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. As Willie Chan of Linear Technology explains, 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. However, they 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 LTM 8028, 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 x 15mm x 4.92mm BGA package. The LTM8028 operates from an input range of 6V up 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 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 ten percent 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 50mV increments 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%.
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 analog 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 LTC 2185, a 16 bit ADC, was evaluated to see the difference between using (1) a typical LDO, (2) a typical switching regulator, and (3) the LTM8028 low noise µModule regulator.
The sampling process produces 250kHz spurs at baseband. As a result, the SINAD drops to 71.84dB, around 4dB compared to an LDO. This reduces the LTC2185 to nearly 12 bit performance. In demanding applications where tenths of dBs are significant, losing 4dB of SINAD because of a noisy regulator is unacceptable.
In addition to degrading the SINAD of the ADC, these spurs may land on neighbouring channels or on other signals of interest, making it impossible to receive meaningful data from those channels. With the LTM8028, only a few extraneous spurs exist near the desired frequency and the SINAD performance is only 0.03dB worse than the LDO baseline. The spurious content that was very pronounced in the spectrum of the switching regulator is virtually eliminated. As a result, there will not be any performance degradation of the LTC2185 when using a LTM8028 regulator.
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