In mobile phones and other mobile devices, white LEDs can provide perfect backlight for small-size color screens. However, most mobile phones use a single lithium battery for power supply, and it is difficult for a single lithium battery to directly drive white LEDs. Generally, the working voltage range of lithium battery is 3 ~ 4.2V, while the conduction voltage drop of white LED is 3.5 ~ 4.2V (20mA). Therefore, the white LED cannot be directly driven after the lithium battery voltage is reduced.

In order to provide sufficient forward voltage drop for white LED, capacitor based charge pump or inductance based boost circuit can be used. Considering efficiency and battery life, inductance based converter may be the best choice, but additional inductance will increase the system cost. Moreover, due to EMI and RF interference, inductive boost circuit needs careful design and layout. In contrast, the charge pump solution has the advantages of low price and easy use, but it has low efficiency and shortens the service life of the battery.

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

How to use charge pump design technology to improve the efficiency of white LED

Fractional charge pump and its effect on efficiency

The basic architecture of the first generation white LED driven charge pump is double voltage or 2x topology. The working efficiency of the double voltage charge pump is:

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

Where IQ is the static current of the circuit, because IQ is very small, the above formula can be approximately equivalent to:

PLED/PIN≈VLED/(2VIN)

In order to improve efficiency, the output of the second generation white LED driven charge pump is no longer an integral multiple of the input voltage. If the battery voltage is sufficient, the LED driver will produce 1.5 times the voltage output, and the conversion efficiency of the 1.5 times the voltage charge pump is:

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

It can be seen from the above formula that the efficiency of 1.5 times the pressure charge pump is significantly improved. Assuming that the battery voltage is 3.6V and the LED voltage is 3.7V, the efficiency increases from 51% of the double voltage charge pump to 69%.

The double voltage mode introduced by the third generation charge pump further improves the efficiency. When the battery voltage is high enough, the battery is directly connected to the LED through a low-voltage differential current regulator. At this time, the efficiency can be expressed by the following formula.

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

When the battery voltage is sufficient to drive the white LED, the efficiency of the voltage doubling mode exceeds 90%. If the battery voltage is 4V and the LED on voltage drop is 3.7V, the efficiency can reach 92%.

Maximum efficiency at different battery voltages

The 1-fold voltage conversion mode has the highest efficiency, but can only be used when the battery voltage is higher than the led forward voltage drop. In order to obtain the highest efficiency, the design of white LED driver requires comprehensive consideration of battery and led voltage. When the battery voltage (or LED voltage) changes, the working mode of the driver needs to be changed accordingly. However, if the operating mode is changed when the battery voltage is high (rather than under necessary conditions), the switching loss may put the circuit into an inefficient mode. When the battery voltage drops, it is best to keep the driver in the efficient mode (e.g. 1-fold voltage mode) as much as possible. For the power switch, in order to obtain low loss, the chip area and cost will be increased.

In order to keep the voltage doubling mode working at the lowest possible battery voltage, the voltage drop of the voltage doubling mode regulator FET and current regulator should be reduced as much as possible. The voltage drop determines the series loss and the minimum input voltage of the voltage doubling mode that can be maintained. The minimum input voltage is expressed by the following formula:

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

The traditional positive charge pump white LED solution uses PMOS FET as a bypass switch to connect the battery and led. The on resistance RDS (on) of FET is about 1 ~ 2 Ω. 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.

Source; International LED network

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