Application of two monolithic DC/DC converters

When a relatively high voltage rail (12V) must be stepped down to a relatively low level (3.3V, 1.8V), the converter traditionally used is a DC/DC switching controller driving an external MOSFET. In many applications, replacing the typical controller-MOSFET-diode combination with a monolithic regulator can save space, design time, and cost. The problem is that the 12V rail is too high for many monolithic buck converters, which typically cannot be used with inputs above 6V. In addition, switching losses make operation above about 1MHz practically impossible, thus eliminating the possibility of using the smallest inductor, so some of the size advantages of monolithic regulators are not realized.

The LTC3601 and LTC3604 are high performance monolithic synchronous step-down regulators capable of delivering up to 1.5A and 2.5A respectively. These devices operate over a wide input voltage range of 3.6V to 15V, which covers the battery chemistries used in handheld devices, PCs and automobiles. Their unique constant frequency/controlled on-time architecture provides a minimum on-time of 20ns, making them ideal for high step-down ratio applications that require high switching frequency and fast transient response while maintaining high efficiency.

To reduce external component
count, lower cost, and save design time, the switching frequency and loop compensation can be set with simple pins. Figure 1 shows a typical application. To operate at 2MHz, the oscillator frequency setting pin (RT) is connected to the internal 3.3V regulator output pin (INTVCC). When the compensation pin (ITH) is connected to INTVCC, the default compensation is applied, resulting in a clean load transient response (Figure 2).

Figure 1: Wide input range to 3.3V/2.5A applications

Figure 2: Fast transient response of the circuit in Figure 1

The operating frequency range is 800kHz to 4MHz and is programmable with an external resistor from RT to ground. For switching noise sensitive applications, the LTC3601 and LTC3604 can be externally synchronized within the same frequency range regardless of the state of RT. No external PLL components are required to achieve synchronization.

Some applications require shifting the switching frequency during operation, usually to avoid interference from adjacent wireless receivers. Figure 3 shows that even when the synchronization frequency introduced by the MODE/SYNC pin changes rapidly, the output voltage deviates very little.

Figure 3: Synchronous switching frequency can be shifted at any time with minimal change in VOUT

Both ICs can operate in selectable Burst Mode® mode for excellent efficiency at light load currents (Figure 4), or in forced continuous mode, which trades off light load efficiency for minimal output ripple and constant frequency operation. Even so, ripple is typically only 20mV in Burst Mode operation.

Figure 4: Burst Mode operation produces high efficiency at light loads, while low RDS(ON) switching maintains high efficiency at maximum loads

A built-in internal 400us soft-start timer prevents current surges in VIN at startup. Longer soft-start times can be achieved by ramping the TRACK pin up, or by connecting a capacitor from the TRACK pin to ground (tSS = 430,000 x CTRACK/SS). The open-drain PGOOD pin monitors the output and pulls low if the output voltage deviates from regulation by ±8%. Additional VIN overvoltage and short-circuit protection contribute to an overall rugged IC.

High Frequency, Low Duty Cycle, No ProblemMany
microprocessors require low 1.x voltage rails, but they are also used in applications that require high switching frequencies to keep passive components small and keep critical frequency bands clear of RF interference. The problem is that achieving this magical combination of high step-down ratio and high switching frequency can be elusive because such a short minimum on-time is required. Figure 5 shows the schematic of the LTC3604 used in a 4MHz, 12V to 1.8V application. The 38ns on-time required for this application is much greater than the 20ns minimum on-time of the LTC3604.

Figure 5: The LTC3604 operates at high frequency (4MHz) and low duty cycle,
providing a compact footprint and allowing high step-down ratios.

The design of Figure 5 takes advantage of several features of the LTC3604. Normally the minimum input voltage is 3.6V, but here undervoltage lockout is increased to 6V by adding a resistor divider from VIN to the RUN pin. Soft-start time is increased to 4.3ms by adding a 10nF capacitor from the TRACK pin to ground. The switching frequency is synchronized to an externally provided 4MHz frequency. If this external source fails, the internal oscillator (also set to 4MHz) takes over. Finally, loop compensation is implemented externally.

Conclusion
The LTC3601 and LTC3604 are members of a new generation of monolithic DC/DC converters that can handle relatively high input voltages and low duty cycles. The compact size, high performance and design that requires very few external components make these devices ideal for compact applications. Both ICs are available in compact and thermally enhanced 3mm x 3mm QFN and MSOP packages.