Using the highly integrated LT3759 wide input voltage application solution

The LT3759 operates over an input voltage range of 1.6V to 42V and uses a single feedback pin to control either the positive or negative output, reducing inventory and simplifying design. The device also incorporates many popular features such as soft-start, adjustable frequency and synchronization into a small footprint. The LT3759 is available in a 5mm x 4mm 12-pin MSE package and can be used in a variety of configurations such as boost, SEPIC, flyback and Cuk topologies.

Wide input voltage range and internal LDO

The LT3759's wide input range simplifies power supply designs that must be compatible with a wide range of input supplies. Because the LT3759 includes two internal low dropout (LDO) voltage regulators powered by the VIN and DRIVE pins, there is no need to add an external regulator or use a slow charge hysteresis start scheme, allowing for simple startup and biasing. The LT3759's internal INTVCC current limit function prevents the IC from dissipating excessive on-chip power.

Figure 1: A SEPIC converter generates a 12V output from a 2.5V to 15V input.

Output voltage detection made easy

The LT3759 features a novel FBX pin architecture that simplifies the design of both negative and positive converters. It includes two internal error amplifiers (one to sense the positive output and the other to sense the negative output) and allows the FBX pin to be connected directly to a voltage divider from either a positive or negative output, eliminating any confusion about positive or negative output sensing and simplifying board layout. You only need to determine the output polarity and topology, and the LT3759 does the rest.

Adjustable/synchronizable switching frequency

Converters are often required to operate at a specific frequency, especially in RF communications applications, which are sensitive to spectral noise in certain frequency bands. Also, if the area available for placing the converter is limited, operating at a higher frequency can also allow the use of smaller components, thereby reducing the required board space and output ripple. If power loss is a concern, switching at a lower frequency can reduce switching losses and thus improve efficiency. The switching frequency ranges from 100kHz to 1MHz and can be set using a single resistor connected between the RT pin and ground. In addition, the device can also be synchronized to an external clock via the SYNC pin.

Precision UVLO and Soft-Start

Input supply undervoltage lockout (UVLO) for sequencing or startup overcurrent protection can be easily implemented by driving the UVLO with a resistor divider from the VIN supply. When VIN is at the desired UVLO rising threshold voltage, the divider output will produce 1.22V at the UVLO pin. The UVLO pin has an adjustable input hysteresis, allowing the IC to ignore a programmable input supply voltage drop before disabling the converter. During the UVLO process, the IC is disabled and the VIN quiescent current drops to 1μA or less.

The SS pin allows access to a soft-start function that reduces the peak input current and avoids output voltage overshoot during startup or recovery from a fault condition. The SS pin suppresses inrush current by reducing the switching peak current. In this way, the soft-start function allows the output capacitor to be charged gradually to its final value.

Figure 2: Efficiency of the converter shown in Figure 1

A 2.5V~15V to 12V SEPIC converter

Figure 1 shows a 2.5V~15V input, 12V/2.5A output SEPIC power supply using LT3759. The typical efficiency of the converter is shown in Figure 2. Figure 3 shows the switching waveform during the output short circuit. As can be seen from the figure, once the output voltage is shorted to ground, the switching frequency will immediately return to one-third of the normal frequency. This feature enhances the short-circuit performance of Cuk and SEPIC converters.

Figure 3: Short-circuit process of the converter shown in Figure 1

A 1.8V~ 4.5V to 5V/2A step-up converter

Figure 4 shows a converter that converts from a 4.5V to as low as 1.8V input to a 5V/2A output. For applications where the converter output voltage is higher than the input voltage, the LT3759 is configured as a boost converter. The 500kHz switching frequency allows the use of small inductors and output capacitors.

Figure 4: A boost converter generates a 5V/2A output from a 1.8V to 4.5V input.

in conclusion

The LT3759 is a general purpose IC that integrates a rich set of unique features in a tiny 5mm x4mm 12-pin MSE package. The device accepts a wide input voltage range of 1.6V to 42V and features low shutdown current and frequency foldback under light load conditions. The LT3759 is ideal for wide input voltage applications, from single-cell Li-Ion battery-powered systems to automotive, industrial and telecom power supplies. Its high level of integration allows for a simple and low component count solution for boost, SEPIC and inverting converters.