| General > General Technical Chat |
| Buck converter that draws a constant current |
| (1/4) > >> |
| sveinb:
Hey! I have this challenge: My circuit has a USB interface and some MEMS microphones on the same circuit board. They are powered by separate switched voltage regulators, both of which are powered by the 5 V USB supply voltage. The USB interface pulls more current when transmitting than not, and it transmits about 30 % of the time, in an 8 kHz cycle. This causes a low but audible 8 kHz noise to be picked up by the microphones. I've identified two routes that this noise takes: 1. The 5V supply voltage gets an 8 kHz ripple which leaks to the microphone supply voltage and 2. The current variation on the USB voltage rail causes mechanical vibration of the circuit board which is in turn picked up by the microphones. I cannot move the components to different circuit boards. A good deal of ripple is acceptable on the USB voltage rail, but not on the MEMS voltage rail. What I want is a voltage supply for the USB interface which draws a constant current from the 5V source. This would eliminate both paths of the noise. But how can I make such a circuit? It should be a switched regulator (for efficiency), and the USB supply voltage should remain between 1.6 V < VDDUSB < 2.0 V. |
| tom66:
This sounds like a classic XY problem. You are trying to stop the current on the USB interface varying to reduce the noise, but why not address why the varying current on the USB rail causes the noise in the first place? Most likely you have a common mode grounding issue, which is causing a radiated emission interfering with the sensitive microphone supply. The microphonic effect is more difficult but might be addressed with mechanical changes. Suggestions would include: * Ensuring the USB current is balanced and common mode paths are eliminated where possible. This means the shield on the USB should be connected to the PCB ground (directly, not with any "filter" components) and the PCB ground also connected to the USB cable/connector ground. If your product uses a pigtail connector, the shield pigtail needs to be as short as possible and kept wrapped on the cable length as much as possible. * Keeping sensitive circuits away from any sources of common mode noise. * Adding a shield over the noise generating part of the circuit, which is grounded all around the edges, to provide a path for common mode current. You could also consider shielding the microphone/regulator circuit though you will need a port for the audio. * Adding additional low frequency filtering to the sensitive supplies. Using ideally non-microphonic components for these parts. * Choosing an LDO with better low-frequency rejection to reduce the noise on the microphone supply. * Select a MEMS microphone with better PSRR, if possible. * For the mechanical problem, adding a way to mechanically isolate the microphone, such as putting it on a cut-out section of the PCB, or adding epoxy in the area of the board with components that appear to be generating the noise (if it's coil whine for instance). Or perhaps you can build a baffle into the product case which provides support, or mount the microphone on a different PCB (you said you can't change the position of the microphone, but can you have another PCB with the microphones on it, within the same volume of the existing board, connected by a wire?) There are plenty of digital devices which manage to keep audio noise low, think about a modern smart phone for instance - you will not hear the CPU switching on and off during a phone call but it will be doing that a great deal as audio compression doesn't use much CPU - and a modern phone has plenty of MEMS microphones in it. They do not use any smart tricks like constant current regulators, they just segregate the noisy parts and minimise paths for common mode noise by shielding sections as needed. |
| tszaboo:
I don't think the idea you are trying to do would work. You can replace your original regulators with an LDO, it would be my first choice trying to eliminate the noise. Other than that, some regulators go into low power modes, like pulse skipping more, or decrease the frequency when light load. Don't use those. I don't think efficiency is paramount for an USB powered gadget. |
| sveinb:
I should maybe add that the electrical noise path is mostly a problem when the device is connected through long USB cables with active repeaters. In that case we are talking about 20-30 meters of cable, with the inductance and resistance that comes with that. Suggestions that people should use shorter USB cables are to be expected, but not useful :-) |
| tom66:
--- Quote from: sveinb on January 25, 2024, 01:17:56 pm ---I should maybe add that the electrical noise path is mostly a problem when the device is connected through long USB cables with active repeaters. In that case we are talking about 20-30 meters of cable, with the inductance and resistance that comes with that. Suggestions that people should use shorter USB cables are to be expected, but not useful :-) --- End quote --- This suggests the problem is due to common mode noise from the cable system returning through the product, OR, your cable is so resistive that the voltage drop is significant when the USB transmit is active. If it is the former, proper shielding discipline (including using shielded cables) should help. If it is the latter, you could solve the problem by adding an LC filter that rolls off well before the 8kHz operating frequency, though the components may be reasonably large. |
| Navigation |
| Message Index |
| Next page |