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| DC-DC converter in mixed signal application |
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| ZeroResistance:
What factors do we need to consider when selecting a DC-DC switching converter to power a mixed signal application. The application would have very sensitive analog circuits along with the usual MCU and few other digital peripherals. 1. Is it a standard practice to have a linear regulator after the output of the DC-DC converter to reduce noise / ripple? 2. If the switching frequency of the DC-DC converter is outside the bandwidth of the signal of interest does that make it any less stringent on the design. TIA |
| floobydust:
DC-DC converters, like the 1W or 3W SIP modules are fairly noisy, they are around 300kHz, assuming it's an isolated module. Most have poor regulation as they are simple oscillators, and the load+line regulation can be as bad as 7.5% so a linear Vreg is used downstream. Other modules are 1% if you shop around. You get output noise which is easy get rid of with an LC filter, before any linear voltage regulator. LDO's and such rarely have bandwidth above 10kHz, so 300kHz incoming is not good, it just passes through the voltage regulator IC. I've had bipolar i.e. +/-15V units die or fail to startup without adding reverse diodes across the outputs. If your load current could be from +15V to -15V (not common GND), add diodes. You get input-side noise current, which can make a mess of your rails on that side. Note the datasheets have "EMC recommended circuit" which is a CLC Pi filter. The unknown enemy is capacitive coupling across the transformer windings (primary and secondary) with switching harmonics extending many MHz. You will find this out at the EMC lab, when the product fails conducted emissions tests. It can radiate out the primary or secondary side, depending on your design. The radiated B-field from the module (transformer) induces noise in nearby pcb components and traces, so leave some room or put insensitive parts around it. |
| perieanuo:
1.no 2.no Envoyé de mon iPad en utilisant Tapatalk |
| ZeroResistance:
--- Quote from: floobydust on June 17, 2019, 05:46:22 am ---DC-DC converters, like the 1W or 3W SIP modules are fairly noisy, they are around 300kHz, assuming it's an isolated module. Most have poor regulation as they are simple oscillators, and the load+line regulation can be as bad as 7.5% so a linear Vreg is used downstream. Other modules are 1% if you shop around. You get output noise which is easy get rid of with an LC filter, before any linear voltage regulator. LDO's and such rarely have bandwidth above 10kHz, so 300kHz incoming is not good, it just passes through the voltage regulator IC. I've had bipolar i.e. +/-15V units die or fail to startup without adding reverse diodes across the outputs. If your load current could be from +15V to -15V (not common GND), add diodes. You get input-side noise current, which can make a mess of your rails on that side. Note the datasheets have "EMC recommended circuit" which is a CLC Pi filter. The unknown enemy is capacitive coupling across the transformer windings (primary and secondary) with switching harmonics extending many MHz. You will find this out at the EMC lab, when the product fails conducted emissions tests. It can radiate out the primary or secondary side, depending on your design. The radiated B-field from the module (transformer) induces noise in nearby pcb components and traces, so leave some room or put insensitive parts around it. --- End quote --- These are some pretty amazing points that you highlighted. +/- 7.5% regulation is pretty bad, I have some datasheets that point out they could be as high as 20%. So looks like a linear LDO is somewhat a must. The noise that you highlighted where you said that it can't be removed with a linear LDO since their bandwidth is limited, so a filter at the input of the LDO is required correct? Are there any capacitor switched versions of these power supplies available would they be lower noise than these? (but I guess they won't do isolation... |
| ZeroResistance:
I see that most DC-DC converters operate around 100Khz, are there specifically LDO's with a wide band PSRR so that 100khz ripple / noise gets minimised? |
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