The field of electronic communications is rapidly expanding into every aspect of everyday life. Detecting, transmitting, and receiving data requires the use of numerous devices such as fiber optic sensors, RF MEMS, PIN diodes, APDs, laser diodes, high voltage DACs, and more. In many cases, these devices require hundreds of volts to operate, necessitating the use of DC-DC converters to meet stringent efficiency, space and cost requirements.

Analog Devices' LT8365 is a versatile monolithic boost converter that integrates a 150 V, 1.5 A switch, making it ideal for high-voltage applications in communications, including portable devices. High voltage outputs can be easily generated from inputs as low as 2.8 V and as high as 60 V. The chip has an optional spread spectrum function to help eliminate EMI, as well as many other commonly used features, see the data sheet for details. The converters shown in Figures 1 and 2 are used to provide positive and negative voltage rails from a 12 V input source for high voltage DACs, MEMS, RF switches, and high voltage op amps. Operating in discontinuous conduction mode (DCM), these converters provide up to 10 mA of current, and +250 V and –250 V output voltages with conversion efficiencies of approximately 80%.
Boost ratio > 1:40

One advantage of implementing DCM operation in a boost converter is that a high boost ratio can be achieved regardless of the duty cycle. Additionally, both the value and physical size of the inductor and output capacitor can be reduced, reducing the overall size of the solution used on the PCB. The circuit shown in Figure 3 can be easily deployed into spaces less than 1 cm2.
Figure 1. 2-Level Boost Converter from 12 V Input to 250 V Output

Figure 2. 2-Stage Inverting Converter from 12 V Input to –250 V Output

Figure 3. Boost Converter from 3 V Input to 125 V Output

In some cases, the available input source voltage may be very low, but a high output voltage is required. At this point, the converter shown in Figure 3 can be used to drive multiple avalanche photodiodes, PIN diodes, and other devices that require high bias voltages. These boost converters can generate a 125 V output from a 3 V input with a load current of up to 3 mA.
Figure 4. 2-Stage Boost Converter with 3 V Input to 250 V Output

The converter shown in Figure 4 takes a 3 V input, steps up a 125 V output to a 250 V output, and supports about 1.5 mA. In the communications field, there are many devices that require such high bias voltages from low input voltage sources.

How high or how low can it be?

In situations where extremely high voltages are required, whether positive or negative, boost converters can use multiple stages to boost the output by a factor of 2, 3, or even more. The converters shown in Figures 1 and 2 demonstrate how the switching voltage can be doubled in both directions (positive and negative). The 3-level boost converter shown in Figure 5 can generate 8 mA, 375 V output from a 12 V input.
3-Level Boost Converter with 12 V Input to 375 V Output NOTE: The available output current must decrease as the output voltage increases because the switching current capability does not change. For example, a single-stage converter designed to deliver 20 mA will deliver about 10 mA when a 2nd stage is added. When adding more stages, always ensure that the peak switch current is always within the guaranteed switch current limit.
Output Voltage Detection Simplified

The LT8365 provides a single FBX pin to sense the output voltage. As in all the schematics shown in this article, the output voltage is sensed by a simple resistor divider connected to the FBX pin, regardless of output polarity.

in conclusion

The LT8365 supports applications requiring compact, efficient, high output voltage boost conversion for input voltages as low as 2.8 V, which is very common in communications. It can also be used as an inverting converter, and in common topologies it can be used, for example, as a CUK and SEPIC converter. The LT8365 is available in a small thermally enhanced 16-lead MSOP package.