A Battery Buck-Boost Charge Controller for Multi-Chemistry Battery Charging Applications

Texas Instruments recently released their new SMBus multi-chemistry battery buck-boost charge controller for rechargeable battery systems that require high-efficiency solutions.

The bq25708 from Texas Instruments is a synchronous narrow voltage DC (NVDC) buck-boost charge controller capable of charging 1-to-4-cell batteries of chemistry types Li⁺, LiFePO₄, NiCd, NiMH, and lead acid. It accepts input voltages ranging from 3.5 VDC to 24 VDC and thus supports a wide range of power sources including legacy USB (2.0 and 3.0), USB 3.1 (Type C), and traditional AC-to-DC adapters.

 

Figure 1. Texas Instruments’ bq25708 IC and application diagram. Images taken from the datasheet (PDF) and TI.com.

 

This IC is designed to be particularly effective when paired with a CPU (notebooks and tablet PCs are included among the recommended applications): it can monitor a system’s power conditions and communicate with a host CPU in order to instruct it to throttle back, or otherwise optimize its performance, when the need arises, such as when there's a reduction in available power.

High Efficiency

Depending on various parameters, this converter is capable of achieving impressively high efficiencies. For instance, when the input voltage (V_IN) is at 12 V, V_OUT is configured to generate either 11.1 or 14.8 V, and the output current is between 2 and 3 A, the converter's efficiency approaches 97%, as can be seen in the figure below.

During light-load conditions, an efficiency of 87% is achieved when V_IN is 12 V, V_OUT is set to 9.2 V, and I_OUT is 50mA. As stated in the datasheet, the high efficiencies under light-load conditions are possible, in part, because of this IC's ability to switch to PFM (pulse frequency modulation) during a light-load situation.

According to section 8.3.3.3 (Pulse Frequency Modulation) of the datasheet, the minimum frequency is 25 kHz.

 

Figure 2. Low-load and high-load currents show rather impressive efficiencies. Plots taken from the datasheet (PDF).

Dynamic Power Management

This battery charge controller features Dynamic Power Management (DPM), which controls the flow of current depending on the system's power needs. For instance, if the controller is in the process of charging the battery and the system's current requirements suddenly increase, the controller will react by reducing the amount of current flowing to the battery as a means of preventing a power-adapter overcurrent condition. Or, as another example, if the system power requirements exceed the power rating of the power adapter, then the controller will allow the battery to discharge its energy to the system, which, on a temporary basis, permits the battery to act as a supplemental power source. Nice!

In addition, the charge controller is able to monitor the total platform power (system power) from both the power supply adapter and the battery; this is the crucial information that allows the controller to "know" when the system power begins to exceed the total power available from both the power adapter and the battery. And when such a condition occurs, the bq25708 uses its PROCHOT (processor hot) signal, which is an active low signal, to "tell" the processor to throttle back (i.e., optimize its performance according to the power available to the system).

Requires Many External Components

As can be observed in the figure below, this seemingly powerful, smart, and versatile charge controller requires many external components. Fortunately, within Section 9 (Application and Implementation) TI has provided—albeit with a warning stating that TI does not warrant the accuracy or completeness of the application—guidance on designing with the bq25708 as well as choosing some external components, such as the inductor, input and output capacitors, and MOSFETs. They have also provided layout guidelines.

 

Figure 3. The bq25708 requires many external components. Diagram taken from the datasheet (PDF).

An Evaluation Module...Coming Soon?

Though the datasheet makes reference to an evaluation module (called the bq2570xEVM-732) as well as to the schematic presented in the module’s user guide, I have been unable to confirm that this EVM actually exists. I can only assume that the module is still in the design and/or test phase at TI. The figure below is a depiction of the evaluation module's test setup.

 

Figure 5. A test setup using the evaluation module, from the user guide (PDF).

 

Have you had a chance to use TI’s new SMBus multi-chemistry battery buck-boost charge controller, or have you been able to find or purchase its evaluation module? If so, leave a comment and tell us about your experiences.