Low output noise regulator for data acquisition

July 27, 2014 //By Willie Chan
Low output noise regulator for data acquisition
Low output noise, fast transient response and high efficiency are just a few of the stringent power supply demands made by applications featuring high data rate FPGA I/O channels and high bit count data converters. 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.

An easier 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 µModule 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 –40°C to 125°C temperature range.

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

PCB Trace Voltage Compensation Using SENSEP

The resistance of PCB traces between the µModule 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 analog control that adjusts the output voltage by up to ±10%.

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