【 Weak current College 】 negative charge pump white LED driver design 】

Overview
The current number of portable consumer electronics such as cell phones, PDA, MP3 player, notebook, with a display screen, although different kinds of application for the display and size will vary, but for the majority of designers who need to design the back light circuit. White led is considered a small handheld device color display backlight of ideals.
Drive white LEDs is the simplest way to use the voltage source is driven by a ballast resistor LED. The drive mode has the advantage that select the voltage source is very large, regulator and LED between only need one connection endpoint. But the disadvantages are also clear: first, inefficient, and this is mainly due to the loss of ballast resistor; second, the LED current of steady flow capacity, control is not accurate, because the temperature drift, and LED the mismatch LED forward voltage changes, will make the final of the led current produces a larger change, thus affecting the backlight brightness control.
Therefore, the ideal way of white LED driver is the use of constant current driver, it can avoid white led as drift caused by current fluctuations, or does not match the result of the LED brightness, can produce a controllable LED forward current. This drive does not require output stabilized voltage, simply control the current flows through the LED can be controlled, constant brightness control.

Comparison of common topology
LED luminous intensity and flows through the LED current, current, the higher the intensity. Common digital cameras and cell phones in general needs 2 to 3 LEDs for backlighting, you generally need a PDA in 3 to 6 LED backlight. By way of parallel or series of LED drivers, both of these methods has advantages and disadvantages: tandem programme led current, circuit control simple, but requires a higher driving voltage; parallel programme of relatively simple circuit, and the required driving voltage is lower, but LED higher number, you need more than one control channel, while current of consistency is also poor.
LED drive on from the topology, there are mainly based on the inductance of DC/DC drives and based on capacitive charge pump drive, of course, there are few LED drive adopts linear regulators-driven architecture. Since the drive based on inductance can provide a wider range of output voltage, high efficiency, so in many designs are based on the inductance of the drive structure driven multiple-line LED. But based on capacitive charge pump drive eliminates the desired external inductor, is of small size, simple design and low cost comparison of characteristics, is also popular. Because based on the charge pump led driver can only produce multiples of the input voltage (e.g. 1.5 times, twice), limited driving voltage based charge pump led driver used in multiple parallel drive LED. As regards the use of linear regulator for led driver framework, because less efficient, and can only work to the buck, so the application range is limited and cannot be used in a single-cell Li + battery-powered handheld devices. This article focuses on using inductively DC/DC drives and based on capacitive charge pump drive both common topologies.
In order to meet the application requirements of portable products, MAXIM offers a variety of topologies of led drivers, including on the basis of inductance of the led drive to MAX1553-MAX1554 as representative, also including MAX1561, MAX1582 etc devices; and based on capacitive charge pump drives to MAX1570 as representative, other products have MAX1576 MAX1575, etc.
MAX1553-MAX1554 is an efficient, 40V boost converter, which can be used to drive 2-10 series white led for cellular phones, PDAs and other handheld devices high-efficiency backlight display. The synchronous boost converter with built-in 40V, low RDSON n-Channel MOSFET switch, greatly improving the conversion efficiency and effective to extend battery life. The device has two analog/PWM mode of brightness adjustment method, independent of the input can also be used for on/off control. Soft-start function can inhibit the startup process of the surge current. The device also has adjustable overvoltage protection circuit that detects output over-voltage, you can turn off the internal MOSFET, thereby reducing the output voltage.
MAX1570 fractional charge pump capable of constant current drive of up to 5 white LED, to obtain a uniform brightness. MAX1570 use 1 x/1.5 fractional charge pump and low-current regulator, in the whole Li + battery voltage range to maintain maximum efficiency. MAX1570 work in 1MHz fixed frequency, allows the selection of small external components. Optimized for current regulation structure guarantee low EMI and low input ripple. Devices you can use an external resistor sets the full LED currents, two digital input control on/off or choose three levels of brightness in the level. Devices also use pulse width modulation (PWM) signal conditioning LED brightness.
The LED drive based inductance and capacitance of charge pump led driver circuit structure compared to the more complex; power inductor selection on circuit performance impact is large, for many of the designers is a difficult; besides inductance volume are large, compared to the footprint board space. Based on capacitive charge pump led driver only needs a few relatively simple capacitance, design, saving board space. However, based on the inductance of led driver with charge pump led driver in terms of efficiency, compared with more obvious advantages of led current MAX1553 within basic can keep 80% efficiency (see Figure 4a) and efficiency with the current change to the smaller fluctuations, while MAX1570 charge pump led driver of efficiency in the work of the LED current range have greater volatility, and the efficiency of light will be less than 80%.
Visible, based on capacitive charge pump led driver while saving design simple, board space, etc., but it's relatively low efficiency often limits the use of the device, especially for efficiency-sensitive applications such asHandheld's cell phone, PDA and other products, people often want the battery has a sufficient amount of battery life. In response to this demand, MAXIM introduced a new type of negative charge pump led driver, and traditional positive charge pump led driver, the device efficiency.