Wearable technology has come a long way from the first Walkman. There are tons of new sub-categories with thousands of different products. However all of them have one thing in common: the need for power and thus batteries. While cassette players could be recharged in seconds by exchanging the battery modern devices usually have customised batteries which are installed internally and need to be charged within the wearable.
For users it’s as easy as plugging in a cable – for designers it’s about much more. In order to offer a comfortable, safe and convenient user experience it is necessary to consider different charging methods. There are two general classes of chargers: linear chargers and switch-mode chargers. As these two differ in cost, thermal performance, size, application area, features, electromagnetic interference (EMI), bill-of-material (BOM), charge time as well as flexibility we will take a look at both in order to give you some guidance when implementing a charger in your application.
Before we get started with the pros and cons let’s get an overview on the basic layout. Switch-mode chargers use two power FETs (high-side and low-side) in order to control the current going into an inductor (see graphic below).
Switch-mode chargers are more complex than linear-mode chargers, require an external inductor and additional capacitors as well as larger application area with higher BOM count. On the other hand they offer higher efficiency and better thermal performance and are well-suited for applications requiring higher change currents.
The comparably simple linear-mode chargers drop the adapter voltage down to the battery voltage using a pass transistor (see graphic below). The non-power path architecture in which the battery pack and the system are connected to the same VOUT pin provides a simpler and lower-cost charger solution.
Further linear chargers require less application area, lower BOM count and eliminate the need for inductor and additional capacitors. However thermal performance and efficiency can’t keep up with switch-mode chargers.
In a first conclusion this translates into superior flexibility and easy adjustment but inferior features within low-power applications where size, BOM and cost are important for switch-mode chargers. Linear-mode chargers can collect points in size-restricted applications but lose some due to limitations like inadvertent charge termination and system operation with a deeply discharged battery.
Thermal performance is a critical feature of wearables due to the usually close proximity to the skin and the body of the user. Therefore it is necessary to consider power losses as those lead to thermal rise of the PCB in order to keep the thermal performance within limits during the charging process. In general switch-mode chargers offer higher efficiency and better power dissipation compared to linear chargers. However the performance of linear chargers improves as the current drops below 300 mA. As shown below, in an example with 300-mA charge current, a battery voltage of 4.0 V, and 5-V input from the adapter the difference between switch-mode and linear-mode chargers is negligible in this range.
With that being said it is important to consider the complete layout of the device as many other factors like package type or board layers influence the thermal performance.
As established before, linear chargers work without an inductor and are comparably simple which makes them a good fit for applications that are restricted in size and need low BOM cost. However with technological advancements and more sophisticated ICs (e.g. TI’s bq24250) switch-mode chargers get smaller and can be an option.
EMI is not really an issue for linear chargers thus they are well suited for low-noise applications. Switch-mode chargers are more likely to emit electromagnetic radiation due to the rapid current changes of the MOSFETs as well as ripple in the switching converter output. In order to reduce the disturbances of the EMI and its effects on areas on the PCB you can add shielding, change the switching frequency or relocate the charger relative to the PCB – however it’s likely that this will affect cost and size.
Regarding flexibility switch-mode chargers with Inter-Integrated Circuit (I2C) interface offer the advantage of programmability which helps designers to define parameters like charge current, input current limit, regulation voltage and termination level. Therefore those battery chargers can be repurposed in multiple products which helps to save costs and preserve scalability of applications. Linear chargers commonly offer less features. However over the last years programmability has been introduced to this family of chargers using external components. Latest generations (e.g. TI’s bq24072 and bq24232) enable designers to adjust pre-charge current, current limit, safety timer and termination current level. However those features add cost and BOM count.
If charging time and/or higher voltages from adapters are a concern switch-mode chargers might be the better solution. The reason is that they can typically handle higher input voltages and that they are more likely to handle the transitions between the different charge cycle phases (re-charger (trickle), fast charge (constant current), taper (constant voltage)) smoothly especially if they are equipped with charge time optimisation technology from TI.
So what’s the best charger design for low-power applications? In general linear chargers will help you to reduce complexity, costs and EMI. Switch-mode chargers will help you to improve efficiency and thermal performance and offer more features and flexibility.
It is not possible to crown a winner of this comparison as the best fit will be the option that suits your requirements and offers the best outcome for your end product.
This article is based on “Battery-charging considerations for low-power applications” by Tahar Allag published in the Texas Instruments Analog Applications Journal (AAJ) 2016 2Q. You can read the full article and download the AAJ here.
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