Charge Pump Design Technique Reduces Cost and Size of White LED Backlight Drivers

In mobile phones and other mobile devices, white LEDs can provide perfect backlighting for small 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, when the lithium battery voltage is reduced, it will not be able to directly drive white LEDs.

In order to provide enough 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 increases system cost. Moreover, due to EMI and RF interference, the inductor-based boost circuit requires careful design and layout. In comparison, the charge pump solution has the advantages of being cheap and easy to use, but it is less efficient and shortens the battery life.

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

Fractional Charge Pumps and Their Impact on Efficiency

The basic architecture of the first generation of white light LED driver charge pumps is a voltage doubling or 2x topology. The operating efficiency of a 2x charge pump is:

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

Among them, Iq is the static current of the circuit. Since 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 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:

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

It is obvious from the above formula that the efficiency of the 1.5x charge pump is significantly improved. Assuming the battery voltage is 3.6V and the LED voltage is 3.7V, the efficiency is increased from 51% of the 2x charge pump to 69%.

The 1x voltage mode introduced by the third generation charge pump further improves efficiency. When the battery voltage is high enough, the battery is directly connected to the LED through a low dropout 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 1x mode exceeds 90%. If the battery voltage is 4V and the LED conduction voltage drop is 3.7V, the efficiency can reach 92%.

Obtaining the highest efficiency at different battery voltages

The 1x voltage conversion mode is the most efficient, but it can only be used when the battery voltage is higher than the forward voltage drop of the LED. In order to obtain the highest efficiency, the design of white light LED drivers requires comprehensive consideration of the battery and LED voltages. When the battery voltage (or LED voltage) changes, the driver's operating mode needs to be changed accordingly. 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 a low-efficiency mode. When the battery voltage drops, it is best to keep the driver in an efficient mode (such as 1x voltage mode) as much as possible. For power switches, 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 adjustment tube FET and current regulator should be reduced as much as possible. 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:

VLED+Bypass PFET RDS(ON)×ILED+VOPOUT

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.

When the input voltage is insufficient, the positive charge pump generates an output of 1.5 times the voltage or 2×VIN to drive the anode of the white light LED. In order to use a 1x voltage structure in the positive charge pump, we must use an internal switch to bypass VIN and the anode of the white light LED. When the input voltage is insufficient, the negative charge pump can generate a -0.5VIN output to drive the cathode of the white light LED. When operating in 1x voltage mode, the negative charge pump structure does not need to bypass -0.5VIN to ground because the current regulator directly controls the LED current to flow from VIN to GND. This expands the operating voltage of the 1x voltage mode: VLED+VDROPOUT

Figure 2 shows the current path of the negative charge pump in 1x mode. There is no P-channel MOSFET bypass switch, and the WLED regulation current flows directly into GND through VIN. If the total ILED current is 100mA and the on-resistance of the P-channel MOSFET is 2Ω, the bypass switch voltage drop is 200mV. Because lithium batteries mainly operate at 3.6~3.8V, for a typical Li+ battery discharge curve, a 200mV voltage drop, 1x mode negative charge pump can significantly improve efficiency.

Get the 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 will switch 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. Independent LED switches can switch to -0.5x mode at different times and different VIN.

in 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 driving solution, as shown in Figure 3.