Dropping the LDO: New Buck Converter from TI Integrates Ferrite Beads
Last week, TI announced its newest product: a low-noise buck converter bolstered by integrated ferrite beads.
When designing analog or mixed-signal systems, power supply noise is typically a paramount consideration.
Noise in this context can be defined as thermal noise generated from components in a power supply such as resistors and transistors. Designing to ensure proper operation of noise-sensitive analog devices, like ADCs, will influence many aspects of system operation.
Power Supply Considerations in Sensitive Analog Circuits
Generally, in sensitive analog circuits, linear power supplies are preferable for their low-ripple characteristics. Linear regulators are characterized by their low output voltage ripple and high bandwidth. This is partly because linear regulators don't include elements that switch on and off frequently.
On the other hand, power efficiency is always an important design consideration, and for that, switch-mode power supplies (SMPS) take the cake. SMPS are more efficient than linear power supplies, but this comes at the cost of increased noise due to switching components.
To quantify this, SMPS usually have mV of output noise voltage whereas linear power supplies tend toward uV output ripple.
Common noise-sensitive power supply architecture. Image used courtesy of Texas Instruments
To work around this difference, a common design practice is to use SMPS as a DC-DC converter and feed it into a linear dropout regulator (LDO). From there, the designer can pass the device through a ferrite bead and finally supply power to the noise-sensitive device. The LDO helps to minimize ripple and noise, while the ferrite bead helps lower EMI and filter out power supply noise.
While this process does work, it comes at the cost of increased power loss (because of the introduced LDO) and board space.
Texas Instruments' New Buck Converter
Well aware of the flaws in this kind of system, TI set out to find a solution. Last week, the company announced the "industry's first low-noise buck converter with integrated ferrite bead compensation."
TI's new products, the TPS62912 and TPS62913, work to mitigate the ripple from the DC-DC converter by leveraging a ferrite-bead filter, integrating ferrite-bead compensation and remote-sense feedback.
The new components combine the inductance of the ferrite bead with an extra output capacitor to eliminate high-frequency devices in the output voltage ripple. According to TI, this feature can shrink ripple by approximately 30 dB.
New TPS62912 and TPS62913. Image (modified) used courtesy of Texas Instruments
These new products are said to offer a low noise of 20 µVRMS for frequencies ranging from 100 Hz to 100 kHz and a low output-voltage ripple of 10 µVRMS. With such low noise values, the hope is that engineers will have the ability to remove the LDO component from their designs.
Conclusions
One question about this product is its frequency range. Oftentimes a buck converter will have switching frequencies of 600 kHz and higher depending on the desired application and other passives in the circuit. TI says this product includes low-noise results from 100 Hz to 100 kHz, so it’d be interesting to see what the noise results are at higher frequencies.
By integrating the ferrite bead into the design, TI has been able to effectively create a power supply that combines the efficiency of SMPS with the low-noise characteristics of a linear regulator. This means that using this product should help to save power and space compared to current design methods.
