October 4, 2018
|By: Christine Young
Blogger, Maxim Integrated
What do gaming consoles, factory automation systems, smart home speakers, and portable tools have in common? They’re all examples of electronic devices that are powered by single-cell lithium-ion batteries. And as such, they present designers with a vexing challenge: lengthen battery lifetime without expanding battery capacity in these small form factor products.
Consumers of these products have embraced their compact sizes and also expect their devices to last a long time between charges. This expectation has led designers to seek switching components that meet some stringent requirements for the power supplies that deliver reliable DC power to the final products. Ideally, power-supply components for small electronics using single-cell batteries should meet these four key criteria:
The role of quiescent, or standby, current cannot be overlooked when evaluating power-supply components. Many of these battery-powered devices are in standby mode until some periodic action triggers them to wake up and perform a task or transmit data to the cloud. While typically nominal, quiescent current of a power supply is usually the biggest contributor to a system’s standby power consumption. Designing power supplies with low-quiescent-current switching regulators is, therefore, a good way to extend battery life in small electronics.
Figure 1: Wireless speakers and smartphones are among the single-cell, lithium-ion battery-powered devices that can benefit from a high-efficiency buck converter.
TThe voltage output of a single-cell lithium-ion battery is generally between 3.0V and 4.2V. If the desired voltage rail falls within this range, then a buck converter can help ensure stable voltage rails. Maxim has a new buck converter designed to meet the requirements of compact electronics powered by single-cell, lithium-ion batteries. The MAX77324 is a 1.5A high-efficiency buck converter available in a 1.22mm x 0.85mm WLP-6 package for a 6.89mm2 total solution size. It provides 93% peak efficiency, 40µA quiescent current, 1µA shutdown current, and an automatic SKIP mode to improve light-load efficiency. The device automatically enters SKIP mode at low output currents, which reduces switching losses by effectively reducing the switching frequency and increasing the converter efficiency. This feature results in optimal efficiency over the widest operating range and, therefore, the longest battery life. Also enhancing battery life are the buck converter’s low on resistance for its high- and low-side MOSFETs.
Figure 2 provides an efficiency curve that shows how the MAX77324 achieves better than 87% conversion efficiency over a typical lithium-ion battery input voltage range and over output current range as low as 4mA up to 1.5A. Whether the converter operates at 1.0VOUT or 1.2VOUT, this level of performance (shown for the 1.8VOUT configuration) remains consistent.
Figure 2: Efficiency curve for MAX77324.
Should any intermittent faults occur, the device includes protection features such as undervoltage lockout (UVLO), which prevents operations in case of low input voltage conditions; soft-start, which limits the inrush current during startup; active output discharge; and short-circuit and thermal shutdown protections. The device also uses Maxim’s proprietary Quick-PWM™ quick-response, constant-on-time pulse-width modulation (PWM) control scheme. This control scheme handles wide input/output voltage ratios, providing immediate response to load transients while maintaining a pseudo-constant switching frequency. As discussed earlier, most portable electronics stay on all the time, alternating between standby and full-power mode. The transition between these two modes usually translates into a rapid increase or decrease in output current. The ability for a converter to quickly and accurately react to surges in output current demand is measured by its load transient response. In one use case, the MAX77324 demonstrated the ability to react to a 750mA load step and load release with a minimal output voltage deviation of about 2.5%
Feature sets are continuing to grow while form factors shrink for a variety of electronic products. Meantime, the tiny batteries inside face the tough challenge of keeping these devices running for extended periods of time, despite capacities limited by size. If you want to keep customers happy, enhance product runtime by choosing small, highly efficient power supply components with low quiescent current.