Easy-to-assemble optical module power supply solution

    High-speed, high-density optical module power solutions face numerous challenges, including high efficiency, excellent heat dissipation, compact size, and low emissions. High-speed, high-density optical modules are widely used as interfaces connecting fiber and copper networks, data centers, and most endpoints in optical networks. As more components are integrated into modules, power solutions face even greater challenges, requiring higher efficiency, better thermal performance, a smaller footprint, and lower emissions.

    Optical modules might not be the first system we think of when it comes to the telecom infrastructure market. Because they are so small, they can be easily forgotten—especially compared to the larger base stations we see everywhere we drive down the road. However, the two systems do have something in common: they must be robust and reliable to minimize network downtime.

    Base stations and optical modules face space constraints—cramming a lot of power, processing, and functionality into a limited space—even though optical modules often fit better on printed circuit boards (PCBs) due to their ultra-small form factors. Finally, designers of both base stations and optical modules value the ease of assembly offered by integrated circuits (ICs) packaged in traditional quad flat no-lead (QFN) packages. Wafer chip-scale packages (WCSPs) are generally undesirable due to their more complex manufacturing requirements. They also exhibit poor thermal performance and a correspondingly higher temperature rise, which reduces reliability.

    Increasing data rates and channel counts in optical modules require higher currents and new architectures to maintain a small solution size, especially for rails requiring currents exceeding 3A. At such high currents, thermal performance and reliability become issues again. How much heat can a small optical module dissipate? If it can't all be dissipated to the surrounding environment, how hot will the IC get? Will it be too hot? How can new solutions deliver higher currents while being small, robust, and easy to assemble?

    The first step toward smaller power supplies is to increase the switching frequency. However, this also increases power losses and temperature rise. Fortunately, frequency isn't the only knob we can turn to achieve smaller power supplies. Splitting the high current into two lower-current phases allows us to use two smaller inductors instead of one large one. This saves cost and PCB space while improving efficiency. Higher efficiency also means less heat to dissipate, easing thermal challenges. I'll discuss this in more detail in a minute.

    Besides size, robustness is a key characteristic of optical modules. One way to achieve robustness in optical modules is to have the module itself check the performance of the data signal and report maintenance needs or outright system failures. Robustness is also possible when the host processor self-adjusts to optimize its performance. Power ICs such as the TPS62480 help address this issue in two unique ways: voltage margining and thermal monitoring.

    The voltage select (VSEL) pin simply changes the output voltage between two customizable levels. The host processor toggles the VSEL pin to alter the output voltage to compensate for strong or weak silicon or to adjust performance for different operating modes. Both of these actions reduce the module's power consumption and, consequently, its temperature rise.

    If the temperature rise of the TPS62480 power supply remains excessive, its Thermal Good (TG) feature activates. If the temperature approaches the IC's maximum rating, the TG pin goes low to alert the host processor. Once the host processor receives this early warning signal, it can reduce processing power or data rates or notify the system host of a potential maintenance issue. Figure 1 shows a typical schematic including the VSEL and TG features.

    Figure 1: The TPS62480 provides a VSEL pin to easily adjust the output voltage between two levels, and a TG pin to alert the host of elevated temperatures.

    Finally, the TPS62480 is packaged in the easy-to-assemble QFN-style HotRod™ package. This innovative packaging technology packs a 6A power supply into a 2.5mm x 3mm package, providing a total solution size of less than 80mm². Because it resembles a standard QFN, the package has low thermal resistance, resulting in a low temperature rise. Combined with the high efficiency of the two-phase approach, the low temperature rise enables full power operation without derating in ambient temperatures above 85°C. Figure 2 shows the derating curve.

    Figure 2: The TPS62480's high efficiency and good thermal performance enable it to achieve full 6A output current even above 85°C ambient temperature.

    Optical modules now have the opportunity to achieve the required higher current power supplies while maintaining small size, robustness, and ease of assembly.