Designing wide input DC/DC converters for smart lock applications
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Hello. My name is Katelyn Wiggenhorn, and I am an Applications Engineer at Texas Instruments. I work in our buck converters and controllers business unit. Today I'm going to discuss Smart Lock systems and the corresponding challenges of the power subsystem.
On this slide, you can see the end equipment reference diagram of an electronic Smart Lock. Smart Locks use a wireless interface for communication to the lock. In this example, the Smart Lock uses an 848 kilobits per second interface. This communication interface is sensitive to noise and EMI. Interference.
Consequently, the input power-- off of a 12-volt rail, in this case-- must uphold the signal integrity of the communication interface. In this example, the communication signal will be operating at 848 kilobits per second, so the power supply design must not cause signal integrity issues in the 800 kilohertz frequency band. So the primary challenge is to design both a low-noise board design, with a low-noise converter.
With this in mind, the primary cause of signal degradation in this case would be near-field coupling from the switch mode power supply. The switch mode power supply switch node is a switching voltage, and it will radiate in an E-field at this switching frequency, which could be anywhere between 400 kilohertz to 2 megahertz depending on the switching frequency of the converter that you are using. And there will also be an E-field that is radiating from the switch node ringing. And in this case, it's typically between 100 megahertz to 300 megahertz.
In this image, it's around 300 megahertz. So your near-field coupling is going to be possible-- you're going to possibly see it at distances less than lambda over 4. And so for 300 megahertz noise, this is going to be lambda over 4 equals about 250 millimeters, and for 800 kilohertz noise, it's actually closer to 94,000 millimeters.
So this distance is greater than the PCB length-- the distance from the DC/DC converter to the digital signal. So in order to minimize the risk of having any sort of signal degradation due to the switch mode power supply, let's look at adding a filter design to DC/DC buck converter. For this example, we're going to use the LMR36006, which is a 60-volt converter with a 600 milliamp load current capability.
Here you can see the schematic without any filter. So we're going to start by taking the conducted emission test results and doing this without any input filter, in order to get a baseline measurement of the performance. Here you can see the standard setup of the conducted emissions.
So the evaluation board is connected to the listen at the input, and the listen is sensing the input. And the distance between the listen to the evaluation board is about 20 centimeters, and the height from the table to the evaluation board is about 5 centimeters. Here you can see the results from the conducted emission scan.
This scan goes up to about 30 megahertz. And you can see that the switching frequency is actually at 1 megahertz for this device, as seen by the fundamental spike at 1 megahertz. Now, what you're looking at in blue-- the blue signal is the average signal, and the yellow signal is the peak emission signal.
And what we notice is that the peak at the 1 megahertz is around 55 dB microvolts To avoid any signal degradation by this DC/DC buck converter, we decided to set the limit of the conducted emissions to 40 dB microvolts, which corresponds to 100 microvolts of noise in the time domain. This will be negligible, as seen in this image on a 3.3 volt or 5 volt digital signal.
So now, in order to filter this out, we'll design a filter such that it filters the peak emissions between the 800 kilohertz to about 1 megahertz frequency span to be below that 40 dB microvolt. So designing a pi filter with that constraint in mind, we end up with a small pi filter consisting of a 2.2 microhenry inductor, and two 2.2 microfarad capacitors.
So now let's take a look if that helps us reach our design requirement. Here you can see the results of the new design with the input pi filter. And again, the yellow signal is the peak emissions, and the blue signal is the average emissions. And now, across the entire frequency span, you can see that we're below the 40 dB microvolt requirement.
So in order to meet the Smart Lock requirements of having a communication chip on the same PCB design as a switch mode power supply, a small input pi filter can mitigate any risk of having interference between the two chips. For more information on the devices in this presentation or the electronic Smart Lock system, please use the links on this slide. Thank you for listening.
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