Tiny DC/DC Buck Controller Provides High Efficiency and Low Ripple

Introduction

To secure a foothold in today’s congested circuit boards, a power controller must deliver the most functionality in the smallest package. With a blend of popular features squeezed into a SOT-23 or 3mm × 2mm DFN, the LTC3772 makes a power supply designer’s job easy. This versatile DC-DC controller supports a wide input voltage range, 2.5V to 9.8V, and maintains high efficiency over a variety of output current levels. Its 550kHz switching frequency trims solution size by permitting the use of small passive components. Its No RSENSE constant frequency architecture also eliminates the need for a sense resistor.

Circuit Description

Figure 1 shows a typical application for the LTC3772. This circuit provides a regulated output of 2.5V from a typical input voltage of 5V, but it can also be powered from any input voltage between 2.75V and 9.8V (depending on the voltage rating of the P-channel power MOSFETs). This wide input range makes the LTC3772 suitable for a variety of input supplies, including 1- and 2-cell Li-Ion and 9V batteries, as well as 3.3V and 5V supply rails. The internal soft-start ramps the output voltage smoothly from 0V to its final value in 1ms (Figure 2).

Figure 1. Typical application delivering 2.5V at 2A.

Figure 2. The output voltage rises smoothly without requiring a soft-start capacitor as seen in this startup waveform for the converter in Figure 1.

At low load currents (≤10% of IMAX), the LTC3772 enters Burst Mode operation. Compared with other power saving schemes, this variant of Burst Mode operation surrenders a modicum of efficiency to obtain very low output voltage ripple. Typically producing just 30mV for a typical application using ceramic output capacitors, the LTC3772 is ideal for noise-sensitive portable applications. Figure 3 illustrates inductor current and output voltage waveforms for Burst Mode operation.

Figure 3. The LTC3772’s Burst Mode operation maintains light load efficiency while holding output voltage ripple to just 20mV in this application.

The LTC3772 uses the drain to source voltage (VDS) of the power P-Channel MOSFET to sense the inductor current. The maximum load current that the converter can provide is determined by the RDS(ON) of the MOSFET, which is a function of the input supply voltage (which supplies the gate drive). The maximum load current can also be changed using the current limit programming pin IPRG, which sets the peak current sense voltage across the MOSFET to one of three states; each voltage is associated with its own inductor current limit. With IPRG floating, the circuit of Figure 1 can reliably provide 2.5V at 2A from a 3.3V input supply. Efficiency for this circuit exceeds 93%, as shown in Figure 4. In drop out, the LTC3772 can operate at 100% duty cycle, providing maximum operating life in battery-powered systems.

Figure 4. Efficiency vs load current for the converter in Figure 1, with input of 3.3V.

Figure 5. Transient performance of the converter in Figure 1, with input of 5V.

OPTI-LOOP Compensation

To meet stringent transient response requirements, some switching regulators use many large and expensive output capacitors to reduce the output voltage droop during a load step. The LTC3772, with OPTI-LOOP compensation, is stable for a wide variety of output capacitors, including tantalum, aluminum electrolytic, and ceramic capacitors. The ITH pin of the LTC3772 allows users to choose the proper component values to compensate the loop so that the transient response can be optimized with the minimum number of output capacitors. Figure 4 shows a transient response for the circuit in Figure 1, using just one 47μF output capacitor. The response is quite fast, even though it involves a transition from Burst Mode operation to continuous conduction mode.

Figure 6. A typical LTC3772 application occupies just 1.5 square centimeters.

Conclusion

For single-output designs with load currents as high as 5A from input voltages up to 9.8V, the LTC3772 delivers the most popular features of PFET controllers in a very small package. With small ancillary components and no sense resistor, the overall solution is unmatched where board space is at a premium.

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Theo Phillips