Texas Instruments Establishes New Benchmark for Low Noise Applications

Texas Instruments (TI) announced the introduction of the industry's first -36V low dropout voltage regulator (LDO) TPS7A30, used in conjunction with the positive voltage TPS7A49, to provide designers with a total solution that supports high precision analog applications. The company's power business development engineer Wang Hao said that the current design plan requires "to achieve a higher degree of integration in a smaller package." The reason is obvious, because increasing PCB density helps designers to achieve product differentiation, and higher input voltage helps reduce the need for discrete transient diodes.

Key Features and Benefits of the TPS7A30 and TPS7A49 Wide input voltage range that supports high line-to-rail transient high-stable tolerances: TPS7A30 is -3.0V to -36V, TPS7A49 is +3.0V to +36V;

The nominal accuracy is 1.5% at full load, full input voltage, and ambient temperature, enabling higher accuracy for VCO, PLL, and ADC;

High stability with ceramic output capacitors larger than 2uF enables smaller size and lower cost than tantalum capacitors;

Low noise/power supply rejection ratio (PSRR) significantly improves the effective range of RF applications, filters, and DC/DC glitch processing

– Output Noise 16uVRMS

PSRR is 72dB at –1kHz and 55dB at 10kHz to 700kHz

Wang Hao said that there are many noise sources in the general design system, such as: glitches caused by momentary closure of switching power supply, low-frequency noise from AC to DC, low-frequency noise from 50 to 60 Hz, and 300 kHz from DC/DC conversion. 1MHz band switching noise and so on, "these noise sources exceed 80% of the total system noise, if the control is not good will have a huge impact on the overall system solution."

"The PSRR of the TPS7A30 is 72dB at 1kHz, 55dB from 10k to 700kHz, and it can be kept above 40dB at 1M." Wang Xi said, "Our PSRR values ​​are marked from 0Hz to 1MHz, and the general LDO After 100kHz, it is not marked because after 100kHz, the PSRR will attenuate very much."

But he also reminded users that one must pay attention to the notion that PSRR is not the higher the better. Some of the LDO specifications have a nominal PSRR of 82dB. This is good, but it has little significance. Because the highest dB is obtained at a low frequency of 1 kHz, the higher the frequency is, the harder it is to do PSRR, and at high frequencies, the more attention is paid to the filter noise value over a wide frequency range. Wang Hao also highlighted the advantages brought by using this type of LDO with ceramic capacitors of 2uF or more. “Previous LDOs are often used together with tantalum capacitors, but tantalum capacitors are relatively expensive and their performance is not stable. Capacitance values ​​change significantly with temperature characteristics. Moreover, tantalum capacitors are more likely to foam at high temperatures and low temperatures.

The reason for choosing a wide input voltage range of ±36 volts is that the maximum withstand voltage of most ICs at present is ±12V, but in practical industrial applications, in order to ensure that the -16V, -20VLDO are facing the instantaneous pulse voltage It can also work normally, and the front-end regulator needs special treatment. Relatively speaking, -36V will be more reliable.

When the TPS7A49/TPS7A30 is used in combination, an additional filter is usually not required. However, he added that although the PSRR of TI products is already very high, if customers have some special applications that need to filter strong signal ripples to less than 1 millivolt, then adding a filter circuit to this frequency point is also Yes.

Wang Wei frankly stated that since LDO's related applications are very persistent, many very professional customers have been doing so for many years. If they ask “Which LDO is best for the system?”, it may not be able to give a satisfactory answer. He believes that the selection of LDO should first consider the following factors, voltage range, current intensity, package cooling, manufacturing accuracy, and so on. “But the LDO's PSRR requirements are relatively high. Should we also add some special considerations? That is, what is the accuracy of the system? What is the PSRR requirement for which frequency band for the LDO? How much can the positive and negative 15V or 12V standard voltage withstand? The power load? These factors should all be considered when using LDOs."