Using Charge Pumps to Reduce Cost and Size of White LED Backlight Drivers

In mobile phones and other mobile devices, white LEDs can provide perfect backlighting effects for small-sized color screens. However, most mobile phones are powered by a single lithium battery, which is difficult to directly drive white LEDs. Usually, the operating voltage range of lithium batteries is 3 to 4.2V, while the on-state voltage drop of white LEDs is 3.5 to 4.2V (20mA). Therefore, after the lithium battery voltage is reduced, it will not be able to directly drive white LEDs. In order to provide sufficient forward voltage drop for white LEDs, a capacitor-based charge pump or an inductor-based boost circuit can be used. Considering efficiency and battery life, an inductor-based converter may be the best choice, but the additional inductor will increase system cost. Moreover, due to EMI and RF interference, inductor-based boost circuits require careful design and layout. In comparison, charge pump solutions have the advantages of being cheap and easy to use, but they are less efficient and shorten battery life.

With the improvement of charge pump design technology, new white LED driver chips, such as chips from companies such as Maxim, can not only obtain the efficiency of inductor boost circuits (about 85%), but also maintain the advantages of simplicity and low cost of traditional charge pump design.

Fractional charge pump and its impact on efficiency

The basic architecture of the first generation of white light LED driver charge pump is the voltage doubling or 2x topology. The working efficiency of the 2x charge pump is:
Among them, Iq is the static current of the circuit. Since Iq is very small, the above formula can be approximately equivalent to:
In order to improve efficiency, the output of the second-generation white light LED driver charge pump is no longer an integer multiple of the input voltage. If the battery voltage is sufficient, the LED driver will produce a 1.5 times voltage output, and the conversion efficiency of the 1.5 times voltage charge pump is:
从上式可明显看出：1.5倍压电荷泵的效率显著提高了。假设电池电压为3.6V，LED电压为3.7V，效率从2倍压电荷泵的51％提高到69％。
第三代电荷泵引入的1倍压模式进一步提高了效率。当电池电压足够高时，通过一个低压差电流调节器直接将电池连接到LED，此时，效率可以通过下式表示。
When the battery voltage is sufficient to drive the white LED, the efficiency of the 1x mode is over 90%. If the battery voltage is 4V and the LED conduction voltage drop is 3.7V, the efficiency can reach 92%.
Get the highest efficiency at different battery voltages

The 1x conversion mode has the highest efficiency, but it can only be used when the battery voltage is higher than the LED forward voltage drop. To achieve the highest efficiency, the design of white LED drivers requires a comprehensive consideration of the battery and LED voltages, and the driver's operating mode needs to be changed accordingly when the battery voltage (or LED voltage) changes. However, if the operating mode is changed when the battery voltage is high (not under necessary conditions), switching losses may cause the circuit to enter an inefficient mode. When the battery voltage drops, it is best to keep the driver in an efficient mode (such as 1x mode) as much as possible. For the power switch, in order to obtain low losses, the chip area and cost will increase.

In order to keep the 1x mode working at the lowest possible battery voltage, the voltage drop of the 1x mode pass tube FET and current regulator should be minimized. The voltage drop determines the series loss and the minimum input voltage that can be maintained in the 1x mode. The minimum input voltage is expressed by the following formula:
The traditional positive charge pump white LED solution uses PMOS FET as a bypass switch to connect the battery and LED, as shown in Figure 1. The on-resistance RDS(ON) of FET is about 1~2Ω. Smaller on-resistance will be limited by chip area and cost. The smaller the on-resistance, the larger the chip area and the higher the cost.
当输入电压不足时，正压电荷泵产生1.5倍压或2×VIN的输出，用来驱动白光LED的阳极。为了在正压电荷泵中采用1倍压结构，我们必须使用一个内部开关旁路VIN和白光LED的阳极。当输入电压不足时，负压电荷泵能够产生-0.5VIN输出，驱动白光LED的阴极。工作在1倍压模式时，负压电荷泵结构不需要旁路-0.5VIN到地，因为电流调节器直接控制LED电流从VIN流入GND。由此扩展了1倍压模式的工作电压：VLED+VD R O POUT

图2显示了1倍压模式下负压电荷泵的电流路径，没有P沟道MOSFET旁路开关，WLED调节电流直接通过VIN流入GND。如果ILED总电流为100mA，P沟道MOSFET的导通电阻为2Ω，则旁路开关压降为200mV。因为锂电池主要工作在3.6～3.8V，对于典型的Li+电池放电曲线，200mV压差、1倍压模式的负压电荷泵可以显著提高效率。
Achieving Highest Efficiency at Different LED Forward Voltage Drops In
traditional 1x/1.5x positive charge pump white LED drivers, the LED anode is connected to the charge pump output. If the LEDs are mismatched, that is, the forward voltage drop of each LED is different, if (VIN-VLED) is not enough to support the maximum forward voltage drop, the driver switches to 1.5x mode. In this case, only one LED may not meet the forward voltage requirement, and the charge pump must abandon the efficient 1x mode. Negative charge pumps are different. They can select 1x mode or -0.5x mode through multiplex switches. Therefore, if a certain LED requires a higher voltage drop, it is not necessary to switch all channels to -0.5x mode. For example, the MAX8647/48 driver only turns on the LED channel that needs to be driven by the negative charge pump when the input voltage cannot drive the LED with the highest forward voltage, and the other LEDs remain in 1x mode. Individual LED switches can switch to -0.5x mode at different times and different VIN.

Conclusion

The negative charge pump white light LED driver can switch the working mode of each channel separately, which significantly improves the working efficiency compared with the 1x/1.5x positive charge pump LED driver solution, as shown in Figure 3.