STMicro Releases Variants of Buck Converters for More Efficiency Options

The new step-down, synchronous DC-DC converters come in many configurations to save space and ease integration into automotive OEM designs.

STMicroelectronics (ST) recently announced new DC-DC converters designed to simplify integration and improve efficiency.

The new devices include both isolated (A6983I; datasheet linked) and non-isolated converters (A6893; datasheet linked). Isolated converters generally use a transformer to electrically separate the input and output circuitry. Non-isolated converters do not provide this feature but generally are available at a lower cost. Non-isolated converters also have lower losses because there is no transformer. STMicroelectronics says that the non-isolated A6893 achieves 88% typical efficiency at a full load and can supply up to 3 A of load current. 

Low-Consumption Mode for High Power Efficiency

A buck converter operates by switching on and off to regulate the output voltage and “step down” the input voltage while increasing the current to the load. However, this switching introduces losses and ripple. In low-consumption mode (LCM), the A6983 device saves energy by skipping switching cycles when the connected device isn't using much power—determined by a threshold called ISKIP. Instead of constantly switching, it adjusts the output voltage using feedback voltage and resistance. When the power demand is low, and the output voltage is at the desired level, the device stops switching altogether, ensuring efficient and stable operation. 

The LCM device's switching activities are interrupted when two conditions are met: the inductor current is less than ISKIP, and the output voltage is greater than VFB_LCM, which is 1.8% higher than VFB. By interrupting the switching cycle, the device does not switch as often over time and is more efficient.

In LCM variants, the switching cycle is interrupted when VFB exceeds VFB_LCM. 

Isolated and Non-Isolated Variants

The non-isolated A6983 is available in a few different variants, such as the A6983C low-consumption variant and the A6983N low-noise variant. ST optimized the A6983C for light-load operation. Light-load operation in a buck converter occurs when the load current is considerably lower than the converter's designed or nominal load current.

Block diagram of the A6983. 

The low-noise variant (LNM) A6983N operates differently. Instead of interrupting switching like the LCM, the LNM ensures that switching is kept constant to minimize output ripple. This is referred to as a forced PWM operation. The tradeoff is that the LNM is generally less efficient than the LCM, as shown below: 

Efficiency comparison of LNM and LCM.

The A6983I features isolation using a transformer, eliminating the need for an optocoupler used in traditional isolated designs. In many buck converters, the optocoupler maintains stability by isolating the input and output sides. Both the isolated and non-isolated variants of the A6983 come in a small 3 mm x 3 mm QFN package and feature low quiescent current at 25 uA as well as a power-saving shutdown mode of less than 2 uA.

A Clear Target at the EV Market

By introducing capabilities such as light-load operation in its LCM, ST joins other converters on the EV market, such as Texas Instruments' new GaN-based DC-DC converter, that target high efficiency. ST says that by tightly integrating all circuitry in this small package and offering the LCM and LNM variants of the A6983, customers can easily integrate these designs into applications such as traction inverters, which are essential in EVs.