“Ultra-Low” Current Consumption: A Battery Charger from STMicroelectronics

This article will look at the most important features of the STBC15 which is STMicroelectronics newly released ultra-low current consumption linear battery charger.

The STBC15 is a linear battery charger with ultra low current consumption. A tiny current requirement makes the device suitable for low-energy sources such as energy harvesting applications. The STBC15 can be used for charging low-capacity cells such as thin film batteries. This charger supports a programmable charging current and floating voltage. With its several protection mechanisms, such as over-discharge and reverse current protection, we can prevent the battery from being damaged. The STBC15 also supports a peak mode of operation which makes the device suitable for RF transmission applications. In the rest of the article, we’ll briefly discuss some of these features.

You can use the following application schematic for the STBC15 charger.

 

Figure 1 Image courtesy of STMicroelectronics.

 

Here, CHGIN and OUT are the input supply voltage and the output, respectively. Vset0, Vset1, Iset0, and Iset1 are used to set the programmable parameters of the charging algorithm. PKM and SM pins can make the device enter the peak mode and shelf mode of operation, respectively. The BAT pin goes to the positive battery terminal. $$CHGIN_{OK}$$ and PG are the charging source status and power good signals.

Programmable Constant Current Constant Voltage (CC/CV) Algorithm

The STBC15 uses CC/CV battery charging algorithm. A typical CC/CV algorithm is shown in Figure 2.

 

Figure 2 Image courtesy of MIXDES.

 

At the beginning of the charging process a constant current (CC) is applied until the battery is charged to a voltage specified as the “floating voltage” in the charger datasheets. Then, the charger goes into the constant voltage (CV) phase. In this phase, a gradually decreasing current is applied to the battery so that the battery voltage remains very close to the floating voltage (see Figure 1). When the current decreases to a value about 10% of the current in the CC phase, the battery is considered to be fully charged. Below, you can see the finite state machine (FSM) that the STBC15 uses to perform the CC/CV algorithm.

 

Figure 3 The CC/CV FSM of STBC15. Image courtesy of STMicroelectronics.

With the STBC15, you can choose the value of the charging current and the floating voltage. To this end, different combinations of Vset0, Vset1, Iset0, and Iset1 can be used. For example, with STBC15, you can have the floating voltages shown in Table 1 below:

 

Table 1 taken from the device datasheet.

 

In the schematic of Figure 1, both Vset0 and Vset1 are connected to logic high (CHGIN) and, as shown in the figure, the floating voltage is 4.2 V. It’s worth to mention that the floating voltage is a critical parameter of the charging process. The charger must be able to keep the battery voltage within the floating voltage with 1% or better accuracy. For the STBC15, the floating voltage can be maintained with an accuracy of 0.5%.

Note that a specific version of STBC15 called STBC15L is designed for charging Li-Ion batteries. For this version, the floating voltages are as shown in Table 2.

 

Table 2 the floating voltage for STBC15L (from device datasheet).

 

Similarly, you can set the charging current using the Iset0 and Iset1 inputs. For example, connecting these two terminals to logic high (CHGIN) as shown in Figure 1, gives us the maximum charging current of STBC15 which is 40 mA.

Battery Protection Functions

The STBC15 supports over-discharge protection. When the battery is discharged to a voltage threshold specified by the $$V_{BAT-OVD}$$ parameter, the charger disconnects the battery.

The STBC15 also has a reverse current protection which prevents undesired battery discharge when the input voltage is lower than the battery voltage. Note that preventing a reverse current from the battery to the input voltage source becomes particularly important when the input voltage is provided by a photovoltaic panel.

Shelf and Peak Modes of Operation

By applying a positive pulse to the SM pin, the device goes to the shelf mode and disconnects the battery. In this case, the current consumption reduces to less than 10 nA. This ultra low power mode allows the final product to have a long shelf storage period without discharging the battery.

As discussed in the previous section, when the battery discharges below $$V_{BAT-OVD}$$, the over-discharge protection mechanism disconnects the battery. However, in some applications, such as RF transmission bursts, the battery voltage may temporarily go below the typical values of $$V_{BAT-OVD}$$. To avoid false triggering of over-discharge condition, the STBC15 allows us to lower the threshold voltage $$V_{BAT-OVD}$$ by entering the peak mode operation. By setting the PKM input of the STBC15 to logic high, the minimum of the $$V_{BAT-OVD}$$ parameter reduces from 3.2 V to 2.2 V.

For more information about the STBC15 charger, please refer to the device datasheet.

 

If you have any experience with this device or other similar parts, please let us know in the comments below.