Introduction

A wide variety of handheld and portable applications are powered by Li-Ion batteries or AA cells. The wide input voltage range of a single Li-Ion battery (2.7V–4.2V) or 2 AA cells (1.8V–3.0V) requires a DC/DC converter that can step-up or step-down the input voltage to provide a fixed output voltage such as 3.3V or 2.5V. The new LTC3240 step-up/step-down DC/DC converter is ideally suited for such applications and can provide up to 150mA in a tiny 6-lead 2mm × 2mm DFN package.

For input voltages greater than the regulated output voltage the LTC3240 operates as a low dropout regulator. When the input voltage decreases to within about 150mV of the regulated output voltage, the LTC3240 automatically switches to step-up mode. In step-up mode, the LTC3240 operates as a constant frequency (1.2MHz) voltage doubling charge pump. The LTC3240 requires only three tiny external ceramic capacitors for an ultra small application footprint as shown in Figure 1.

 

 

The LTC3240 features low no load operating current (65µA typical) and ultra low shutdown current (<1µA). Built-in soft-start circuitry prevents excessive inrush current during start-up. The thermal-shutdown and current-limit circuitry allow the parts to survive a continuous output short-circuit.

Application for Li-Ion or Three AA Battery Input to 3.3V Out

A typical application circuit for LTC3240-3.3 is shown in Figure 2. The input can be a single Li-Ion battery or three AA cells. Figure 3 shows the output voltage variation for the entire input voltage range at a load current of 30mA.

 

 

 

 

A new or recharged battery starts out at its highest terminal voltage. As the battery discharges, its terminal voltage continues to drop until the next recharge. The LTC3240 optimizes the output efficiency by continuing to operate in the step-down (LDO mode) for most of the battery life. When the battery voltage gets low enough it automatically switches into charge-pump mode to squeeze out maximum energy from the battery before its next recharge cycle. Figure 4 shows the efficiency of LTC3240-3.3 as a function of input voltage.

 

 

In step-up (charge pump) mode, the LTC3240 uses a unique architecture to optimize the charge transferred to the output in each clock cycle, thus minimizing the output ripple. The part only needs a 4.7µF, 0603 size ceramic capacitor to obtain a 32mV maximum ripple voltage (<1% for 3.3V output) at 150mA (See Figure 5).

 

 

To extend battery life at light loads, in charge pump mode, the part operates in high efficiency Burst Mode operation. In this case, the LTC3240 delivers a minimum amount of charge for a few cycles, and then enters a low current state until the output drops low enough to require another burst of charge.

Conclusion

The LTC3240 step-up/step-down charge pump DC/DC converter provides fixed regulated output voltage with currents up to 150mA from a wide input voltage range in a small 6-lead 2mm × 2mm DFN package. It is ideally suited for efficient DC/DC conversion in space-constrained applications such as battery-powered handheld electronics.