Efficient dual-channel synchronous step-down DC/DC regulator saves board area

The shrinking size of cell phones, PDAs and other portable electronic products has driven demand for smaller components. One way to reduce DC/DC regulator board area is to increase the switching frequency of the regulator to allow the circuit to be constructed with smaller, cheaper capacitors and inductors. Another approach is to integrate the transfer switch and MOSFET in a small single-chip package. The LTC3548 DC/DC regulator uses the above two methods to reduce the circuit board area.

The LTC3548 is a dual-channel synchronous step-down DC/DC regulator in a 10-pin MSOP/DFN package. It uses a current mode architecture and is targeted at low power applications. The device operates from an input voltage range of 2.5V to 5.5V and has a fixed switching frequency of 2.25MHz, allowing the use of small capacitors and inductors with a height of only 1mm. The LTC3548, the latest member of the LTC3407 and LTC3407-2 dual-channel regulator families, improves voltage ripple during burst mode operation and supports both 400mA and 800mA outputs. The device is available in small MSOP and DFN packages, allowing the two DC/DC regulators to occupy less than 0.2 square inches of board area, as shown in Figure 1.

The LTC3548's outputs are independently adjustable from 0.6V to 5V. For battery-powered applications with input voltages above and below the output voltage, the LTC3548 can be used in a single-inductor, positive buck/boost converter configuration. Two built-in 0.35Ω switches provide high efficiency at maximum output current, while internal compensation minimizes external component count and board area.

Figure 1: Two DC/DC regulators occupy less than 0.2 square inches of board area.

Efficiency is very important in battery-powered applications, and the LTC3548 maintains high efficiency by utilizing automatic, power-saving burst mode operation, which reduces gate charge losses under low load current conditions. Under no-load conditions, both converters draw only 40µA total, while in the off state the devices consume less than 1µA, making the LTC3548 ideal for low-current applications. In addition, the LTC3548's ripple voltage in burst mode operation has improved to only about 1/3 of the ripple voltage of the LTC3407 and LTC3407-2 (see Figures 2 and 3).

Figure 2: Ripple voltage comparison of LTC3548 and LTC3407-2 in burst mode operation.

Figure 3: Output voltage ripple versus load current for the LTC3548 and LTC3407-2 in burst mode operation.

The LTC3548 uses a constant-frequency, current-mode architecture useful in noise-sensitive applications. While burst mode is a high-efficiency solution for low-current applications, sometimes noise rejection performance takes precedence. To reduce noise issues, the LTC3548 can also operate in pulse-skipping mode to reduce ripple noise in low-current applications. Although pulse-skipping mode is not as efficient as burst mode at low currents, it is still more efficient at moderate loads (see Figure 4). During the dropout condition, the internal P-channel MOSFET switch conducts continuously, thereby maximizing the available battery life.

Microprocessor systems can use the power-on reset output to ensure proper startup operation. Internal overvoltage and undervoltage comparators on both outputs pull the /POR output low if the output voltage falls outside the ±8.5% fluctuation range of regulation. After regulation is achieved, the /POR output is delayed for 262,144 clock cycles (approximately 175ms), but when either output becomes unregulated, it is immediately pulled low.

Figure 4: Burst mode operation and pulse skipping mode efficiency.

The low cost and low ESR of ceramic capacitors make them ideal for use in switching regulators. In addition, unlike tantalum capacitors, ceramic capacitors also have a good failure mechanism. Unfortunately, the very low ESR of ceramic capacitors can cause loop stability problems. The ESR of solid tantalum capacitors creates a loop "zero" in the 5kHz to 50kHz range, which is very helpful in providing acceptable loop phase margin.

Ceramic capacitors, on the other hand, remain capacitive at frequencies above 300kHz and often resonate with the ESL before the ESR begins to take effect. In addition, inexpensive ceramic capacitors are susceptible to temperature and voltage effects, requiring design engineers to ensure loop stability over the entire operating temperature range. Therefore, great care must be taken when using only ceramic capacitors as input and output capacitors. The LTC3548 has been designed with the effects of ceramic capacitors in mind and is compensated internally to address these difficult design issues. X5R or X7R type high-quality ceramic capacitors should be used in the design to minimize the temperature and voltage coefficients.

Figure 5: Dual-channel buck regulator application circuit provides 2.5V regulated output (400mA) and 1.8V regulated output (800mA).

Figure 5 shows a typical application circuit diagram of the LTC3548 using only ceramic capacitors. When the input voltage is 2.5V to 5.5V, this circuit can provide 2.5V regulated output and 1.8V regulated output, with output currents up to 400mA and 800mA respectively.

The LTC3548 is a dual-channel monolithic step-down regulator with a switching frequency of 2.25MHz, which minimizes the component cost and board area requirements of DC/DC regulators. The LTC3548's small size, high efficiency, low external component count, and design flexibility make it ideal for portable applications.