Optical modules are probably not the first system you might think of in the telecom infrastructure market. Because they’re so small, optical modules easily get forgotten – especially when compared to the larger base stations that you see everywhere when driving down the road. However, both systems do have something in common: they must be robust and reliable in order to minimize network downtime.
Base stations and optical modules are space-constrained – cramming a whole lot of power, processing and functionality into a limited space – although optical modules are generally more pressed for printed circuit board (PCB) space because of their super-small form factor. Finally, both designers of base stations and optical modules value the ease of assembly that comes with integrated circuits (ICs) packaged in traditional quad flat no-lead (QFN) packages. It is generally not desirable to employ wafer-chip scale packages (WCSPs), because they require more complex manufacturing. They also have poorer thermal performance and a corresponding greater temperature rise, which reduces reliability.
The increasing data rates and channel counts in optical modules demand ever-higher currents and new architectures to maintain a small solution size, especially for rails that need currents above 3A. And with currents this high, thermal performance and reliability are issues once again. How much heat can you dissipate in a small optical module? If you can’t dissipate it all into the ambient environment, how hot will the IC get, and will that be too hot? How can new solutions deliver the higher currents while at the same time being small, robust and easy to assemble?
The first step in achieving smaller size for a power supply is to increase the switching frequency. But this also increases the power loss and temperature rise. Fortunately, frequency is not the only knob you can turn to get smaller. Splitting the high current into two lower-current phases allows you to use two smaller inductors rather than one larger inductor. This saves cost and PCB space while increasing efficiency. Read my blog, Are two inductors are better than one?, to learn more about this topic. Higher efficiency also means there’s less heat to dissipate, which eases thermal challenges. I’ll talk more about this in a minute.
In addition to size, robustness is a key optical-module trait. One way to obtain robustness in an optical module is for the module itself to check the performance of the data signal and report a maintenance need or outright system failure. Robustness is also possible when the host processor self-adjusts to optimize its performance. A power-supply IC such as the TPS62480 assists with this in two unique ways: voltage margining and thermal monitoring.
A voltage select (VSEL) pin enables a simple change of the output voltage between two customizable levels. The host processor toggles the VSEL pin to change the output voltage to compensate for its strong or weak silicon, or to adjust its performance for different operating modes. Both of these enable a power-consumption reduction in the module, which reduces the corresponding temperature rise.
If the temperature rise in the TPS62480 power supply is still too high, its thermal-good (TG) feature acts. If the temperature gets near 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 the processing power or data rate, or notify the system host of a possible maintenance issue. Figure 1 shows the typical schematic that includes the VSEL and TG features.
Figure 1: The TPS62480 provides a VSEL pin to easily adjust the output voltage between two levels, as well as a TG pin to alert the host of an elevated temperature
Finally, the TPS62480 is packaged in a QFN-style, easy-to-assemble HotRod™ package. This innovative packaging technology fits a 6A power supply into a 2.5mm-by-3mm package and delivers a total solution size of less than 80mm2. Because it’s like a standard QFN, the package’s thermal resistance is low – and a low temperature rise results. Combined with the high efficiency of the two-phase approach, the low temperature rise enables operation at full power above 85°C ambient with no derating. Figure 2 shows the derating curve.
Figure 2: The TPS62480’s high efficiency and good thermal performance enable its full 6A output current even above an 85°C ambient temperature
Optical modules now have the opportunity to achieve the higher-current power supply required, along with small size, robustness and ease of assembly.
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