Save Board Space with a High Efficiency Dual Synchronous, 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

Introduction

The ever shrinking nature of cell phones, pagers, PDAs and other portable devices drives a corresponding demand for smaller components. One way to shrink DC/DC regulator circuitry is to increase the switching frequency of the regulator, thus allowing the use of smaller and cheaper capacitors and inductors to complete the circuit. Another way is to combine the switcher and MOSFETs in one small, monolithic package. The LTC3548 DC/DC regulator does both.

The LTC3548 is a 10-lead MSOP/DFN, dual, synchronous, step-down, current mode, DC/DC regulator, intended for low power applications. It operates within a 2.5V to 5.5V input voltage range and has a fixed 2.25MHz switching frequency, making it possible to use low-profile capacitors and inductors that are only 1mm high. The LTC3548 is the latest in the LTC3407 and LTC3407-2 family of dual regulators and features an improved Burst Mode ripple and two outputs of 400mA and 800mA. It is available in small MSOP and DFN packages, allowing two DC/DC Regulators to occupy less than 0.2 square inches of board real estate, as shown in Figure 1.

Figure 1. Two DC/DC regulators occupy less than 0.2in2 of board space.

The outputs of the LTC3548 are independently adjustable from 0.6V to 5V. For battery-powered applications that have input voltages above and below the output voltage, the LTC3548 can be used in a single inductor, positive buck-boost converter configuration (see data sheet for details). Two built in 0.35Ω switch provides high efficiency at maximum output current. Internal compensation minimizes external components and board space.

Efficiency is extremely important in battery-powered applications, and the LTC3548 keeps efficiency high with an automatic, power saving Burst Mode operation, which reduces gate charge losses at low load currents. With no load, both converters together draw only 40µA, and in shutdown, the device draws less than 1µA, making it ideal for low current applications. The LTC3548 features an improved Burst Mode ripple voltage, which is only about one third of the ripple for the LTC3407 and LTC3407-2, as shown in Figure 2 and Figure 3.

Figure 2. Comparison of ripple for Burst Mode operation of the LTC3548 and LTC3407-2.

Figure 3. Burst Mode operation output voltage ripple vs load current for the LTC3548 and LTC3407-2.

The LTC3548 uses a current-mode, constant frequency architecture that benefits noise sensitive applications. Burst Mode is an efficient solution for low current applications, but sometimes noise suppression is a higher priority. To reduce noise problems, a pulse-skipping mode is available, which decreases the ripple noise at low currents. Although not as efficient as Burst Mode at low currents, pulse-skipping mode still provides high efficiency for moderate loads, as seen in Figure 4. In dropout, the internal P-channel MOSFET switch is turned on continuously, thereby maximizing the usable battery life.

Figure 4. Efficiency of Burst Mode operation and pulse skip mode.

A Power-On Reset output is available for microprocessor systems to insure proper startups. Internal overvoltage and undervoltage comparators on both outputs will pull the POR output low if the output voltages are not within ±8.5%. The POR output is delayed by 262,144 clock cycles (about 175ms) after achieving regulation, but will be pulled low immediately when either output is out of regulation.

A High Efficiency 2.5V and 1.8V Step-Down DC/DC Regulator with all Ceramic Capacitors

The low cost and low ESR of ceramic capacitors make them a very attractive choice for use in switching regulators. In addition, ceramic capacitors have a benign failure mechanism unlike tantalum capacitors. Unfortunately, the ESR is so low that it can cause loop stability issues. A solid tantalum capacitor’s ESR generates a loop zero at 5kHz–50kHz that can be instrumental in giving acceptable loop phase margin. Ceramic capacitors, on the other hand, remain capacitive to beyond 300kHz and usually resonate with their ESL before the ESR becomes effective. Also, inexpensive ceramic capacitors are prone to temperature and voltage effects, requiring the designer to check loop stability over the operating temperature range. For these reasons, great care is usually needed when using only ceramic input and output capacitors. The LTC3548 was designed with ceramic capacitors in mind and is internally compensated to handle these difficult design considerations. High quality X5R or X7R ceramic capacitors should be used to minimize the temperature and voltage coefficients.

Figure 5 shows a typical application for the LTC3548 using only ceramic capacitors. This circuit provides a regulated 2.5V output and a regulated 1.8V output, at up to 400mA and 800mA, from a 2.5V to 5.5V input.

Figure 5. Dual output step-down application yields 1.8V at 800mA and 2.5V at 400mA.

Conclusion

The LTC3548 is a dual monolithic, step-down regulator that switches at 2.25MHz, minimizing component costs and board real estate requirements for DC/DC regulators. The small size, efficiency, low external component count, and design flexibility of the LTC3548 make it ideal for portable applications.

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Damon Lee