COVER ST RY R STORY


frequencies of switching regulators. Higher efficiency, lowminimumon- and off- times result in higher harmonic content due to the faster switch transitions. For every doubling in switching frequency, the EMI becomes 6dB worse while all other parameters, such as switch capacity and transition times, remain constant. The wideband EMI behaves like a first order high pass with 20dB higher emissions if the


switching frequency increases by 10 times. Savvy PCB designers willmake the hot loops small and use shielding ground layers as close to the active layer as possible. Nevertheless, device pin-outs, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate aminimumhot loop size.


To further complicatematters, in typical planar printed circuit boards, themagnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts since the higher the harmonic frequencies are themore ef fective unwantedmagnetic coupling becomes. The tried and true solution to EMI issues is to use a shielding box for the complete circuit. Of course, this adds costs, increases required board space,makes thermalmanagement and testingmore difficult, as well as introducing additional assembly costs. Another frequently used method is to slow down the switching edges. This has the undesired effect of reducing the efficiency, increasing minimumon- off- times and their associated dead times and compromises the potential current contr ol loop speed.


, ,


NEW SOLUTIONION TO THESE EMIHESE EMI Linear Technology’s recently introduced


A NEW SOL ISSUESISSUES


LT8614 Silent Switcher regulator, delivers the desired effects of a shielded box without using one, and so eliminates the abovementioned drawbacks. (See Figure 1.)


This device also has a world class low IQ of only 2.5µA operating current. This is the


total supply current consumed by the device, in regulation with no load. Its ultralow dropout is only limited by the internal top switch. Unlike alternative


solutions, the LT8614’s RDSON is not limited bymaximumduty cycle andminimum


off-times. The device skips its switch-off cycles in dropout and performs only the s to keep the age voltage


internal top switch boost st minimumrequired off cycle


sustaine d.


At the same time, theminimum operating input voltage is only 2.9V typical (3.4Vmaximum), enabling it to supply a 3.3V rail with the part in dropout. The LT8614 has higher efficiency than the LT8610/11 at high currents since its total switch resistance is lower. It can also be


Figure 4: Ch1: LT8610, Ch2:


LT8614 switchtc node rising edge both at


at


8.4VIN, 3.3VOU at 2.2A


Figure 4 g / ELECTRONICS ELECTRONICS OUT


Linear Technology (UK) Ltd. www.linear.com


www.linear.com 01628 477 066


ELEC RO ELECTRONICS CS | OCTOBER 201 BER 2016 13 13 Figure 3 Fi


FEA


FEAT R E ATURE


To compare the LT8614 Silent Switcher technology against another current state- of-the-art switching regulator, the part wasmeasured against test was performed in


a GTEMcell using the LT8610. The


the same load, input voltage and the same inductor on the standard demo


Silent Switcher techno improvement is attain One can see that up boards for both parts.


the already very good


ed using the LT8614 to a 20dB


EMI performance of logy compared to


Figure 2 Figure 2


synchronised to an external frequency operating from200KHz to 3MHz.


The AC switch losses are low, so it can be operated at high switching frequencies withminimal efficiency loss. In EMI- sensitive applications, such as those commonly found inmany industrial environments, a good balance can be attained and the LT8614 can run either at low hundreds of KHz or above 2MHz. In a set-up with 700KHz operating switching frequency, the standard LT8614 demo board does not exceed the noise floor in a CISPR25measurement.


The Figure 2measurements were taken in an anechoic chamber under the following conditions: 12V in, 3.3V out at 2A with a fixed switching frequency of 700kHz.


Figure 2: Figure 2:


Blue trace is the noise floor; red trace is the LT


CISPR25 radiated measurementmeasurement in anin an anechoic chamber


anechoic chamber


Blue trace is the noise floor; red trace is the LT8614 board atT8614 board at CISPR25 radiated


the LT8610, especially in themore difficult tomanage high frequency area. This enables simpler andmore compac t designs where the LT8614 switching power supply needs less filtering


compared to other sensitive systems in the overall design.


In the time domain, the LT8614 shows very benign behaviour on the switch node edges, as shown in Fig 4ns/div this Silent Swit


cher regulator shows ure 4. Even at


very low ringing (see Ch2 in Figure 3). The LT8610 has a good damped ringing (Ch1, Figure 3) but one can see the highe r energy stored in the hot loop compared to the LT8614 (in Ch2).


All time domainmeasurements in Figure 3: Figure 3:


Blue trace is the LT8614, purple trace is the T


3.3VOUT


Blue trace is the LT purple trace is the LT8610; both 13.5V N, at 2.2A load


LT8610; both 13.5VIIN 3.3VOU at 2.2A load


LT8614,


Figures 3 and 4 were done with 500MHz Tektronix P6139A probes with close probe tip shield connection t plane, both on the sta


ndard demo boards. o the PCB GND


The LT8614’s lowminimumon-time of 30ns enables large step-down ratios even at high switching frequencies. As a result, it can supply logic core voltages with a single step-down frominputs up to 42 V. It is well known that EMI considerations for industrial environments require careful attention during the initial design process in order to ensure that they will pass EMI testing once the systemis completed. Until now, there was not a sure way to guarantee that this could easily be


attained with the right power IC selection. This has now changed due to the


introduction of the LT8614. This regulator reduces EMI fromcurrent state-of-the - art switching regulators bymore than 20dB, while increasing conversion


efficiencies with no drawbacks. A 10 fold improvement of EMI in the frequency range above 30MHz is attained without compromisingminimumon- and off- times or efficiency in the same board area. This is accomplished with no special components or shielding, representing a significant breakthrough in switching regulator design. This is just the sort of breakthrough product that allow s


take their products to industrial automation


noise performance.


systemdesigners to the next level of


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