With the rapid development of digital and portable products such as mobile phones, PDAs, DSCs, GPS, etc., as well as the improvement of LCD manufacturing technology. Color LCD displays are increasingly becoming an important part of digital and portable products. Color screen mobile phones, PDAs, etc. are also more and more popular and favored by people. A color LCD display needs to use white light as a backlight for full color display. It is ideal for small color LCD displays to use white LEDs as the backlight source. This is because it has the characteristics of simple circuit, small space, high efficiency, low price and long life. The forward voltage VF of the white LED is higher than that of the general colored LED, VF=3∽4V. The white LED backlight power supply generally consists of several white LEDs. For example, mobile phones and digital cameras generally only need 2 to 4 white LEDs, while PDAs need 3 to 6 according to the area of ​​their display screens. The requirements for the backlight source are: to meet the brightness requirements of the backlight and the brightness is uniform (no part of the light is allowed to be bright and the other part is dark), and the brightness can be easily adjusted. However, the battery voltage of portable electronic products (such as PDAs, GPS, digital cameras, new-generation mobile phones, etc.) is generally lower than 3-4.2V, so a boost power supply is required to drive the white LED.
There are basically two driving methods for white LED drivers:1) Adopt boost charge pump drive circuit, because its output voltage is less than twice the input voltage (generally output voltage +5V), so it drives LEDs in parallel, as shown in Figure 1-a;
2) It is driven by a step-up DC/DC converter circuit. Since its output voltage ranges from 13V to 28V (or even higher), the way it drives LEDs is in series, as shown in Figure 1-b. The series connection makes the current flowing through each LED the same. The uniformity of light emission is good; while the parallel connection method is affected by the discreteness of the forward voltage drop VF, and it is easy to cause unequal currents of each LED when supplying power at a constant voltage, resulting in poor uniformity of light emission. Although you can adjust the current-limiting resistor to adjust the brightness. But this is very troublesome. Some manufacturers have developed ICs that drive LEDs connected in parallel, because the IC has a constant current between the LEDs connected in parallel. It can meet the requirements of LED brightness uniformity. However, due to the complexity of the IC circuit, the cost is relatively high.
1-a Parallel LED driver
1-b Series LED driver
With the development of portable color LCD display products and the rapid increase in output, new white LED drivers have sprung up on the market. The variety and quantity of white LED drivers have formed an important branch of power ICs. Now portable products are getting cheaper and the competition is getting fiercer. Therefore, for manufacturers such as mobile phones, PDAs, and DSCs, cost down pressure is also greater. Therefore, in response to market demand, BCD recently introduced a low-cost, high-performance, constant-current, series-type boost white LED driver-AP3008. In terms of function and performance, it can meet the requirements of LCD backlight uniformity, and in terms of price, it can meet the low-cost requirements of customers. It is the most cost-effective product among the current white LED drivers.
The characteristic of series-connected LEDs is that the current ILED of each LED is the same, so when high-quality LEDs of the same batch number are used, their luminescence is more uniform. Here take AP3008 that BCD company launched in August 2005 as an example to illustrate its working principle. And introduce its application circuit.
Figure 2 AP3008 internal structure block diagram
The internal structure block diagram of AP3008 is shown as in Fig. 2. AP3008 is a step-up DC-DC converter in current mode. It consists of 1.25V reference, error amplifier, 1.2MHZ oscillator and ramp generator, current sampling amplifier circuit, PWM comparator, R/S trigger, output driver and power switch tube, overvoltage protection (OVP), etc. .
AP3008 has a built-in under-voltage lockout (UVLO) circuit to make the circuit work more reliable. Has a wide operating voltage range. The lowest working voltage can reach 2.5V, and the highest working voltage can reach 15V.
AP3008 has a built-in error amplifier frequency compensation network and a slope compensation circuit of the current loop, which makes the circuit work more stable and has fewer peripheral components, which is very convenient for users.
The over-voltage 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 over-voltage protection (OVP) circuit operates and shuts down the PWM switching circuit. If you make the circuit work again, you can apply a signal from low to high on the Shutdown pin or Vin pin. This method of monitoring the peak voltage of the PWM pulse inside the IC to achieve OVP protection is better than the method of comparing the external feedback output voltage with the internal setting value of the IC. The built-in OVP function not only saves a feedback line. It also ensures that the feedback signal is free from interference.
The reference voltage at the non-inverting end of the error amplifier is 95mv, which can effectively reduce the loss of the LED current sampling resistor and improve the output efficiency.
The current ILED flowing through the LED is related to the detection current resistance R1 and the reference voltage of 95mv. Their relationship is:
ILED=95mv/R1The current of ILED is generally 5 ∽ 20 mA, and the corresponding resistance R1 is 19 ∽ 4.7 Ω.
What I want to put forward here is that its reference voltage is not directly 1.25V, but 95mv after being divided by resistance, which is a measure to reduce loss. It can be seen from Figure 1-b that the current ILEDX95mv flowing through the LED is the loss on the resistance (the feedback voltage on R1 is 95mv), which is reduced from 1.25v to 95mv (a difference of 13 times), that is, the loss on R1 is reduced 13 times smaller. Improve the conversion efficiency.
In addition, due to the high frequency 36V Bipoler process adopted by AP3008, its withstand voltage is much higher than that of boost DC/DC converters made by some CMOS processes.
image 3
Figure 4
The application circuit of AP3008 is shown as in Fig. 3. A feature of this circuit is that when the input is 3.2V, it can drive 4 white LEDs, the output current can reach up to 20mA; when the input is 5V, it can drive 6 white LEDs, the output current can reach up to 20mA, and its conversion efficiency As shown in Figure 4, when ILED=15mA, the efficiency can reach 83%.
Measures to reduce the space occupied by the printing plateNew portable electronic products require small size, lightness and thinness. The backlight circuit composed of white LED drivers must also meet these requirements. AP3008 is designed to increase the oscillator frequency to 1.2MHZ, so that the inductor can use 1mm low profile height stickers. Chip components, input capacitors, and output capacitors can use 1uF and 0.22uF capacitors respectively; in addition, the product is packaged in SOT23-5 and TSOT23-5, which can make the height of the IC less than 1.1mm and 0.9mm; it can boost the AP 3008 The space occupied by the circuit and the area of ​​the PCB board are greatly reduced.
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