Negative Charge Pumps Achieve

Overview

Offering a small footprint and high light output, white LEDs (WLEDs) provide an ideal backlight solution for small color displays in cellular phones and other portable devices. WLEDs, however, do present one difficulty in devices powered from a single-cell lithium-ion (Li+) battery. The operating voltage from most Li+ cells is 3V to 4.2V, while a WLED’s forward voltage is typically 3.5V to 3.8V (at 20mA). Consequently, the voltage output at the lower end of a Li+ battery’s operating range is not enough to bias WLEDs.

Two approaches are commonly used to generate adequate forward bias for WLEDs: capacitor-charge-pump and inductor-based boost circuits. Traditionally, inductor-based circuits have been the best choice for efficiency and battery life. However, they require the addition of that costly inductor, and necessitate careful layout and design to avoid electromagnetic and RF interference issues. In contrast, charge-pump solutions are simpler to implement and cost less, but they have also typically been less efficient, which can reduce battery runtime.

Negative-Charge-Pump Technology Enables Lower-Cost, Energy-Efficient Applications

Maxim’s negative-charge-pump architecture with adaptive switchover enables WLED driver ICs to achieve inductor-like efficiencies (averaging 85%) while still retaining the simplicity and low cost of an inductorless design.

This innovative topology employs adaptive mode-switching technology to supply, dim, and regulate each LED individually. Delivering a 12% increase in LED efficiency, this approach extends battery life and saves valuable PCB real estate in portable applications. By providing efficiency comparable to inductor-based designs, these devices effectively reduce the price point of energy efficiency.

Efficiency Improvements Among Fractional-Ratio Charge Pumps

The first generation of WLED charge-pump solutions used a basic doubler topology (or 2x mode) at its core. The efficiency of a 2x charge pump is:

P_LED/P_IN = V_LED × I_LED/[(2 × V_IN × I_LED + I_Q × V_IN)]

where I_Q is the circuit's quiescent operating current.

Because the I_Q is usually small compared to the WLED's load current, the efficiency can be closely approximated by:

P_LED/P_IN ≈ V_LED/2V_IN

To improve efficiency, second-generation WLED charge pumps did not always drive the output to a whole multiple of the input. If the battery voltage was sufficient, adequate LED drive voltage could be generated with a 1.5x charge pump. The conversion efficiency of a 1.5x pump is:

P_LED/P_IN = V_LED × I_LED/(1.5 × V_IN × I_LED + I_Q × V_IN)
≈ V_LED/1.5V_IN

As can be seen, the 1.5x pump substantially improves efficiency. With a 3.6V battery voltage and a 3.7V WLED, efficiency jumps from 51% with a 2x pump to 69% with a 1.5x pump.

Third-generation WLED drivers added further improvement with a 1x transfer mode that connects the battery directly to the LEDs through low-dropout current regulators when the battery voltage is high enough. This efficiency is described by:

P_LED/P_IN = V_LED × I_LED/(V_IN × I_LED + I_Q × V_IN)
≈ V_LED/V_IN

When the battery voltage is sufficient to directly drive WLEDs, 1x mode efficiency can be over 90%. With a 4V battery and a 3.7V WLED, efficiency is 92%.

Maximizing Efficiency at Each Battery Voltage

An optimum WLED driver design employs the most efficient power-transfer mode possible for a given battery and LED voltage. The design also changes modes as the battery (or WLED) voltage changes. However, switch losses can force the circuit into a less efficient mode at a higher battery