Operate Your Design from a Single Battery, with the Right Converter
July 18, 2017
|By: Jim Harrison
Guest Blogger, Lincoln Technology Communications
If you are designing a portable device, especially one that has fairly low average power, why not operate from a single battery? I see many applications using two or three primary batteries. That seems like a pretty big waste, when one will do just fine using a readily available, low-cost switching regulator chip.
Those multiple batteries, of course, increase the voltage, but a great, high-efficiency boost converter can do that and keep a nice regulated supply for you as well. It’s really a sweet application idea, providing efficiency and small size.
Let’s assume you need a 3.3V supply and, as always, you want to get as much energy as possible out of your battery. You could use one standard-type primary AAA or AA size battery—non-rechargeable. In either alkaline or lithium, these have much higher energy density than any rechargeable cell.
Alkaline batteries are lower cost than lithium (by 1/3), but have a higher internal resistance and, thus, work better at low discharge rates. Also, at about 210Wh/kg, they have about one-half the energy density of 1.5V lithium primary cells (Li-FeS2). An alkaline cell’s output voltage will sag under a high load because of that high internal Z. A spent alkaline in a digital camera (a high peak load) will leave enough energy to run the kitchen clock for two years. One example of a lithium Li-FeS2 type battery (1.5V) is the Energizer® Ultimate Lithium brand. These have a 400WH/kg energy density and a relatively low impedance.
Two Types of 3.6V Primary Lithium-Ion Batteries
Another type of lithium battery is the Lithium-thionyl chloride (LiSOCI2 or LTC). This type is not normally found in the drug store and is quite a bit more expensive than the Li-FeS2 type, but it offers an energy density of up to 500WH/kg. It has a nominal voltage of 3.60V/cell and is one of the most rugged of the lithium-metal batteries. LiSOCI2 cells offer an even higher energy density than the 1.5V Li-FeS2 type.
I also must mention that the LiSOCI2 3.6V batteries come in two versions: bobbin and spirally wound. The bobbin-type have higher internal impedance, but offer a higher energy density and a very low self-discharge rate (< 1%/yr). This low self-discharge rate means the bobbin-type can operate up to 40 years in a very low-current application.
The use of 3.6V lithium-ion batteries has become common. One of the main drawbacks of this type of battery is that a step-up/step-down DC-DC converter must be used to make a standard 3.3V supply because the battery voltage ranges between 2.7V and 4.2V.
There are a few different topologies that can be used for a combination step-up/step-down converter, the most popular being the single-ended primary inductance converter (SEPIC). This is a non-isolating voltage converter that is unusually non-inverting, but can also be inverting.
One other option that is often overlooked is the step-up converter followed by a low dropout (LDO) regulator. Because a linear regulator is used for the step-down function, it is often assumed that the efficiency is poor. However, when making 3.3V from a single lithium-ion battery, the efficiency of this circuit can exceed that of an equivalent SEPIC circuit, with the added benefit of lower component count, lower cost, and less board space.
Examples of Two Boost Regulators
The MAX17220-MAX17225 from Maxim is a family of boost (step-up) DC-DC converter ICs with three power levels (see Figure 1 for a circuit drawing). They offer 225mA, 0.5A, or 1A peak inductor current and feature a 0.40V minimum VIN. The ICs take ultra-low quiescent current and have what Maxim calls True Shutdown™ that disconnects the output from the input with no forward or reverse current. Their output voltage is selectable with a standard 1% resistor and, with the devices’ built-in power switch, the circuit design requires only four external components.
Figure. 1. MAX17220–MAX17225 Basic Circuit
These ICs offer very small total solution size and high efficiency (up to 95%) throughout their entire load range (see Figure 2 for a block diagram). Their switching frequency is up to 2.5MHz and they are great for battery-powered applications where long battery life is a must. They use a fixed on-time, current-limited, pulse-frequency-modulation (PFM) control scheme. The ICs' 2mm x 2mm 6-pin μDFN or 0.88mm x 1.4mm 6-bump WLP packages make for easy PCB layout. Some versions have post-startup enable transient protection (ETP), allowing the output to remain regulated for input voltages down to 400mV. The minimum startup voltage is 0.88V.
Figure 2. Block Diagram of MAX17220–MAX17225
The MAX1705 and MAX1706 are high-efficiency, low-noise, step-up DC-DC converters with an auxiliary linear-regulator output. As I had mentioned earlier, 3.6V lithium cells usually need a buck-boost topology to cover their lifetime voltage range, but these converters combine boost and linear regulators, and the linear output is a very low noise supply as well. They use a 300kHz synchronous rectifier PWM boost topology to generate a 2.5V to 5.5V output from a battery input, such as 1 to 3 NiCd/NiMH cells or 1 Li-Ion cell. They provide a regulated output over their entire operating voltage range. The MAX1705 has a 1A n-channel MOSFET switch, while the MAX1706 has a 0.5A switch. Minimum VIN is 1.1V and they have 1μA shutdown mode.
The linear regulator in both devices delivers up to 200mA. An efficiency-enhancing track mode reduces the step-up DC-DC converter output to 300mV above the linear-regulator output. The devices deliver 5% better efficiency than similar non-synchronous converters. They feature a pulse-frequency-modulation standby mode to improve efficiency at light loads, and a 1µA shutdown mode. Both devices come in a 16-pin QSOP package and have two shutdown-control inputs for push-on/push-off control, along with an uncommitted comparator for use as a voltage monitor. They come in a 16-pin QSOP package, which occupies the same space as an 8-pin SO, and in standard or industrial temperature ranges. A MAX1705 evaluation kit can be used to consider the ICs for a bunch of different battery-powered designs.