How to improve ultrasound system image quality using ultra-low-noise supplies
Figure 5: The LTM8060 is a four-channel μModule configurable array with 3 A/channel output in a compact package measuring just 11.9mm × 16mm × 3.32mm. Image source: Analog Devices
Figure 3: The LTM8053 member of the Silent Switcher family can deliver 3.5 A continuous/6 A peak current; it accepts an input of 3.4 to 40 volts and can provide output across a wide 0.97-to- 15-volt range. Image source: Analog Devices
regulators feature constant- frequency switching. At the same time, they offer EMI performance with selectable spread spectrum techniques, to provide excellent transient response without introducing the uncertainties associated with spread spectrum. The Silent Switcher regulator family is not limited solely to lower power regulators, either. For example, the LTM8053 is a 40 VIN (maximum), 3.5 A continuous, 6 A peak, step-down regulator that includes a switching controller, power switches, an inductor, and all support components. Only input and output filter capacitors are needed to finish the design (Figure 3). It supports an output voltage range from 0.97 to 15 volts, and a switching frequency range of 200kHz to 3MHz, each set by a single resistor.
DC-DC regulator, making the harmonics harder to control A better solution is to select a switching regulator IC with
an external clock from 200kHz to 2.2MHz. Another technique is to use a switching regulator that employs random spread-spectrum clocking to spread the generated electromagnetic interference (EMI) across a wider band, lowering its peak value at any specific frequency. While this is an attractive solution for some applications that
zero and 80kHz in a 400kHz power supply. Thus, while this approach to lowering EMI “spikes” may help meet relevant regulatory mandates, it may be counterproductive for the special SNR needs of ultrasound designs. Switching regulators with constant frequency help avoid this issue. ADI’s family of Silent Switcher voltage regulators and μModule
helps maintain low EMI along with higher current output. A copper pillar flip-chip package in a Silent Switcher µModule regulator helps to reduce parasitic inductance and optimize spike and dead time, enabling high-density design and large current capability in a small package (Figure 4). If more current is needed, multiple LT8053 devices can be connected in parallel. The Silent Switcher line’s technology and topology are not limited to single-output regulators. The LTM8060 is a quad-channel, 40 VIN Silent Switcher μModule
regulator with a configurable 3 A output array (Figure 5). It operates up to 3MHz and is packaged in a compact (11.9mm × 16mm × 3.32mm), over-molded ball grid array (BGA). One of the interesting aspects of this quad-channel device is that its outputs can be paralleled in different configurations to match different load-current needs, up to a maximum of 12 A (Figure 6). In summary, the Silent Switcher regulators offer many benefits with respect to noise, harmonics, and thermal performance (Figure 7). ■ White noise: there are also many white noise sources in an ultrasound system, which leads to background noise and image “speckles.” This noise comes primarily from the signal chain, clock, and power. Adding a low- dropout (LDO) regulator at the power pin of a sensitive analog
a synchronization feature implemented via a SYNC
connection on one of its package pins. Using this feature, an external clock can distribute a signal to the various regulators so that they all switch at the same frequency and phase. This eliminates the mixing of the nominal frequencies and associated harmonic products. For example, the LT8620 is a high- efficiency, high-speed synchronous monolithic step-down switching regulator that accepts a wide input voltage range up to 65 volts, and consumes only 2.5 microamperes (μA) of quiescent current (Figure 2). Its low ripple “Burst Mode” operation enables high efficiency down to very low output currents while keeping the output ripple below 10 millivolts (mV) peak-to- peak. A SYNC pin allows for user- established synchronization to
are less SNR critical and more concerned with meeting EMI requirements, it introduces uncertainties in the resultant harmonics that will be created across a wider spectrum, making them harder to control. For example, a switching frequency spread of 20% for EMI consideration results in harmonic frequencies between
The LTM8053’s unique packing
Figure 4: The LTM8053 (and other Silent Switcher devices) integrate a copper pillar flip-chip, enabling high- density design and large current capability in a small package while minimizing parasitic inductance. Image source: Analog Devices
Figure 6: The four 3 A outputs of the LTM8060 can be arranged in different parallel configurations to match the application’s DC rail requirements. Image source: Analog Devices
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