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Charge Pump White Led Backlight Use Reduces The Cost And Size Of The Drive

Mobile phones and other mobile devices, white LED Can provide the perfect small size color backlight effect. However, most mobile phones use a single lithium Battery Power, while a single Li Battery Difficult to directly drive white LED. Usually operating voltage range of lithium battery 3 ~ 4.2V, while the white LED's on-voltage drop is 3.5 ~ 4.2V (20mA). Therefore, the lithium battery voltage drops will not directly drive white LED.

Order to provide enough white LED forward voltage drop, you can use based on Capacitance Charge pump or inductive boost circuit based. Taking into account efficiency and battery life, inductor-based converters may be the best option, but the extra inductance will increase system cost. Moreover, since the EMI and RF interference, Inductive boost circuit requires careful design and layout. In contrast, charge pump solution is cheap, easy to use and other advantages, but less efficient and shorten the battery life.

Charge pump design with improved technology, the new white LED driver Chip , Chip companies such as Maxim, not only can get the efficiency inductive boost circuit (about 85%), and can maintain the traditional charge pump design is simple, low-cost advantages.

Fractional charge pump and its efficiency The first generation of white LED drivers

the basic framework is charge-pump voltage multiplier or 2x topology, two times the charge pump's efficiency is:

PLED / PIN = VLED × ILED / (2 × VIN × ILED + Iq × VIN)

Which, Iq is the quiescent current, because the Iq is very small, the type can be approximately equivalent to:

PLED / PIN VLED / (2VIN) Order to improve efficiency, the second generation of the output charge pump white LED driver is no longer an integer multiple of the input voltage. If the battery voltage is sufficient, LED drive will generate 1.5 times the voltage output, 1.5 times the charge pump conversion efficiency of:

PLED / PIN = VLED × ILED / (1.5 × VIN × ILED + Iq × VIN) VLED / (1.5VIN)

Is apparent from the above formula: 1.5 times the charge pump efficiency significantly increased. Suppose the battery voltage of 3.6V, LED voltage of 3.7V, the efficiency of charge pump from 2 times 51% to 69%.

The introduction of a third-generation charge pump voltage doubler mode to further improve efficiency. When the battery voltage is high enough, by a low dropout current regulator connected directly to the battery to the LED, this time, the efficiency can be expressed.

PLED / PIN = VLED × ILED / (VIN × ILED + Iq × VIN) VLED / (VIN)

When the battery voltage is sufficient to drive white LED, 1 times the voltage mode efficiency of over 90%. If the battery voltage of 4V, LED turn-on voltage drop is 3.7V, the efficiency of up to 92%.

At different battery voltages for maximum efficiency

1 times the voltage conversion mode of the most efficient, but only for the battery voltage is higher than the LED forward voltage drop of the case. In order to obtain maximum efficiency, white light LED driver design requirements taken into account and LED battery voltage, when the battery voltage (or LED voltage) changes need to drive a corresponding change in work pattern. However, if the battery voltage is higher (but not necessary conditions) to change the mode of Switch Loss may make the circuit into a low efficiency mode. When the battery voltage drops, it is best to keep as much as possible to drive at high mode (for example, a double voltage mode), for power Switch , The low loss in order to get the chip area and cost will increase.

Times in order to maintain a pressure mode to work in the battery voltage as low as possible to minimize the adjustment of a double voltage mode control FET and the current regulator pressure drop. Decided to drop the series loss and can maintain a minimum Voltage mode input voltage. Minimum input voltage is expressed as follows:

VLED + Bypass PFET RDS (ON) × ILED + VDROPOUT

Traditional charge pump white LED solution is used as a bypass switch connected PMOSFET battery and LED, as shown in Figure 1. FET on-resistance RDS (ON) is about 1 ~ 2 . Smaller resistance will be limited by chip area and cost. The smaller resistance, larger chip area, the higher the cost.

When the input voltage is low, positive pressure charge pump produces 1.5 times or 2 × VIN voltage output to drive the white LED's anode. In order to pressure the use of a double voltage charge pump structure, we must use an internal bypass switch VIN and white LED's anode. When the input voltage is low, the negative pressure generated-0.5VIN output charge pump to drive the white LED's cathode. Working in a double voltage mode, the vacuum structure does not require bypass-0.5VIN charge pump to ground, because the current regulator direct control of the LED current flows from VIN GND. 1 times the pressure of this model extends the operating voltage: VLED + VDROPOUT

Figure 2 shows a voltage doubler mode, Negative Charge Pump current path, there is no P-channel MOSFET bypass switch, WLED regulate current flow directly through the VIN GND. If ILED total current 100mA, P-channel MOSFET on-resistance is 2 , the bypass switch voltage drop is 200mV. Because lithium primary work in the 3.6 ~ 3.8V, the typical battery discharge curve of Li +, 200mV dropout, a Negative Charge Pump Voltage mode can significantly improve efficiency.

LED forward voltage drop at different maximum efficiency obtained

1 times the pressure of the traditional / 1.5 times the pressure is charge pump white LED driver, LED's anode connected to the charge pump output. If the LED does not match, that guide each LED's voltage drop is not the same time, if (VIN-VLED) sufficient to support the maximum voltage drop is the wizard, it will switch to 1.5 times the drive voltage mode. This case may only have one LED turn-on voltage can not meet the requirements of the charge pump must give up the high pressure mode 1 times. Negative charge pump is different, you can choose a multi-channel switch times were -0.5 times the voltage mode or voltage mode. Therefore, if a LED requires a higher pressure drop, you do not need all the channels to -0.5 times the pressure mode. For example, MAX8647/48 drive, when the input voltage can not drive up the LED turn-on voltage, the only open the required negative pressure charge pump LED driver channel, other times the LED remains a pressing mode. Independent LED switch at different times, under different VIN switch to -0.5 times the pressure mode.

Conclusion Negative Charge Pump White LED

drivers were switched to working mode of each channel, with a double voltage / 1.5x charge pump LED driver programs are compared, significantly improve efficiency, shown in Figure 3.

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