TI plans 80V DC-DC converter chip with active filter for EMI

TI plans 80V DC-DC converter chip with active filter for EMI

Technology News |
By Nick Flaherty

Alongside the 42V LM25149-Q1 with the AEF active filter, the company has also integrated passive filters into another DC-DC converter, the LM25149. TI is also working on pin-to-pin compatible 80V versions of both devices.

The two also use a dual-random spread-spectrum (DRSS) technology, and the combination allows designers to reduce the area of the external EMI filter in half, lower the conducted EMI of the power design by as much as 55 dBµV across multiple frequency bands, or achieve a combination of reduced filter size and low EMI.

“This is the first buck controller with an integrated active EMI filter,” said  Ganesh Srinivasan, product line manager for wide Vin buck switching regulators.  “This is a breakthrough technology. The active EMI filter senses the noise at the input and feeds a signal out of phase to fully cancel it. With the active filter and DRSS it is also possible to get the same performance and reduce the size of the bulky EMI  filters by 50 percent in area and 75 percent in volume,” he said.

AEF implementation

“The AEF amplifier is configured to respond very quickly, to get attenuation up to a frequency of about 5 MHz,” said Srinivasan.

The active EMI filter with voltage sense and current cancellation (VSCC) uses an operational-amplifier (op-amp) circuit as a capacitive multiplier to replace the filter capacitor (CF) in the passive design. The active filter sensing, injection and compensation impedances use relatively low capacitance values with small component footprints to design a gain (Gop). The effective active capacitance is set by the op-amp circuit gain and an injection capacitor (CINJ).  

The AEF senses the noise voltage, amplifies it and injects a cancellation current into the system. Having a large gain creates a low-impedance path to shunt noise and the measured noise voltage across the load, making it possible to reduce the voltage noise

 Depending on the gain and bandwidth of the active circuit, AEF enables attenuation at the fundamental switching frequency and low-order harmonics, while relatively small-sized passive components (with lower parasitic contributions) provide supplementary attenuation at higher frequencies where the gain of the active stage rolls off.

In terms of the response time specifically, this is enabled by the high gain-bandwidth (GBW) product of the integrated AEF amplifier.

The 42V LM25149-Q1 and LM25149 buck controllers help engineers meet the CISPR25 Class 5  requirements by mitigating conducted EMI across multiple frequency bands. The integrated AEF helps detect and reduce conducted EMI in the low-frequency band of 150 kHz to 10 MHz, enabling engineers to attenuate EMI by up to 50 dBµV at a switching frequency of 440 kHz, relative to a design with the AEF disabled, or as much as 20 dBµV when compared to a design with a typical passive filter. In both design scenarios, the DRSS technology helps mitigate EMI by an additional 5 dBµV across low- and high-frequency bands.

The silicon-based device also operates at 2.2MHz, above the radio band, to further avoid emissions.

The LM25149-Q1 and LM25149 controllers further increase power density by enabling interleaved dual-phase operation and by integrating the bootstrap diode, loop compensation and output-voltage feedback components, which in turn reduces design complexity and cost. Engineers also have an option to use external feedback and loop compensation to further optimize their designs.

There is also a 3.5V to 36V evaluation module to test out the low Iq and both types of EMI improvement. Pre-production quantities of the 42V LM25149-Q1 and LM25149 are available now in a 3.5-mm-by-5.5-mm thermally enhanced, 24-pin very thin quad flat no-lead (VQFN) package. Pricing starts at US$1.42 and US$1.20 in 1,000-unit quantities, respectively.

The LM25149-Q1EVM-2100 evaluation module is available for US$75. TI expects both devices to be available in volume production in the fourth quarter of 2021.

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