To provide sufficient forward voltage drop for white LEDs, a capacitor based charge pump or an inductor based boost circuit can be used. Inductor-based converters may be the best choice for efficiency and battery life, but additional inductors increase system cost. Moreover, inductive boost circuits require careful design and layout due to EMI and RF interference. In contrast, charge pump solutions offer the advantages of being inexpensive and easy to use, but with lower efficiency and shorter battery life.
With the improvement of charge pump design technology, new white LED driver chips, such as Maxim's chips, can not only obtain the efficiency of the inductor boost circuit (about 85%), but also keep the traditional charge pump design simple, low cost, etc. Advantage.
Fractional charge pump and its effect on efficiency
The basic architecture of the first generation of white LED-driven charge pumps is double voltage or 2x topology. The efficiency of the 2x charge pump is:
PLED/PIN=VLED×ILED/(2×VIN×ILED+Iq×VIN)
Among them, Iq is the quiescent current of the circuit. Because Iq is very small, the above formula can be approximated as:
PLED/PIN≈VLED/(2VIN)
To increase efficiency, the output of the second-generation white LED-driven charge pump is no longer an integer 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 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 1.5 times the charge pump is significantly improved. Assuming a battery voltage of 3.6V and an LED voltage of 3.7V, the efficiency is increased from 51% of the 2x charge pump to 69%.
The 1x mode introduced by the third generation charge pump further increases 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 equation.
PLED/PIN=VLED×ILED/(VIN×ILED+Iq×VIN)VLED/(VIN)
When the battery voltage is sufficient to drive a white LED, the efficiency of the 1x mode is over 90%. If the battery voltage is 4V and the LED's turn-on voltage drop is 3.7V, the efficiency can reach 92%.
Get the highest efficiency at different battery voltages
The 1x voltage conversion mode is the most efficient, but can only be used when the battery voltage is higher than the LED forward voltage drop. For maximum efficiency, white LED driver design requires a combination of battery and LED voltages, and the operating mode of the driver needs to be changed accordingly as the battery voltage (or LED voltage) changes. However, if the operating mode is changed when the battery voltage is high (not necessary), switching losses may cause the circuit to enter an inefficient mode. When the battery voltage drops, it is best to keep the driver in a high efficiency mode (for example, 1x mode). For power switches, the chip area and cost will increase in order to achieve low loss.
In order to keep the 1x mode operating at the lowest possible battery voltage, the voltage drop of the 1x voltage mode regulator FET and current regulator should be reduced as much as possible. The voltage drop determines the series loss and the lowest input voltage of the 1x mode that can be maintained. The minimum input voltage is represented by:
VLED+Bypass PFET RDS(ON)×ILED+VDROPOUT
The traditional positive charge pump white LED solution uses a PMOS FET as a bypass switch to connect the battery to the LED, as shown in Figure 1. The on-resistance RDS(ON) of the FET is approximately 1 to 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.
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