Industry News Smart Cities

The field of electronic communications is rapidly expanding into every aspect of ordinary life. Detection, transmission, and reception of data require a wide array of devices such as optical sensors, RF MEMS, PIN diodes, APDs, laser diodes, and high voltage DACs, to name just a few. In many cases, these devices require several hundred volts to operate, calling for dc-to-dc converters that meet stringent efficiency, space, and cost requirements.

Analog Devices’ LT8365 is a versatile monolithic boost converter that integrates a 150V, 1.5A switch, making it ideal for high voltage applications found in the communications field, including portable devices. High voltage outputs are easily produced from inputs as low as 2.8V and as high as 60V. It features optional spread spectrum frequency modulation, which can help mitigate EMI, and many other popular features detailed in the data sheet.

The converters shown in Figure 1 and Figure 2 have been used to provide the positive and negative voltage rails for high voltage DACs, MEMS, RF switches, and high voltage op amps, from a 12V input source. These converters operate in discontinuous conduction mode (DCM) and deliver as much as 10mA, with +250V and –250V output voltages with a conversion efficiency of about 80%.

Figure 1. 12V input to 250V output 2-stage boost converter.

Figure 2. 12V input to –250V output 2-stage inverting converter.

Step-Up Ratios >1:40
One benefit of DCM operation in a boost converter is the ability to achieve a high step-up ratio independent of duty cycle. Additionally, the values and physical sizes of the inductor and output capacitor can be reduced, which leads to a smaller overall footprint solution on the PCB. The circuit in Figure 3 can easily fit in an area less than 1cm2.
There are situations when only a very low input source is available and a high output voltage is needed. The converter shown in Figure 3 could be used to drive a variety of avalanche photo diodes, PIN diodes, and other devices requiring high bias voltages. This boost converter produces 125V from a 3V input source with up to 3mA of load current.

Figure 3. 3V input to 125V output boost converter.

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

Figure 4. 3V input to 250V output 2-stage boost converter.

How High or Low Can You Go?
For situations where very high voltage is needed, whether positive or negative, a boost converter can use multiplier stages to boost the output 2×, 3×, or more. The converters in Figure 1 and Figure 2 show how to double the switch voltage in both directions, positive and negative. The 3-stage boost converter in Figure 5 delivers 375V at 8mA from a 12V input source.
Note that the available output current must decrease as output voltage increases, since the switch capability does not change. As an example, a single-stage converter designed to deliver 20mA will deliver about 10mA when a second stage is added. As additional stages are added, always ensure the peak switch current stays within the guaranteed switch current limit.

Figure 5. 12V input to 375V output 3-stage boost converter.

Output Voltage Sensing Simplified
The LT8365 offers a single FBX pin to sense the output voltage. A simple resistor divider connected to the FBX pin senses the output voltage, independent of output polarity, as observed on all the schematics presented in this article.
Conclusion
The LT8365 enables applications that require compact, efficient, high output voltage boost conversion from input voltages as low as 2.8V, which is common in the field of communications. It can also be used as an inverting converter and in popular topologies such as CUK and SEPIC converters. The LT8365 is available in a small, thermally enhanced, 16-lead MSOP package.

About the Author
Jesus Rosales is an applications engineer in Analog Devices’ Applications Group in Milpitas, CA. He joined Linear Technology (now a part of ADI) in 1995 as an associate engineer and was promoted to applications engineer in 2001. He has since supported the boost/inverting/SEPIC family of monolithic converters and some offline isolated application controllers. He received an associate degree in electronics from Bay Valley Technical Institute in 1982. He can be reached at jesus.rosales@analog.com.

Increasing demands for frequency bandwidth, throughput, and dynamic range in communications systems, together with the need for higher antenna frequencies used in millimetre wave 5G, have all placed further demands on the quality of the local oscillator (LO) or clock used in communications or mixed-signal systems, respectively.

The integrated phase-locked loop (PLL) and voltage controlled oscillator (VCO), the ADF4371, and the similar ADF4372, showcase Analog Devices’ efforts to address the needs of these demanding applications.

Figure 1. ADF4371 block diagram.

Frequency Coverage
To maximise frequency coverage, the ADF4371/ADF4372 VCO covers an octave range from 4GHz to 8GHz, and, by using frequency dividers at the output, dividing by 1/2/4/8/16/32/64 allows full frequency coverage at the main RF8 output of between 62.5MHz to 8000MHz. A second identical output is provided to allow a user to drive a converter clock. The open-loop VCO phase noise is –109dBc/Hz at 100kHz offset for the 8GHz output frequency.
Until recently, generating high frequencies required the use of external frequency multipliers, typically fabricated on GaAs processes, and they often demanded additional filtering, along with amplification, to overcome the effect of the filtering.
To achieve higher frequencies, the ADF4371/ADF4372 contain an integrated frequency doubler, which provides 8GHz to 16GHz output at the differential RF16 pins. The ADF4371 also features a frequency quadrupler that generates from 16GHz to 32GHz at the RF32 differential output. To minimise the generation of unwanted multiplier products, the ADF4371/ADF4372 contain tracking filters that optimise the power level of the desired frequency while suppressing the unwanted multiplier products. On the doubled output, the VCO feedthrough is –45dBc. On the quad output, the suppression is approximately –35dBc.

Figure 2. RMS jitter at 6.144GHz.

Figure 3. RMS jitter at 12.288GHz.

Leading PLL Performance for Converter Clocks
Improvements to the PLL circuitry mean the ADF4371/ADF4372 devices have a PLL figure of merit (FOM) as low as –234dBc/Hz that, when together with a correspondingly low 1/f noise of –127dBc/Hz (normalised to 1GHz output frequency at 10kHz offset), allows users to generate clocks with an rms jitter number as low as 40fs (1kHz to 100MHz integration limit), making them very suitable for use in the most demanding converter clock applications. A simple low-pass filter with small resistors is recommended in order to minimise the resistor noise, which may appear in the loop. A high frequency (250MHz or 125MHz using the reference frequency doubler) ultralow noise reference source is essential for achieving such low noise. The phase frequency detector (PFD) can operate up to a maximum of 250MHz in integer-N applications. The doubled VCO differential output at RF16 can be used to interface directly to some ADI converters without the need for external balun circuitry, which would increase cost and performance. No degradation in performance from 6.144GHz to 12.288GHz is expected.

Communications and Instrumentation LOs
For wireless and instrumentation applications, the ADF4371/ADF4372 contain 39-bit resolution sigma-delta modulators, which enable frequency generation with sub-millihertz resolution with 0Hz error. In this case, the ADF4371 PFD operates up to a maximum of 160MHz PFD frequency. In these applications, the ADF4371/ADF4372 deliver <48fs rms jitter. The ADF4371 also has industry-leading PLL spurious performance, with PFD spurious as low as –100dBc and in-band, unfiltered integer boundary spurious as low as –55dBc. This performance level greatly simplifies frequency planning and reduces time to market. Many fractional-N PLL and VCO devices have unpredictable fractional-N spurious mechanisms, which can lead to additional unplanned characterisation and frequency planning, which add complexity and cost.

Small Size
The ADF4371/ADF4372 PLL/VCO devices are available in 7mm × 7mm, 48-lead land grid array (LGA) packages. Minimal additional decoupling is required, meaning exceptional performance exists in a small footprint solution.
To achieve the best performance, the use of high quality low dropout (LDO) regulators such as the ADM7150 or LT3045 are recommended. The VCO can be supplied with either 3.3V or 5V, and the remaining circuitry is powered from a 3.3V rail. The ADF4371 can be simulated in ADIsimPLL™ to assist the user in designing the appropriate external component circuitry required to implement a full PLL system.

Conclusion
Industry-leading frequency coverage, performance, and small form factor combine on the ADF4371 to address the high demands of new communication and instrumentation systems.

About the Author
Ian Collins graduated from University College Cork with a degree in electrical and electronic engineering, and he has worked in the RF and Microwave Group at Analog Devices since 2000. He is currently an applications manager in the Microwave Frequency Generation Group, which focuses mainly on phase-locked loop (PLL) and voltage controlled oscillator (VCO) products. When not spending time at work or with his young family, Ian enjoys photography and the theatre (both on- and off-stage), reading, and listening to music. He can be reached at ian.collins@analog.com.