Save Board Space with a High Efficiency Dual Synchronous, 600mA, 1.5MHz 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 LTC3407 DC/DC regulator does both.

The LTC3407 is a 10-lead, 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 1.5MHz switching frequency, making it possible to use tiny capacitors and inductors that are under 1.2mm in height. The LTC3407 is available in small DFN and MSOP packages, allowing two 600mA 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 LTC3407 are independently adjustable from 0.6V to 5V. For battery-powered applications that have input voltages above and below the output voltage, the LTC3407 can be used in a single inductor, positive buck-boost converter configuration. A built in 0.35Ω switch allows up to 600mA of output current at high efficiency. Internal compensation minimizes external components and board space.

Efficiency is extremely important in battery-powered applications, and the LTC3407 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 LTC3407 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 2. In dropout, the internal P-channel MOSFET switch is turned on continuously, thereby maximizing the usable battery life.

Figure 2. Comparison of Burst Mode operation and pulse skip mode. This particular data is for the circuit shown in Figure 3.

A Power-On Reset (POR) 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 ouput is out of regulation.

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 LTC3407 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 3 shows a typical application for the LTC3407 using only ceramic capacitors. This circuit provides a regulated 2.5V output and a regulated 1.8V output, both at up to 600mA, from a 2.5V to 5.5V input. Efficiency for the circuit is as high as 95% as shown in Figure 4.

Figure 3. Dual step-down regulator provides 1.8V and 2.5V at 600mA.

Figure 4. Efficiencies of the circuit in Figure 3.

2mm Height Li-Ion, Single Inductor, Buck-Boost Regulator and Buck Regulator

Lithium-Ion batteries are popular in many portable applications because of their light weight and high energy density, but the battery voltage ranges from a fully charged 4.2V down to a drained 2.8V. When a device requires an output voltage that falls somewhere in the middle of the Li-Ion operating range, such as the 3.3V I/O supply, a simple buck or boost converter does not work. One solution is a single inductor, positive buck-boost regulator, which allows the input voltage to vary above and below the output voltage.

In Figure 5, regulator 2 is configured as a single inductor, positive buck-boost regulator to supply a constant 3.3V with 200mA–400mA of load current, depending on the battery voltage. The circuit is well suited to portable applications because none of the components exceed 2mm in height.

Figure 5. Single inductor, positive buck-boost regulator and a buck regulator with maximum height < 2mm.

The efficiency varies with the input supply, due to resistive losses at high currents and to switching losses at low currents. As shown in Figure 6, the typical efficiency across both battery voltage and load current is about 75% for the 3.3V output, and about 90% for the 1.8V output.

Figure 6. Efficiencies for the circuit in Figure 5.

Low Profile, 1.2mm Height, Lithium-Ion Dual Supply

In some applications, minimizing the height of the circuit takes prime importance. New low profile capacitors and inductors can be combined with the already low profile 1.1mm maximum height of the LTC3407’s 10-lead MSOP package. Figure 7 shows a circuit designed to meet a 1.2mm maximum height requirement, and to occupy less than 0.2in2. The circuit provides 2.5V and 1.5V outputs, each with up to 600mA of load current. Two 4.7µF ceramic capacitors are used on each supply, due to the lack of availability of low profile 10µF ceramic capacitors.

Figure 7. Low profile (1.2mm) Lithium-Ion dual step-down regulator.

The efficiency is slightly lower due to the higher series resistance of the low profile inductors. A peak efficiency of 91% for the 1.5V output and 95% for the 2.5V output is achieved with these components, as seen in Figure 8.

Figure 8. Efficiencies for the circuit in Figure 7.

Conclusion

The LTC3407 is a dual monolithic, step-down regulator that switches at 1.5MHz, 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 LTC3407 make it ideal for portable applications.

Author

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Damon Lee