Maxim Integrated Products has introduced the MAX8647 negative charge pump. This innovative negative charge pump architecture eliminates line impedance from the battery to the LED. When the battery is discharged, the circuit delays mode switching between 1 and 1.5 times the voltage. The proprietary adaptive mode switching technique is for six-way white or RGB.
The LEDs are independently controlled. Therefore, even with a large mismatch in LED forward voltage drop, the MAX8647 can still achieve a significant 12% increase in efficiency. This outstanding power management solution is ideal for complex handheld devices that require long battery life and overall lighting management. Typical applications include cell phones, smartphones, and portable media players, where battery life is extremely valuable per mAh.
Traditional charge pump design limits LED efficiency
Ideally, the designer would like to use all battery voltages to drive directly (ie, no voltage drop in 1x mode) without losing any efficiency, white and RGB LEDs. Obviously to achieve this goal, it is impossible to use a "positive" charge pump between the battery and the LED. This configuration creates an additional voltage drop in the power supply loop that reduces the drive voltage on the LED. When the drive voltage is insufficient, the charge pump turns on. Therefore, the battery voltage at which the positive charge pump starts operating is higher, reducing efficiency.
Using 1x mode will extend battery life. But to achieve zero voltage drop, a typical competitive solution requires the removal of a positive charge pump, which is not possible with this architecture.
Driving all LEDs with the same voltage will consume more power
The competition scheme does not supply power to each LED separately. The circuit monitors all LED outputs and the positive charge pump turns on when any of the LED currents falls below a preset value. When there is a large mismatch in the forward voltage of the system LED, the highest LED vf will trigger the charge pump to boost the battery voltage. Thus, current regulators for those LEDs with lower vf will consume additional voltage and power. Therefore, the more mismatched vf and the greater the number of LEDs, the greater the power consumption. Video phones, smartphones, and multimedia players typically use five or more LEDs, and mismatch issues can further exacerbate power consumption.
Innovative negative charge pump and independent LED switching solve the problem of efficiency
The MAX8647 negative charge pump architecture eliminates line impedance from battery to LED. Therefore, when the battery is discharged, the device delays switching between 1 and 1.5 times the pressure mode. The adaptive switching technology supplies, dims, and stabilizes the individual LEDs. The end result of this new technology is an increase in LED efficiency by 12%.
The MAX8647 negative charge pump has built-in adaptive mode switching with several important advantages. Increased efficiency by 12% and extends battery life. Increased efficiency is especially important for applications such as cellular phones, where backlighting is a major part of its power dissipation. Higher efficiency corresponds to longer talk time. Correspondingly, the same talk time can be achieved with a smaller battery, thereby reducing the cost of the manufacturer. The MAX8647 can also replace inductive boost LED drivers, which often require large pcb space.
Optimized lighting management
The MAX8647's range of applications includes not only the aforementioned complex handheld devices, but also all display backlighting and general lighting management. The i2c interface allows control of individual LEDs for flexible multi-zone lighting management, 32-level brightness adjustment, or 32,768 colors with RGB LEDs. A quiescent current as low as 70μa and a dimming current of 100μa (minimum) contribute to the lowest power “normally open†tft display. The device's current output is accurate to ±1%, minimizing battery current consumption while ensuring the desired brightness for each LED. The MAX8647 also features temperature derating and overvoltage protection to extend LED life.
The MAX8647's sister product, the MAX8648 negative-charge pump, also features adaptive mode switching for each LED. The difference is that the MAX8648 uses serial pulse dimming to divide the LED control into three groups.
The MAX8647/8648 are available in a tiny 16-pin tqfn package (3mm x 3mm, 0.8mm high, maximum). The device operates over the extended temperature range of -40°c to +85°c. An evaluation board (MAX8648evkit) is available to speed up the design process. The MAX8647 starts at $1.95, while the MAX8648 starts at $1.70 (2500 tablets, FOB USA).
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