LDOs Replace Switching Regulators in Low-Power 4G IoT Modems
In the limbo dance of the Internet of Things, a startup has come out of stealth mode, taking cellular networking to new lows in power and price. Riot Micro is sampling a modem tailored for the latest 4G IoT standards. It claims that its RM1000 chip draws milliamps to microamps of power and could sell for well below the industry’s target of a $5 module.
Carriers around the world are just starting to turn on various flavors of LTE-based cellular IoT networks. They aim to leapfrog an emerging crop of emerging low-power wide-area networks such as LoRa, Sigfox, 802.11ah Wi-Fi, and others.
The Riot Micro chip is designed to handle both the Narrowband IoT and LTE Cat M1 (aka eMTC) standards.
“The modems are very, very similar with a lot of commonalities between the two; and they don’t run simultaneously, it’s one or the other,” said Peter Wong, chief executive of the startup.
Rather than implement baseband functions in software on a DSP, Riot carved fast Fourier transforms and baseband filters into its physical layer block. Separately, it stripped an LTE protocol stack down to just the essentials needed for the IoT specs.
The result is a 55-nm TSMC chip that processes NB-IoT and Cat M1 using a 26-MHz ARM M0 controller, Mbytes of embedded SRAM, and new PLLs. Competing chips tend to use a controller running at greater than 100 MHz and using as much as 16 Mbytes of RAM, often some of it external.
The slow clock rate lets Riot use low-dropout regulators for power management in place of more expensive dc-dc converters that competitors need to compensate for the noise of faster logic.
The result is a baseband that draws about 20mA from a 3.7-V battery when handling 23-dBm transmissions. A full chipset that includes a third-party power amp and RFIC draws about 100 milliamps.
In sleep mode, the part draws less than 2uA. Most of the chip sleeps while code watches a paging channel for signals. Riot claims that the chip has less latency going in and out of sleep mode than rival devices given that it does not have to load DSP code.
“We think [that] we can bring the cost of cellular close to that of Bluetooth so we can help drive the deployment of the IoT,” said Wong.
The RM1000 evaluation boards use Skyworks power amps and RFICs from ACP AG of Switzerland. However, they could also work with chips from Qorvo or other suppliers.