How to design a negative charge pump white LED driver?
Currently, many portable consumer electronic products, such as mobile phones, PDAs, MP3 players, notebooks, etc., have displays, although different applications may vary in the type and size of the display, but for the majority of designers, A backlight circuit needs to be designed for it. White LEDs are considered an ideal backlight for color displays in small handheld devices.
The easiest way to drive a white LED is to use a voltage source to drive the LED through a ballast resistor. The advantage of this type of drive is that there is a lot of room for selecting a voltage source, and only one connection end point is required between the regulator and the LED. But the shortcomings are also obvious: First, the LED current has poor current-stable capability, and the control is inaccurate. The change of the LED forward voltage due to temperature drift and LED mismatch will cause a large change in the final LED current. Controlling the brightness of the backlight; second, the efficiency is low, which is mainly caused by the loss of ballast resistance.
Therefore, the ideal white LED driving method is to use constant current driving, which can avoid the current fluctuation caused by the temperature drift of the white LED, or the uneven brightness caused by the LED mismatch, can generate a controllable LED forward current. At this time, the driver does not need to output a stable voltage, and only needs to control the current flowing through the LED to be constant, thereby achieving controllable brightness control.
Common topology comparison
The luminous intensity of the LED is related to the current flowing through the LED. The higher the current, the higher the light intensity. Two to three LEDs are typically used as backlights in common digital cameras and cellular phones, while three to six LED backlights are typically required in PDAs. LEDs can be driven in parallel or in series. These two methods have their own advantages and disadvantages: LED current is consistent in the series scheme, circuit control is simple, but high driving voltage is required; parallel circuit is simpler, required driving voltage It is also low, but when there are a large number of LEDs, multiple control channels are required, and the current consistency is also poor.
LED drivers are divided into topological structures, which can be divided into inductor-based DC/DC drivers and capacitor-based charge pump drivers. Of course, there are also a few LED driversthat use a linear regulator drive architecture. Since inductor-based drivers can provide a wide range of output voltages with high efficiency, inductor-based driver structures are used to drive multiple series LEDs in many designs. The capacitor-based charge pump driver eliminates the need for external inductance and is popular because of its small size, simple design, and low cost. Since charge pump-based LED drivers can only produce multiples of the input voltage (eg, 1.5x, 2x), the limited drive voltage allows charge pump-based LED drivers to be used to drive multiple LEDs in parallel. As for the LED driver architecture with linear regulator, due to its low efficiency and only working under buck conditions, the application range is limited and cannot be used for handheld devices powered by a single-cell Li+ battery. This article focuses on two common topologies for inductor-based DC/DC drivers and capacitor-based charge pump drivers.
To accommodate the needs of portable applications, MAXIM offers a wide range of topological LED drivers, including inductor-based LED drivers, represented by the MAX1553-MAX1554, including the MAX1561, MAX1582, and capacitor-based charge-pump drivers. The MAX1570 is representative, and other products include the MAX1575, MAX1576, and others.
The MAX1570 fractional charge pump can drive up to five white LEDs at a constant current for uniform brightness. The MAX1570 utilizes a 1x/1.5x fractional charge pump and low dropout current regulator to maintain maximum efficiency over the entire Li+ battery supply voltage range. The MAX1570 operates at a fixed 1MHz frequency, allowing the use of small external components. The optimized current regulation structure ensures low EMI and low input ripple. The device can use an external resistor to set the full-scale LED current, and the two digital inputs control on/off or select one of the three levels of brightness. The device can also use a pulse width modulation (PWM) signal to adjust the brightness of the LED.
The MAX1553-MAX1554 is a high-efficiency, 40V boost converter that can drive 2-10 series white LEDs to provide a highly efficient backlit display for cellular phones, PDAs and other handheld devices. The boost converter features a 40V, low RDSON N-channel MOSFET switch that greatly increases conversion efficiency and extends battery life. The device features an analog/PWM mode of brightness adjustment, and an independent enable input can be used for on/off control. The soft start function effectively suppresses the inrush current during the startup process. The device also features an adjustable overvoltage protection circuit that turns off the internal MOSFET when an output overvoltage is detected, reducing the output voltage.
The inductor-based LED driver is more complicated than the capacitor-based charge pump LED driver. The selection of the power inductor has a great influence on the circuit performance, which is a difficult point for many designers. In addition, the inductor volume is also large, it takes up space on the board. Capacitor-based charge pump LED drivers require only a few capacitors and are simple to design, saving board space. However, the inductor-based LED driver has a significant advantage in terms of efficiency compared to the charge pump LED driver. The MAX1553 can maintain approximately 80% efficiency over the LED operating current range, and the efficiency fluctuates less with current. The efficiency of the MAX1570 charge-pump LED driver has large fluctuations in the operating current range of the LED, and the efficiency at light loads is less than 80%.