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In recent years, LEDs have been greatly developed in backlight applications, not only in the backlight applications of small-size LCD displays, but also in the mid-size LCD display backlight applications that require higher performance and longer working hours. Such as GPS, P-DVD, digital photo frame (DPF), and netbooks.
LED backlight technology has many advantages, including longer life and lower power consumption. More importantly, LEDs are fully environmentally friendly and will not cause environmental pollution.
LEDs are current-based devices that require a dedicated driver IC whose brightness is proportional to the current flowing through it. The current used to control the LED by the driver IC generally has two methods: a constant voltage source and a constant current source. The former cannot accurately control the brightness of the LED, while the latter can well avoid the influence of the discreteness of the LED, thereby accurately controlling the brightness of the LED. The constant current source control mode is currently the mainstream mode of LED driving. The following is an introduction to the LED backlight drive circuit of Wuxi Xinpeng Microelectronics Co., Ltd.
Linear constant current driving, also known as LDO current sharing driving, is the lowest cost and has no EMI problem, but its main problem is that the brightness will change significantly when the battery voltage drops below a certain value. Therefore, this driver IC method is generally used in low cost portable products. The core AP3600, AP3601, AP3602, etc. are the driver ICs.
The charge pump mixed mode drive solves the problem that the output current drops significantly when the battery voltage is relatively low. Depending on the input voltage, the chip operates in linear, 1.5X or 2X mode. This method has the advantages of wide input voltage range, no inductance, small size, etc., and is widely used in small LCD screens such as mobile phones. These driver ICs include AP3324, AP3325, etc. of the core microelectronics.
Compared with the charge pump mode, the PWM constant current drive mode can obtain a higher voltage than the input, and has a higher drive capability, can support a larger number of LEDs, and can be applied to larger products (such as GPS, P-DVD). LCD backlight modules for products such as DPF and netbooks. The IP3127, AP3293, etc. of the core microelectronics are PWM constant current drive ICs.
The AP3127 and AP3293 are step-up backlight LED driver circuits designed for medium-sized LCD screens. They use an operating frequency of up to 1MHz, allowing the use of small external inductors and capacitors. The LEDs are connected in series to ensure the same current flowing through each LED for consistent brightness. The 250mV feedback voltage minimizes the power dissipated in the current setting resistor, optimizing the efficiency of the white LED driver application as much as possible.
The AP3127's switching transistor has a peak current of up to 500mA and can withstand voltages up to 22V. It can drive a string of 6 LEDs and three strings of up to 21 LEDs in a SOT23-6L package.
With a peak current of up to 1,000mA, the AP3293 can withstand voltages up to 40V, driving a string of 10 LEDs and three strings of up to 24 LEDs in a SOT23-6L or WDFN8 package.
Since the feedback voltage of the AP3127/3293 is only 250mV, the power consumption on the resistor is small, the efficiency is greatly improved, and the battery life is prolonged.
Since the output current is directly controlled instead of the voltage, when the load resistance becomes large, the output voltage must be increased to maintain a constant current. Once the load is open, the output voltage rises immediately until it exceeds the maximum allowable voltage of the chip, breaking through the chip. Conventional driver chips typically require a Zener diode to be placed in parallel with the LED at the output to limit the output voltage. The AP3127/3293 is designed with open-circuit protection in mind and incorporates an overvoltage protection circuit that eliminates the need for external diode protection.
In addition, the chip supports PWM dimming, and uses the visual pause principle of the human eye to periodically control the on-current of the white LED to switch back and forth between zero current and rated operating current with a certain frequency and duty ratio, thereby adjusting brightness. The advantage of this dimming method is that it does not produce a color shift compared to the dimming method that changes the magnitude of the drive current. In the application, in order to ensure that the LED is not visible to the human eye, the PWM dimming frequency is generally greater than 100 Hz. The AP3127/3293 supports a dimming frequency range of 20 Hz to 100 kHz, which greatly facilitates system design.
For 10-inch to 12-inch netbooks, Chippen Microelectronics is developing the AP3295. Its input voltage is 4.5V~24V, which makes the LED backlight module compatible with the original CCFL input voltage or directly powered by battery or adapter. Thereby reducing the power conversion link and improving the overall efficiency of the system. This solution also supports PWM dimming. Because the interface is compatible, users only need to change their backlight from CCFL tube to aluminum-based LED strip, and replace high-voltage complex AC driver with low-voltage simple DC driver. Its overall cost is very close to the traditional CCFL backlight module.
The AP3127/3293/3295 feature soft-start, over-voltage, under-voltage, and over-temperature features, all in an ultra-small package. 
December 15, 2023