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| TO220 rail splitter? |
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| David Hess:
There is such a thing as a switching rail splitter. A buck converter with synchronous rectification will produce half of its input voltage at a 50% duty cycle whether sourcing or sinking current. In practice, feedback is used to tightly control the output but this is not absolutely necessary. For practical purposes, an integrated class-D audio amplifier is probably the easiest way to implement this now. Linear Technology made some switching regulators for termination applications which worked this way including some high voltage ones. Note that when the output current is negative, the switching regulator pumps power back into the supply. https://www.analog.com/en/ltc3717 |
| GigaJoe:
exactly what I thought , bump voltage to +40, then use audio power amp to split it, but I not sure D-class in TO220 , not enough pins. it maybe TDA2030A clone , that cost $0.10 then we definitely fit in $2.5 budget :) |
| 0xdeadbeef:
--- Quote from: Hero999 on December 18, 2018, 06:56:23 pm ---I don't see why the LM675 will be inferior to the TLE2426. The TLE2426 is specified to have a tolerance of <1%, which should be easily achievable with the LM675, given close enough tolerance resistors. --- End quote --- First of all I'm not even sure which OpAmp is used there. I picked the LM675 just as an example. Anyway, for sure the resistors on this Chinese board are neither hand selected nor pre-aged or whatever, so this circuit will behave worse than a TLE2426 for sure. About the TLE2426 tolerance: there's a table in the datasheet of the TLE2426 which says the error (i.e. deviation of Vout/Vin from 50%) is practially 0 for input voltages around 5V (Vin) and then linearly raises to 1% at 15V but keeps raising to 3.6% for 40V. So I'd say the initial total error is supposed to be <1% only for voltages below 16V. Anyway, this linear voltage dependency of the total error seems to indicate that it's not caused by the trimming of the resistors. Also note that the temperature dependency is very low which is certainly also a result of the integrated voltage divider. So yes, I'm still skeptical that it's a piece of cake to achieve the same error with a discrete voltage divider, specifically under temperature/load changes and considering aging. |
| Zero999:
--- Quote from: 0xdeadbeef on December 18, 2018, 08:07:01 pm --- --- Quote from: Hero999 on December 18, 2018, 06:56:23 pm ---I don't see why the LM675 will be inferior to the TLE2426. The TLE2426 is specified to have a tolerance of <1%, which should be easily achievable with the LM675, given close enough tolerance resistors. --- End quote --- First of all I'm not even sure which OpAmp is used there. I picked the LM675 just as an example. Anyway, for sure the resistors on this Chinese board are neither hand selected nor pre-aged or whatever, so this circuit will behave worse than a TLE2426 for sure. About the TLE2426 tolerance: there's a table in the datasheet of the TLE2426 which says the error (i.e. deviation of Vout/Vin from 50%) is practially 0 for input voltages around 5V (Vin) and then linearly raises to 1% at 15V but keeps raising to 3.6% for 40V. So I'd say the initial total error is supposed to be <1% only for voltages below 16V. Anyway, this linear voltage dependency of the total error seems to indicate that it's not caused by the trimming of the resistors. Also note that the temperature dependency is very low which is certainly also a result of the integrated voltage divider. So yes, I'm still skeptical that it's a piece of cake to achieve the same error with a discrete voltage divider, specifically under temperature/load changes and considering aging. --- End quote --- The op-amp makes very little difference. The offset errors will be tiny, compared to the supply voltage. The resistors don't need to be hand selected or anything special to get equal performance to the TL2426. A power op-amp with 1% resistors will give similar performance, at low currents and will easily beat it, at high currents. The op-amp isolates the load changes from the potential divider and both resistors should have a similar temperature co-efficient. |
| 0xdeadbeef:
--- Quote from: Hero999 on December 18, 2018, 09:22:25 pm ---The op-amp makes very little difference. The offset errors will be tiny, compared to the supply voltage. --- End quote --- Well, not all OpAmp have small output offsets. E.g. the LT1010 (which is also used as rail splitter and the datasheet shows an according circuit) defines a maximum of 150mV (even 220mV over full temperature range). Also bias currents etc. differ. As a side note, the 1mV offset given for the LM675 seems to be the input voltage offset (i.e. the voltage needed to get 0V at the output). IMHO this is not a definition of the worst case output offset. --- Quote from: Hero999 on December 18, 2018, 09:22:25 pm ---The resistors don't need to be hand selected or anything special to get equal performance to the TL2426. A power op-amp with 1% resistors will give similar performance, at low currents and will easily beat it, at high currents. --- End quote --- Well, this adds another 1% (or so) error to the total error. My understanding is that the trimming of the TL2426 removed this error more or less completely (i.e. to 0.1% or so). --- Quote from: Hero999 on December 18, 2018, 09:22:25 pm ---The op-amp isolates the load changes from the potential divider and both resistors should have a similar temperature co-efficient. --- End quote --- But there are other errors introduced by a real OpAmp. E.g. there are bias currents going into the inputs which are voltage and temperature dependent. Actually I would think that error increasing linearly with the voltage on the TL2426 is mainly caused by the input bias current(s). |
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