Application of LTC3601 and LTC3604 Monolithic DC/DC Converters

Crystal oscillator

Single chip microcomputer STM32L151CCU6

Quartz Oscillator 3225 20M OSC

When a relatively high voltage rail (12V) must be reduced to a relatively low level (3.3V, 1.8V), the conventionally used converter is a DC/DC switching controller that drives an external MOSFET. In many applications, replacing a typical controller-MOSFET-diode combination with a monolithic regulator saves space, design time and cost. The problem is that for many monolithic buck converters, the 12V rail is too high, and such converters typically cannot be used with inputs above 6V. In addition, switching losses make it virtually impossible to operate above about 1 MHz, eliminating the possibility of using a minimum inductor, so some of the advantages of monolithic regulators do not work.

The LTC3601 and LTC3604 are high performance monolithic synchronous buck regulators that deliver up to 1.5A and 2.5A, respectively. Both devices operate over a wide input voltage range of 3.6V to 15V, which covers the battery chemistry used in handheld devices, PCs, and automobiles. Their unique constant frequency/controlled on-time architecture provides a minimum turn-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.

Default configuration requiring minimal components

To reduce external component count, reduce cost, and save design time, switching frequency and loop compensation can be set with simple pins. Figure 1 shows a typical application. To operate at 2MHz, the oscillator frequency set 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 to produce a clean load transient response (Figure 2).

Figure 1: Application from wide input range to 3.3V/2.5A

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

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

Some applications require moving switching frequencies during operation, usually to avoid interference from adjacent wireless receivers. Figure 3 shows that even when the synchronous frequency introduced by the MODE/SYNC pin changes rapidly, the deviation of the output voltage is small.

Figure 3: The synchronous switching frequency can be moved at any time, and the change in VOUT is small

Both ICs can operate in an optional Burst Mode? for superior efficiency at low load currents (Figure 4) or forced continuous mode, which eliminates light load efficiency. Works in exchange for minimum output ripple and constant frequency. Even so, the ripple when operating in burst mode is typically only 20mV.

Figure 4: Burst mode operation produces high efficiency at light loads, while low RDS(ON) switches maintain high efficiency at maximum load

The built-in internal 400us soft-start timer prevents current surges in VIN at startup. A long soft-start time can be achieved by ramping the TRACK pin or by connecting a capacitor (tSS = 430,000 x CTRACK/SS) between the TRACK pin and ground. Open-drain PGOOD pin monitor output, if the output voltage deviates from the stable point by ±8%, the pin is pulled low. Additional VIN overvoltage and short circuit protection helps to form a fully rugged IC.

High frequency, low duty cycle, no problem

Many microprocessors require low-voltage rails of 1.x, but they are also used in applications that require high switching frequencies. These applications maintain low passive components at high switching frequencies and avoid critical frequency bands. Turn on RF interference. The problem is that the incredible combination of high step-down ratio and high switching frequency can be elusive because of the shortest on-time required. Figure 5 shows the schematic of the LTC3604 for use in a 4MHz, 12V to 1.8V application. The 38ns turn-on time required for this application is much larger than the 20ns minimum turn-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 in Figure 5 takes advantage of several features of the LTC3604. Usually the minimum input voltage is 3.6V, but here, by adding a resistor divider between the VIN and RUN pins, the undervoltage lockout is increased to 6V. The 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 supplied 4MHz frequency. If this external source fails, the internal oscillator (also set to 4MHz) will take over, and finally, loop compensation is implemented externally.

in 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 of these external components make them ideal for compact applications. Both ICs are available in a compact and thermally enhanced 3mm x 3mm QFN and MSOP package.

This website tries to open micro- and small-enterprise business advertising business; maintenance point recommended items. The fee is affordable and effective! Welcome to contact in QQ or email!

Why do you want to do online advertising contact?