A new series of real-time clock (RTC) ICs is said to feature the world’s lowest current consumption at just 0.15 μA. Developed by OKI Electric Industry Co., the ML9073/ML9074 series reportedly offers appliance engineers 1.5 times longer battery life than conventional ICs.
According to the Tokyo-based company, devices such as digital cameras, audio players, TVs, and DVD recorders typically have built-in RTCs for their clock functions to keep accurate time even when the power is turned off. However, most RTCs are embedded in application-specific integrated circuits (ASICs) and microprocessor units (MPUs) as IPs, and it is difficult to minimize current consumption because ASICs and MPUs use leading-edge processes. Thus, reducing current consumption when the power is turned off has been a challenge in mobile equipment that runs on small batteries such as button dry batteries.
Sawako Honda of OKI’s SoC business team says that the new series responds to these market needs by using silicon-on-insulator (SOI) complementary metal-oxide–semiconductor (CMOS) technology. The technology utilizes an SOI substrate with an embedded insulator instead of a conventional silicon substrate. This enables lower-voltage operation and decreases the effect of leak current temperature, which is attractive for low-voltage, low-current, and low-leakage operation. It has also helped reduce the size of the chip. “When we apply an SOI structure, there is no need for a ‘well,’” Honda explains. “Therefore, one can place the transistors closer to each other.”
She also notes that while other companies can achieve lower power consumption using SOI processes, OKI reduces power consumption further by using a low-power circuit technology it developed for watches and calculators.
Specifically, the company uses three approaches to reduce power consumption. The first design strategy is lowering the logic power voltage. According to Honda, this is the area in which the effect of reduced power consumption can be seen the most. The company also reduces the capacitance level of the package. “If the capacitance level for the oscillation circuit is smaller, it is more effective in power consumption,” Honda explains. “However, it is particularly difficult to achieve steady performance while reducing the capacitance level and current. With OKI’s know-how, we are able to reduce this level while keeping steady performance.”
The final approach the company takes to lower power consumption is to expand the margin for the transistor’s lower-power voltage operation by tuning the threshold voltage (Vt).
The series also boasts longer battery life, which the company says was achieved by making the value of the timing operation current small. “We were able to reduce the backup time by over 25% compared to other ultra-low-power RTCs,” Honda says. “This enables consumers to use the product longer without charging or changing the battery.”
Designing the low-power devices did have its challenges, such as stabilizing the line quality of OKI’s in-house SOI process. “We were able to overcome this challenge by modifying prototypes of design and process development,” Honda explains.