitor converter (NS) can be used as an example. The device's power supply voltage range is 2.55.5V, the output voltage has a range of nominal values ​​to choose from, it is generally considered to be suitable for use as a white LED driver with a nominal output voltage of 4.1V. The switching frequency of the device is 1MHz, so the switching capacitor with smaller capacity can be used; the maximum output current is 90mA, with the on-chip thermal protection circuit, the static and off-state currents are 475jjlA and 5mA respectively. Pulse width modulation to control the brightness of the LED The signal can be sent from the shutdown control (*D) terminal of the device. The repetition frequency should be above 60Hz to avoid flashing of the LED, but it should not exceed 200Hz to ensure that the switched capacitor has sufficient discharge time.
Input voltage control voltage", input power g * wave capacitors LTC3200-5 (LT) and SC600 (Semtech) are also voltage output type charge pump integrated devices. The former is similar to the LM3354 but has a small size and requires only one switching capacitor. The operating voltage is 3~4.4V, the operating frequency is 2MHz, and the output current can reach 00mA. However, the 5V rated output voltage is slightly higher, so the current limiting resistor voltage drop is also Slightly larger, it affects the overall power conversion frequency. The latter includes 4~4.5V/120mA and 455V/60mA multiple output specifications, the power supply voltage range is slightly wider than 2.76.5V; use the "pin programming" mode to switch the operating frequency, and share the welcome in the range of 8~650kHz. Design a new technology file to adjust the output current. Its biggest feature is that when the power supply voltage exceeds a certain threshold, the internal circuit structure is automatically switched to obtain a higher power conversion frequency. The threshold has a "backlash" of SOffiV to prevent the switching from being too frequent and causing a large output ripple.
As mentioned above, the white LED is powered by a constant current source, which is beneficial to suppress the adverse effects caused by the fluctuation of the power supply voltage. At present, there are many current output type charge pump circuits available, and the LTC3202 is one of them. As shown. It can be seen that the current sampling resistor is one of the current limiting resistors, and the feedback threshold voltage can be set to 200, 400 or 600mV by "pin programming" mode (the fourth programming configuration is "off"); Similar to the other devices described above, if the PWM signal is used to intermittently enter the off state, the brightness of the LED can be smoothly adjusted without causing significant "color shift".
The device operates at 1.5MHz, and the maximum output current is 125mA when the power supply voltage is 2.7-4.5V. It is said that it can drive eight white LEDs LE1D at the same time. The structural characteristics are similar to those of the MAX1912 (Max-imh is also limited by the current). The resistor collects the load current signal to form a current control loop, but its maximum output current is only 60mA, the operating frequency is 750kHz, and the feedback threshold is 200mV. Another type of current output type charge pump such as LM2794 (NS) uses current mirror as the output stage, so Eliminating the current-limiting resistor but increasing the power consumption of the device. The LM2794 has four current outputs of 20mA each; if you want to adjust the brightness of the LED, you can still add a pulse-width modulation signal to its shutdown control terminal, or you can use an external current. The mode changes its feedback threshold. The device power supply voltage ranges from 2.7 to 5.5V, and the internal circuit is automatically switched when the voltage exceeds approximately 4.7V.
Mode operation; the switching threshold also has a hysteresis of about 250mV to prevent output ripple deterioration. It is worth mentioning that the device has a physical size of only 2X2.4X0.84mm3, which is considered to be the smallest of its kind. S8813 (Seiko) is similar to the LM2794 structure, but the constant current principle is pulse frequency modulation. This constant current method may have some advantages for the improvement of the conversion efficiency, but it also brings some trouble to the suppression of the switching noise. The typical operating frequency of the device is about 600kHz. The three types of charge pumps have capacitors as the energy storage components. However, there are not many booster switching power supplies as white LED drivers. In fact, such inductors are used. As a device for energy storage components, the former is often superior to the efficiency of voltage boosting. The switching power supply used as the LED driver is mostly current output type. The basic circuit is as shown in the figure, and the working principle can be compared with the MAX1848 (Maxim).
At present, low-power integrated switching power supplies have many on-chip n-channel MOSFETs as power switches, but the energy storage inductors usually still need to be externally connected. Medium and small resistance current sampling resistor converts the load current flowing through the LED into a feedback voltage and feeds it into the on-chip error amplifier. After comparing with the on-chip reference voltage, the pulse width modulation is used to control the on/off of the power switch, thereby making the load The current is kept substantially constant. The switching frequency of MAX.1848 is 1...2MHZ; when operating from 2.6~5.5V power supply, the maximum output voltage can reach 13V, which is enough to drive 2~3 series white LEDs. If necessary, the device can also drive three channels at the same time. Parallel load, each load includes two LEDs connected in series. Of course, three sampling resistors need to be used at this time, and the current matching of each load is not satisfactory.
Another low-power integrated switching power supply, the LT1937 (LT), has an on-chip power switch withstand voltages up to 36V and can drive a single load of six white LEDs in series. The sampling resistor voltage drop is about lmV. Adjusting the resistance value can set the output current to 5~20mA. The brightness adjustment of the LED can still be used to introduce the additional control current at the feedback node or send the pulse width modulation signal to the shutdown control terminal. These two methods.
(Microsemi). The former can work with 2.2-7V power supply, the highest output voltage is 20V, the output current is 20tnA, and the driving capability is two channels of eight white LEDs; the characteristic is that the on-chip power switch peak current can reach 0.5A, and the on-resistance is only 0.7X1. Therefore, the power conversion efficiency is high, and it is easy to solve the problem of heat dissipation of a small package which is quite troublesome. The latter can drive a single four-channel white light design new technology* with 20mA output current. The advantage is that the power supply voltage can be as low as 1.6V. 4 Application development is self-evident, whether it is car, portable or handheld The design of electronic equipment, power efficiency is a must not be ignored. Important issues; however, for white LED driver circuits, the comparison of conversion performance of various integrated devices does not seem so simple. Conventionally, the power conversion efficiency index provided by the device manufacturer is defined as the ratio of the output power to the input power, but the forward voltage drop of the white LED can often vary by hundreds of millivolts, enough to affect its actual power consumption. The ratio of the total output power of the device. In other words, the useless power consumed by the current limiting resistor or the current mirror output tube voltage drop in the white LED driving circuit often has a great influence on the actual power conversion efficiency, and is particularly remarkable when driving devices of different circuit configurations are used. According to this, it is proposed that determining the power input power of the LED when operating at a specific forward current is a reliable method for comparing the conversion efficiency of the driving circuit.
In general, the use of multiple LEDs in series is beneficial to improve the overall efficiency of the driver circuit, but in practice this method will be limited in two ways. First, the maximum output voltage of the driver circuit is usually limited, so the number of LEDs in the same branch cannot be too much. Second, the consistency of white LED products is still unsatisfactory, and usually in the same branch. It is only necessary to use the same production batch of LEDs from the same manufacturer to ensure that the entire illumination area has a sufficiently uniform illumination.
In fact, the room for improving the conversion efficiency of the driving circuit can be said to be quite limited, but the luminous efficiency of the white LED itself seems to have great potential. The so-called white LED actually uses a short-wavelength violet blue light emitted by a blue LED as its die to excite the fluorescent material applied to the inner wall of the output optical lens, thereby generating a white composite light having a broad spectrum. It can be seen that the violet blue light-emitting diode of the white LED, the fluorescent material and even the optical lens can affect the luminous efficiency. Recently, a white LED manufacturer (Nichia) announced that its new generation NSCW215 series has a luminous efficiency that is 50% higher than that of the previous generation. The potential for LED performance improvement is evident. In addition, the company has made progress in the development of high-power LEDs. At present, LEDs with positive forward currents of several hundred milliamperes have been developed, which are competitive in lighting indicator light sources suitable for use as traffic signals. .
In view of the application prospects of white LEDs in the field of LCD backlighting, relevant component products are now available in the market. If there is a MTG-F24064FMNHSGW liquid crystal display unit (WTS), use 240X64 pixel liquid crystal display and T6963C type display controller (Toshiba) and use white light LED as background light source; if equipped with power converter (optional), The whole component can be powered by a single 5V power supply; the effective display area of ​​the LCD screen is 132x39mm2, and the dot pitch is convenient for comparison and comparison. The main technical characteristics of the integrated driving device described herein are listed in the following table.
Category Model Power Supply Voltage (V) Output Voltage (V) Maximum Output Current (mA) Switching Frequency (kHz) Package Relative Price Remarks Constant Current Source Electrical Output Type Load See Current Output Pump Body Climb Switching Power Supply
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