Data transmission and reception devices require hundreds of volts
| LT8365, Multi-purpose Monolithic Boost Converter
| Supply up to 10mA with ±250V output voltage

The field of electronic communications is rapidly expanding into every aspect of daily life.

Data sensing and transmission require various types of devices, such as optical sensors, RF MEMS, PIN diodes, APDs, laser diodes, and high-voltage DACs. These devices typically require hundreds of volts to operate. Therefore, DC-DC converters are required to satisfy stringent efficiency, space, and cost requirements.

Analog Devices (ADI)'s LT8365 is a versatile monolithic boost converter that integrates a 150V, 1.5A switch, making it suitable for high-voltage applications in the communications field, including portable devices.

High voltage output can be easily generated from low inputs such as 2.8V or high inputs such as 60V. Additionally, spread spectrum frequency modulation is provided as an option to help mitigate EMI.

Many other useful features can be viewed in detail in the datasheet.

The converter shown in Figures 1 and 2 is used to provide positive and negative voltage rails from a 12V input source to a high-voltage DAC, MEMS, RF switch, and high-voltage operational amplifier.

These converters operate in discontinuous conduction mode (DCM) and supply up to 10mA with +250V and –250V output voltages with a conversion efficiency of about 80%.


Step-up ratio, 1:40 or higher
One of the advantages of DCM operation in a boost converter is that it can achieve a high step-up ratio regardless of the duty cycle. In addition, because the value and physical size of the inductor and output capacitor can be reduced, a smaller overall footprint solution can be implemented on the PCB. The circuit in Figure 3 can be easily mounted in an area of less than 1 cm².

There are cases where only very low input sources can be used and a high output voltage is required. The converter shown in Figure 3 can be used to drive various avalanche photodiodes, PIN diodes, and other devices that require high bias voltages. This boost converter generates 125V from a 3V input source and supports a load current of up to 3mA.

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


How much can you raise and lower it?
Boost converters can use multiple stages to double, triple, or more the output when very high voltages are required, whether positive or negative. The converters in Figures 1 and 2 demonstrate how to double the switching voltage in both positive and negative directions. The 3-stage boost converter in Figure 5 generates 375V from a 12V input source and supplies 8mA.

It should be noted that since the switch performance does not change, the available output current decreases as the output voltage increases. For example, a single-stage converter designed to supply 20mA will supply approximately 10mA if a second stage is added. When adding additional stages, always ensure that the peak switch current remains within the guaranteed switch current limit.

Simplified output voltage detection
The LT8365 provides a single FBX pin for output voltage sensing. A simple resistor divider connected to the FBX pin senses the output voltage regardless of the output polarity. This can be seen in all circuit diagrams in this article.


finish
The LT8365 can implement applications requiring ultra-compact, efficient high-output voltage boost conversion from low input voltages, such as 2.8V commonly seen in the communications field. It can also be used as an inverting converter and for popular topologies such as CUK and SEPIC converters.

The LT8365 is available in a small, thermally enhanced 16-pin MSOP package.










This article is a summary of a piece titled 'High Voltage Boost and Inverting Converters for Communications' written by Jesus Rosales, an application engineer at Analog Devices' Applications business unit.