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Employ low output noise regulator for data acquisition

Posted: 12 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:FPGA? data converters? linear regulators? switching regulator? LTM8028?

The power supply demands of applications featuring high data rate FPGA I/O channels and high bit count data converters are quite stringent. Some of which are low output noise, fast transient response and high efficiency. The power supply designer faces the difficult task of meeting all of these requirements with as few components as possible, since no single topology easily meets all three.

For instance, high performance linear regulators achieve the required low output noise and fast transient response, but tend to dissipate more power than a switching topology, resulting in thermal issues. Switching regulators, on the other hand, are generally more efficient and run cooler than linear regulators, but generate significantly more output noise and cannot respond as quickly to transients. Power supply designers often resort to combining the two topologies, using a switching regulator to efficiently step down a relatively high bus voltage, followed by a linear post regulator to produce a low noise output. Although it is possible to produce a low noise supply in this way, it requires careful design to achieve high efficiency and fast transient response.

Another way to reap the benefits of both a linear regulator and a switching regulator is to use the LTM8028, which achieves low noise, fast transient response and high efficiency by combining both regulators into a single part.

Integrated switching and linear regulators
The LTM8028 is a 36VIN, 5A?Module regulator that combines a synchronous switching converter and low noise linear regulator in a 15mm 15mm 4.92mm BGA package. It operates from an input range of 6V to 36V with an output voltage that can be programmed between 0.8V and 1.8V. The combination of the two converters results in tight tolerance of line and load regulation over the C40C to 125C temperature range.

The switching frequency can be adjusted between 200kHz and 1MHz with the RT resistor, or the SYNC pin can synchronise the internal oscillator to an external clock. The 5A current limit can be reduced by utilising the IMAX pin. The PGOOD pin can be used to detect when the output voltage is within 10% of the target value.

Figure 1: ?Module regulator takes a wide ranging 6V to 36V input and produces a low noise 1.8V output with up to 5A output current.

PCB trace voltage compensation using SENSEP
The resistance of PCB traces between the?Module regulator and the load can result in voltage drops that cause a load regulation error at the point of load. As the output current increases, the voltage drop increases accordingly. To eliminate this voltage error, the LTM8028's SENSEP pin can be connected directly to the load point.

Programmable output voltage
The output voltage can be digitally programmed in 50mVincrements by controlling the LTM8028's 3-state inputs: VO0, VO1 and VO2. Additionally, the MARGA pin can be used for output margining via analogue control that adjusts the output voltage by up to 10%.

DC1738A highlights the LTM8028 capabilities
A 1.8V output application is shown in figure 1. The LTM8028 comes in a 15mm x 15mm x 4.92mm BGA package and is featured in the demonstration circuit DC1738A, shown in figure 2.

Noise test comparison using LTC2185 ADC
When powering high speed analogue-to-digital converters (ADCs), it is important to use a power supply that is as clean as possible. Any switching spurs that are present on the power supply rail will translate into AM modulation in the ADC output spectrum. The noise performance of the LTC2185, a 16bit ADC, was evaluated to see the difference between using (1) a typical LDO, (2) a typical switching regulator, and (3) the LTM8028 low noise?Module regulator. A simplified schematic of the test is shown in figure 3, where the DUT is represented by either of the configurations. Figure 4 shows the FFT plots using the three different methods of powering the LTC2185 when sampling a 70MHz tone at 100Msps. The LDO provides a clean power supply, achieving a SINAD of 76.22dB. However, when powered by a typical 250kHz switching regulator, there are spurs around the fundamental with an offset frequency of 250kHz. These are switching regula-tor spurs that are AM modulated around the carrier frequency.

Figure 2: The LTM8028 makes it possible to build a minimal component-count regulator that meets stringent noise, efficiency and transient response requirements.

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