1.2MHz, 2A, Monolithic Boost Regulator Delivers High Power in Small Spaces

Introduction

Even as cell phones, computers and PDAs shrink, they require an increasing number of power supply voltages. The challenge, of course, is how to squeeze more voltage converter circuits into less space—without sacrificing power or efficiency. Boost converters, in particular, are becoming more prevalent, as main supply voltages are lowered to accommodate core logic circuits, while many components require a higher supply voltage. The LTC3426 boost converter meets the challenge with converter-shrinking features, including a low RDS(ON) monolithic switch, internal compensation and a 3mm × 3mm × 1mm ThinSOT package. The LTC3426 operates at high frequency and therefore works with small, low cost inductors and tiny ceramic capacitors.

The LTC3426 incorporates a constant frequency current mode architecture, which is low noise and provides fast transient response. With a minimum peak current level of 2A, the LTC3426 delivers up to 900mA of output current. Figure 1 shows the converters output current capability at 5V as a function of VIN with peak inductor current at 2A. An input supply range of 1.6V to 4.3V makes the LTC3426 ideal for local supplies ranging from 2.5V to 5V. Efficiencies above 90% are made possible by its low 0.11Ω (typ.) RDS(ON) internal switch.

Figure 1. High current outputs are attainable with minimum 2A switch limit.

There is no need for an external compensation network because the LTC3426 has a built-in loop compensation network. This reduces size, lowers overall cost and greatly simplifies the design process. Figure 2 shows the VOUT response to a 250mA-to-500mA load step in a 1.8V to 3.3V application.

Figure 2. Fast transient response to load step of 250mA to 500mA.

The Shutdown input can be driven with standard CMOS logic above either VIN or VOUT (up to 6V maximum). Quiescent current in shutdown is less than 1µA. A simple resistive pull-up to VIN configures the LTC3426 for continuous operation when VIN is present.

3.3V Output 800mA Converter

Some applications require local 3.3V supplies which are utilized periodically yet are required to deliver high currents. The LTC3426 is an ideal solution which requires minimal board space and, when in shutdown, draws less than 1µA quiescent current. Figure 3 shows a circuit which delivers up to 800mA at 3.3V from a 2.5V input. This circuit also works with VIN down to 1.8V with 750mA output. The output voltage is easily programmed by changing the feedback ratio of R1 and R2 according to the formula:

Equation 1
Figure 3. Application circuit for 3.3V output delivers 800mA.

Lithium-Ion 5V Boost Converter

Some portable applications still require a 5V supply. Figure 4 shows a circuit which operates from a single Lithium-Ion battery and delivers at least 750mA from a VIN as low as 3V. When fully charged to 4.2V, over 1A can be supplied. The photograph of a demonstration board in Figure 5 shows just how small the board area is for this application, 10mm × 12mm. Tiny ceramic bypass capacitors and surface mount inductors keep the design small.

Figure 4. Compact application circuit for VOUT at 5V.

Figure 5. Photograph of demo board of circuit in Figure 4—board area is 10mm × 12mm.

Figure 6 shows efficiency exceeding 90% and remaining greater than 85% over a load range from 10mA to 900mA with a fully charged battery.

Figure 6. Up to 92% efficiency in Lithium-Ion battery to 5V output applications.

Component Selection

The LTC3426 requires just a few external components to accommodate various VIN and VOUT combinations. Selecting the proper inductor is important to optimize converter performance and efficiency. An inductor with low DCR increases efficiency and reduces self-heating. Since the inductor conducts the DC output current plus half the peak-to-peak switching current, select an inductor with a minimum DC rating of 2A. To minimize VOUT ripple, use low ESR X5R ceramic capacitors. The average Schottky diode forward current is equal to the DC output current therefore the diode average current should be greater than 1A. A low forward voltage Schottky diode reduces power loss in the converter circuit.

Conclusion

The addition of the LTC3426 to Linear Technology’s high performance boost converter family allows the designer to deliver high current levels with minimal board space. An on chip switch and internal loop compensation reduces component count to provide an inexpensive solution for spot regulation applications.

Author

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Kevin Ohlson