Buck-Boost Converters for Embedded Battery-Powered Applications
Buck-Boost for the Rest of the World
Intersil has been very successful in selling buck-boost converters to the high volume consumer space such as smartphones and tablets. Recently, we’ve released a few select devices to target the lower volume battery applications. This training will focus on Intersil’s ISL9120IR, ISL91107IR and ISL91127IR buck-boost devices in QFN packages.
Buck-Boost in a Battery Application
A single-cell Li-ion battery ranges from 4.2V fully charged down to 2.7V when discharged. If a consistent voltage rail such as 3.3V is needed in a design, then a way is needed to regulate the voltage down when the battery is charged and boost the voltage when the battery is discharged.
A buck-boost converter is essentially just a combination of a buck controller and a boost controller. In buck mode, the transistor Q4 acts as a closed switch and Q3 acts as an open switch. Q1 and Q2 modulate based upon the required load. In boost mode, Q1 is in the closed position, Q2 is open, and Q3 and Q4 modulate depending upon the load. When VIN is close to the same value as the required VOUT, then the device may alternate between bucking and boosting the circuit as shown in the next slide.
Buck to Boost Mode Transition
I know this slide looks scary but don’t be intimidated. Basically, it illustrates the difference in operation between buck, buck-boost and boost modes. When VIN is greater than VOUT, the devices operate in buck mode. When VIN is less than VOUT, the devices operate in boost mode. When VIN is near VOUT, the devices operate in buck-boost mode. You can see that as VIN decreases in this graph, the device moves from pure buck mode into pure boost mode.
Buck-Boost Improves Overall Efficiency
A buck-boost can improve overall efficiency of a device that uses multiple LDOs. Efficiency of an LDO is directly related to the voltage drop across the LDO. In this example if a 2.8V LDO is powered directly from the battery, then the voltage drop is more than a volt when the battery is fully charged. By using the buck-boost to create an intermediate voltage of 3.4V in this example, the efficiency is increased by 3%. The efficiency improvement will be even greater for lower voltage LDO outputs. The other advantage of adding a buck-boost is of course the device would stop functioning when the battery drops below the required input voltage of the LDO, whereas with a buck-boost, the device would run until the battery is fully discharged.
PFM/PWM Mode Transition
Some of Intersil’s buck-boost devices operate in both pulse width modulation (PWM) mode and in pulse frequency modulation (PFM) mode. Why have the two modes of operation? PFM mode has better efficiency at light loads while PWM mode is more efficient at higher current loads. The crossover point is approximately 350mA. Since the Intersil buck-boost devices auto-switch between PFM and PWM modes, our devices are able to optimize the overall efficiency of the device regardless of the load.
Three New Buck-Boost Products
Intersil has introduced three buck-boost regulators in QFN packages—the ISL9120IR, ISL91107IR and the ISL91127IR. These devices are excellent targets for any battery-powered portable device.
ISL9120IR – 800mA Small Buck-Boost
The ISL9120IR is a highly efficient buck-boost converter for low power applications needing less than an amp of output current. It comes in a tiny 3mm x 3mm TQFN package and only requires three external components to create a regulated 3.3V output. Like the ISL91107IR and ISL91127IR that we will discuss in a moment, the ISL9120IR accepts inputs from 1.8V to 5.5V and can create a regulated output from 1.0V to 5.2V. As the battery voltage input drops to near the required output, the ISL9120IR will automatically switch to bypass mode and maximize battery life.
ISL91107IR – 2A Buck-Boost Regulator
The ISL91107IR is the next step up from the ISL9120IR for applications requiring higher currents. It has the same input and output capabilities but can produce outputs in the 2A range. It comes in a small 3mm x 4mm TQFN package.
ISL91127IR – 3A Buck Boost Regulator
The ISL91127IR is the flagship product of these three buck-boost devices. It has the highest efficiency and highest output current of the three devices. It comes in a 4mm x 4mm QFN package with quiescent currents as low as 30µA and output currents up to 3A, and like the previous two devices, only requires three external components to create a 3.3V output.
Maximum Output Current Capability at 3.3VOUT
It’s important to note that the output current rating for each of these three devices is dependent on input and output voltage. While I’ve referred to the ISL9120IR as an 800mA device, the ISL91107IR as a 2A device, and the ISL91127IR as a 3A device respectively, this chart illustrates how the max rating varies with input/output conditions. Depending on the input and output voltages, each device may actually have higher or lower current capability. There is a similar chart in the datasheets for the ISL9120IR and ISL91107IR that will need to be referred to in order to verify that the device will meet design needs.
The buck-boost family from Intersil offers excellent efficiency and the compact size is highly suitable for wearables, IoT devices, portable medical, card readers, or basically any device that draws low to medium power (less than 3A) and is powered by a single-cell Li-ion battery or other chemistry batteries with similar voltage ranges.
Available Tools and Support
Full documentation for the ISL9120IR, ISL91107IR and ISL91127IR are available on the product web pages.
ISL9120IR Evaluation Board
The ISL9120IR kit is readily available and allows the user to customize the output for their specific application or, if preferred, there is a version that offers a fixed 3.3V output.
ISL91107IR Evaluation Board
The ISL91107IR kit is readily available and allows the user to customize the output for their specific application or, if preferred, there is a version that offers a fixed 3.3V output.
The ISL91107IR evaluation board comes in a fixed 3.3V version, has two adjustable output resistors, and jumpers to select whether you want to enable or disable the chip or force it into PFM mode.
ISL91127IR Evaluation Board
The ISL91127IR kit is readily available and allows the user to customize the output for their specific application or, if preferred, there is a version that offers a fixed 3.3V output.
The ISL91127IR evaluation board has adjustable outputs, jumper selectables for enable and mode change, and is available in a 3.3V output version.
With their high efficiency, low power consumption and a small PCB footprint, Intersil’s lineup of buck-boost DC/DC converters help extend battery life and simplify design. The wide input and output voltage ranges provide flexibility to cover a variety of design needs.