Unique Silent Switcher designs offer high efficiency at high switching frequencies >2MHz with ultra-low EMI
In order to achieve high efficiency and low heat dissipation, switching regulators are replacing traditional linear regulators. The switching regulator is typically the first active component on the input power line, and therefore has a significant impact on the Electromagnetic Interference (EMI) performance of the complete circuit.
Getting the balance between high efficiency and low EMI right whilst also meeting standards around radiated EMI emissions is a tough challenge for designers. A non-compliant design can be very costly not just because of the retest fees incurred but also from a time-to-market perspective. In fact, the most common reason power supply designs go over budget and miss their target completion schedule is the PCB failing to EMI compliance testing.
Fortunately, there is help at hand, Matthew Johnson, Divisional Engineering Manager at Anglia introduces several high-performance switching regulators from Analog Devices that bring new levels of performance to synchronous converters. Combining high frequency switching, high efficiency, and ultra-low EMI, these switching regulators offer very compact power solutions.
Silent Switcher®
Typical techniques for reducing radiated emissions can include spread spectrum frequency modulation, slew rate control or use of modules with internal filtering and shielding. With Silent Switcher technology, Analog Devices have looked at the fundamental design of a switching regulator and developed parts from the ground up which meet Radiated EMI standards such as CISPR25 out of the box.
In a buck regulator for example, the Silent Switcher design uses two high side FETs to create opposing magnetic fields in the two high current loops which cancel each other out (Fig.1), the resulting effect is almost like enclosing the circuit in a metal box.
Figure. 1 – Magnetic Field Cancellation in High Current Loops
Another common source of radiation can be the relatively long bond wires inside the device package that connect the die structure to the lead-frame, these can effectively act as miniature antennas radiating EMI. Again Silent Switcher has taken a ground up approach and employed a flip chip design which removes these bond wires completely reducing parasitic inductance and this source of EMI
An extension to the technology, Silent Switcher 2, takes all the previous design principles and adds internal bypass capacitors and an integrated substrate to further improve EMI which is not sensitive to PCB layout. This helps to significantly simplify designs and reduces performance risks even when using two-layer PC boards.
To validate the superior EMI performance, Analog Devices used a scope with a sniffing probe to compare the performance of a Silent Switcher device to a competitor part under the same conditions, this example demonstrated an impressive 40dB reduction in radiated emissions (Fig. 2).
Figure. 2 – Comparison of Radiated Emissions vs. Competitor
More than 50 devices and modules featuring Silent Switcher® technology are already available from Analog Devices and increasing month by month, below are a small selection of these devices to suit a broad range of applications.
LT8337
The LT8337 is a synchronous step-up (boost) switching regulator which boasts very low quiescent current. Featuring Silent Switcher architecture and optional spread spectrum frequency modulation (SSFM) to minimize EMI emissions this device delivers high efficiency (>90%) at high switching frequencies and low output currents. The device features low radiated EMI emissions to meet the requirements of CISPR25 Class 5 Peak Limits (Fig.3).
The LT8337 series integrates 28V, 5A power switches, operating at a fixed switching frequency programmable between 300kHz and 3MHz and synchronizable to an external clock. In addition, the device has output soft-start and output overvoltage lockout protection. The LT8337-1 variant of the device also allows external compensation for fast transient response.
Figure 3. Radiated EMI Performance (CISPR25 Class 5 Peak)
The LT8337 synchronous switching regulator has a wide input voltage range from 2.7V to 28V and a programmable output voltage up to 26V. The devices low VIN quiescent current in Burst Mode operation, and 100% duty-cycle capability for the integrated synchronous MOSFET in Pass Thru operation (VIN > VOUT) makes the LT8337 series ideal for battery-powered systems or any other general purpose step-up applications.
LTC3307B
The LTC3307B showcases Analog Devices ability to produce very small, high efficiency, low noise, monolithic synchronous step-down DC/DC converters. Housed in a low profile 12-lead LQFN package measuring just 2mm × 2mm × 0.74mm the device is still capable of delivering 3A of output current and operates from a 2.25V to 5.5V input supply.
Using constant-frequency, peak current mode control at switching frequencies from 3MHz to 10MHz and minimum on-time as low as 22ns, this regulator achieves fast transient response with small external components. The Silent Switcher architecture minimizes EMI emissions whilst the LQFN packages exposed pad delivers low thermal resistance.
Figure 4. Efficiency and Power Loss in Burst Mode Operation
The LTC3307B can be operated in forced continuous or pulse- skipping mode for low noise, or low ripple Burst Mode operation for high efficiency at light loads (Fig.4). The device regulates output voltages as low as 500mV and includes integrated output over-voltage protection, short-circuit protection, thermal shut-down, clock synchronization, and up to 100% duty-cycle operation for low dropout.
The compact and efficient LTC3307B is suitable for a range of applications such as battery-powered devices, optical networking equipment, servers, automotive, industrial and communications equipment. They are also ideal for circuits using distributed point of load (POL) DC power systems or for power supplies in designs incorporating FPGA, ASIC, and microprocessors.
LT8648S
The LT8648S is ideal for applications with higher power demands, this synchronous step-down regulator features second generation Silent Switcher 2 architecture designed to minimize EMI emissions while delivering high efficiency at high switching frequencies. The device includes the integration of input and boost capacitors to optimize all the fast current loops inside and make it easy to achieve specified EMI performance by reducing layout sensitivity. This performance makes the LT8648S ideal for noise sensitive applications and environments.
The LT8648S features fast, clean, low overshoot switching edges that enable high efficiency operation even at high switching frequencies, leading to a small overall solution. The device is housed in a LQFN36 package measuring 7 x 4mm and can deliver an impressive 15A output current.
Figure 5. Radiated EMI Performance (CISPR25 Test with Class 5 Peak Limits)
`The device features peak current mode control with a 25ns minimum on-time, this allows high step-down ratios even at high switching frequencies. External compensation can also be implemented allowing for fast transient response at high switching frequencies, the device also permits current sharing whilst the CLKOUT pin enables synchronizing other regulators to the LT8648S. The device has low radiated EMI emissions and can meet the requirements of CISPR25 Class 5 Peak Limits (Fig.5).
The LT8648S has 3 operation modes, Burst Mode enables low standby current consumption, forced continuous mode can control frequency harmonics across the entire output load range, or spread spectrum operation can further reduce EMI emissions. Soft-start and tracking functionality is also provided along with an accurate input voltage UVLO configurable threshold.
LTM8051
The LTM8051 is a quad 40VIN, 1.2A step-down μModule® regulator, the outputs can be paralleled in an array for up to 4.8A capability. Again, Silent Switcher architecture is employed in the LTM8051 to minimize EMI while delivering high efficiency at frequencies up to 3MHz and enabling the device to achieve compliance with radiated emissions limits to CISPR22 Class B and CISPR25 Class 5 standards (Fig. 6)
Included in the LTM8051 μModule® package are the controllers, power switches, inductors, and other support components. This high level of integration minimises the number of external components required, simplifying circuit design, and reducing overall cost.
Figure 5. Radiated EMI Performance (CISPR22 Class B & CISPR25 Class 5 Peak Limits)
The device operates over a wide input voltage range from 3V to 40V, output voltages from 0.8V to 8V, and a switching frequency range from 300kHz to 3MHz which can be set by a single resistor. Only the bulk input and output filter capacitors are needed to finish the design.
The LTM8051 is packaged in a compact Ball Grid Array (BGA) package measuring 6.25mm × 11.25mm × 2.22mm and has a specified operating temperature range from -40°C to +125°C.
Design Support
The Silent Switcher switching regulators from Analog Devices are backed up with various development boards and kits, these allow designers to evaluate the full capability and features of the parts whilst speeding up time to market.
Anglia offer support for power supply and other types of designs with free evaluation kits and samples of Analog Devices products via the EZYsample service which is available to all registered Anglia Live account customers.
Anglia’s engineering team are also on hand, they have extensive experience of working with power supply related designs and are able to advise on best practice to meet both efficiency and regulatory requirements. This expertise is available to support customers with all aspects of their designs, offering hands on design support along with access to Analog Devices vast resource of technical application notes and reference designs.
Visit www.anglia-live.com to see the full range of other Analog Devices products available from Anglia.