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I am working with the TL783.

I have 2 connected in series. The input to the first is 24V. The first configured to output 5.5V (R1 = 240, R2 = 820), the second configured to output 3.3V (R1 = 240, R2 = 402). But after powering it up I am getting 3.03V on the output with no load.

My question is: Is there a minimum Vi-Vo that is required to allow the regulator to function properly. The datasheet makes reference to Vi-Vo but never specifies a minimum.

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    \$\begingroup\$ May I ask why did you use such a special purpose regulator for the very trivial task? Especially considering that there are literally thousands parts better suited for this. \$\endgroup\$
    – Maple
    Commented May 12, 2020 at 18:31
  • \$\begingroup\$ I didn't choose it but am considering changing it because it isn't working. Do you have one that may work similar to this but function better? One with the same package size because the board is already built and with 2 resistors for adjusting? \$\endgroup\$
    – Eric33
    Commented May 12, 2020 at 18:36
  • \$\begingroup\$ See figure 1 and figure 10. Quite large dropout. \$\endgroup\$
    – G36
    Commented May 12, 2020 at 18:36
  • \$\begingroup\$ I'd recommend posting your schematics and specifying which package the board is designed for (there are 3 options for TL783) if you want meaningful answer. As well as current requirements for both voltages \$\endgroup\$
    – Maple
    Commented May 12, 2020 at 18:42
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    \$\begingroup\$ Also this part is literally 40 years old. My rule of thumb is to not use anything more than 10 years old unless theres a good reason to. There will almost always be better, cheaper, more available options that have been designed more recently. \$\endgroup\$
    – BeB00
    Commented May 12, 2020 at 19:43

5 Answers 5

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This is to answer your later question in the comments.

There are many suitable linear regulators for this, for example LM317 in TO-220 package. The choice is entirely up to you. It depends on current requirements and PCB footprint.

Having said that, I strongly recommend using DC-DC for the first stage, due to huge voltage difference. There are quite a few converters compatible with TO-220 pinout, for example from Pololu, Murata and MPS. Most of them have fixed voltage though, like 5V 3.3V and 12V. I don't know why you need 5.5V but there are some adjustable options too.

Also, if the only reason you have two-stage power is because you wanted to split heat dumping between two components, then with DC-DC you can go straight from 24 to 3.3 and simply jumper the first footprint.

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    \$\begingroup\$ LM317 is unsuitable for his 2nd regulator as they are in series. Dropout is larger than available delta V. \$\endgroup\$
    – Russell McMahon
    Commented May 13, 2020 at 1:26
  • \$\begingroup\$ @RussellMcMahon The OP already confirmed that they only need 3.3V, so the first regulator can be replaced with a jumper and there will be no dropout issue with DC-DC. OP also mentioned "needed two in series for failure modes" which I don't get at all. Maybe you can explain it to me. \$\endgroup\$
    – Maple
    Commented May 13, 2020 at 4:39
  • \$\begingroup\$ @Maple I am trying to get better at putting all details into my questions, my bad. So for failure modes: I was told one failure mode of this IC is that the input voltage can appear at the output if the chip "fails". So that it why the 1st stage outputs 5.5V, because the stuff that is getting powered can receive up to 5.5V, but nominally it wants 3.3V. So if stage 2 fails, the stuff getting power receives 5.5V, which is ok. \$\endgroup\$
    – Eric33
    Commented May 13, 2020 at 16:08
  • \$\begingroup\$ @Maple He is trying to limit Vout to a safe range under failure. With two regulators, the 2nd failing short gives him a deemed safe 5V out. \$\endgroup\$
    – Russell McMahon
    Commented May 14, 2020 at 7:36
  • \$\begingroup\$ @RussellMcMahon I understood the intention, but was rather skeptical about the result. A lot of 3.3V components cannot tolerate 5.5V. And strictly statistically, the chances of second stage failure are the same as for single regulator directly from 24 to 3.3. \$\endgroup\$
    – Maple
    Commented May 14, 2020 at 12:14
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I guess it's not listed on the front page or even the tables because it's super variable and rather terrible compared to your common linear regulator.

enter image description here

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You are correct that no minimum guaranteed performance is specified, only the footnote that >25V difference is required for >700mA, and the fact that all the specs are given at Vi-Vo = 25V (so nothing is guaranteed for Vi-Vo < 25V). Not even your 24V->5.5V.

Would recommend considering replacing the 3.3V regulator with a part such as the LM1117. It has the same pinout and a similar, but not identical, 1.25V reference voltage. You can either change one of the resistor values a bit or use the fixed voltage version and replace one of the resistors with a 0\$\Omega\$ resistor (and remove the other).

It has a maximum input voltage of 20V so it is not acceptable for the first regulator. Note also that, like most LDO regulators, it has more stringent requirements for the output capacitor ESR to maintain stability than the source/emitter follower type of output stage used on the TL783, LM78xx etc.

The LM317 might be worth looking at for the input regulator, but its dropout voltage is a bit high for the 5->3.3V regulator, especially at low temperatures. It is rated at 40V input so probably okay for 24V nominal unless you are expecting large transients.

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due to mouser.com (just for example) TL783Series is still "active" part and "in stock"; the quick specs there say:
max. Input Voltage 150 V ## that may be a reason for that part still alive
min. Input Voltage 21.5 V ## that is why cascading cannot work
Output Range 1.25 V to 125 V max. 700 mA

the LM1117 for example is limited to 15 V Input Voltage,
other frequently used parts may allow e.g. 30..40..50 Volts

please keep in mind, that not you may not drive the part against all limits at once, and some data may be "abs.max." absolute maximums which are not recommended for normal operation cause they are close to destruction of the part - these are only a way to preselect parts, for which you have to validate usage against the detailed datasheet, its data tables and its diagrams

so it must be: Vi-Vo > (=) V_dropout
and V dropout depends mainly on I out (and on temperature, as every value) here, TL783 with 700mA in the diagram: already about 15V at 60°C, but it may become hotter, secure at about 18V dropout calcuation
a good start are often the "test conditions" in the "electrical characteristics" table of a part
here, for the TL783, ti.com states, Vi-Vo = 25V, Io = 500mA, Tj = 0°C..+125°C
and "(3) ...output current is limited to less than 700 mA at input-to-output voltage differentials of less than 25 V"

the reference Voltage may also vary from part to part, by the individual part used
given here a range of 1.2 to 1.3 V

for example a ~5.5 constant Voltage can be gained from a "ordinary" (µA) 7805, which is limited to Vi <= 25V (!!) but may easily drive the 700mA (up 1.0A, limit even higher) - which is already a maximum/limit(!) for the TL783; by lifting the ground reference level up with a diode by ~0.6V - if this ranges match with your circuit

given Vo ~ Vref ( 1 + R2/R1 )
your data was: 5.5 V with 1+820/240 = 4.417x
typical Vref = 1.27 => 5.61 Volts out
if you got 5.5 V, the individual Vref of the part must be 1.24V (and the resistors may differ by tolerance from stated value? or 1% parts?)

the 3.3V with 1+402/240 = 2.675x
would require a Vref of 1.233 V
with a Vref of only 1.2V => Vout should be at least 3.2V

minimum output current (to maintain regulation): 15 mA

"two in series" with 24V in --> 5.5V out --> to 3.3V out
okay first stage is very close to limits (700 mA possible with more than ~13V Vi-Vo,
and Vi-Vo = 19.5V at 700mA
but second stage will never function properly, absolute minimum is a dropout of about 4 Volts at a current of 15 mA

"two in series for failure modes" - should it not be "two in parallel" for redundancy - and "failure modes"(which?) are normally solved by fuses??? - now I con-fuse-d :-/

another typical application may be to reduce the "huge" voltage gap from about 24V to 6..5 Volts with a DC/DC converter and then use a linear regulator (LDO - low drop out) to get a smooth 3.3V - but that's up to your application

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  • \$\begingroup\$ Just a note: DC-DC is not good for RF circuits (a special approach is required which increases costs to avoid EM interference). Otherwise for significant difference of Vin-Vout at high current no better alternative than DC-DC converter. \$\endgroup\$
    – Polar Bear
    Commented May 13, 2020 at 17:17
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Provided requirements to PS by OP does not give good picture what it used for. If PS supplies power to simple circuit then something like following module should work just fine.

If requirements is little higher then module would be not bad choice.

Unfortunately as PCB already at hand some inventive method will require to adopt modules.

Even if there were a part with same foot print which would represent LDO due high differential of Vin-Vout and required current, dissipated heat will require a massive radiator, Vin=25V Vout=5V I=1A -- P=(25V-5V)*1A=20W. Efficiency (5V*1A)/(25V*1A)*100=20% -- 80% power goes toward waste.

For RF circuit PS will have to be different kind to avoid EM emission and above mentioned modules are not fit.

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