3
\$\begingroup\$

Maybe this is very stupid question but I want to know if I can connect two or more voltage regulator (like LM7L05) to add ampere? Say if 1 is able to source 100 mA then combine 2 to draw 200 mA? Sorry if this sounds stupid!

Note: I know I can search regulator to source more current but in this question, I would like to understand that if I can connect multiple regulators together!

\$\endgroup\$
  • 1
    \$\begingroup\$ Read some details here .. Answer is probably "no" - electronics.stackexchange.com/questions/32690/… \$\endgroup\$ – Anuj Purohit Apr 27 '15 at 10:34
  • \$\begingroup\$ Do you need that much current supplied to mostly one part, or is for a complex system with many stages/units/modules each needing a little power? \$\endgroup\$ – DarenW Apr 27 '15 at 16:48
  • \$\begingroup\$ @DarenW I designed a block to power 5 OptoIsolator but I see a need to power 15 more. I want to stick to LM7Lxx package; therefore was thinking what would happen if I add more LM7Lxx in parallel. I might switch to higher output LM7xxx if required but I don't like them due to reasons known only to me. \$\endgroup\$ – Anuj Purohit Apr 28 '15 at 6:16
4
\$\begingroup\$

You can connect multiple voltage regulators together but you are unlikely to get 200mA consistently from two 100mA voltage regulators. The main reason is the REG1 may be supplying 1mV more than REG2. With the outputs connected REG1 will tend to cause REG2 to shut down. It shuts down because it believes the output is slightly high and backs off the power.

Thus, in this simple configuration only REG1 operates with REG2 never contributing until the current limit of REG1 is reached then things get a little messy and unstable with a little bit extra noise on the output.

You can resistively connect the outputs of REG1 and REG2 and this alleviates the situation but, regulation purity is degraded and, due to the resistors, you will probably only be able to achieve 150mA from the pair. Resistor values of an ohm upwards are needed.

\$\endgroup\$
  • \$\begingroup\$ With select few parts you can use lower resistors, 10 mOhm for LT3080 and some even integrate said resistors (LT3080-1): linear.com/solutions/5759 That page also discusses more obscure methods like balancing with an opamp that measures the imbalance and adjusts a variable regulator, etc. \$\endgroup\$ – Fizz Sep 30 '15 at 13:47
1
\$\begingroup\$

Andy is correct: any two voltage regulators will always have slight manufacturing variation in their voltage set-points, which causes uneven distribution of the load current. So regulator outputs should not be directly connected together.

In the comments you mention the requirement to scale your 5 optocoupler design to power 15 optocouplers. So it may be possible to separate your total system's load current into smaller branches, and use multiple 100mA regulators.

Instead of connecting the second LM7L05 to the first one, why not use a point of load regulator configuration. The first LM7L05 (U101) would supply 5V_BANK1 to power the first 10 optocouplers (U110-U119), and a second LM7L05 (U201) would supply 5V_BANK2 to power another 10 optocouplers (U210-U219). Both U101 and U102 have the same higher-voltage input, but each has its own separate 5V output.

schematic

simulate this circuit – Schematic created using CircuitLab

Since the regulator outputs are independent, the slight variance between 5V_BANK1 voltage and 5V_BANK2 voltage doesn't matter: as long as they are both within the 4.75V to 5.25V spec for each of the devices being powered. And it's also perfectly OK to drive logic signals from one bank to another bank, since they are all "5V logic" signals and they all have the same ground.

This point-of-load regulator architecture is commonly used in large systems. Power distribution is handled with relatively higher voltages, and a very small regulator serves each independent sub-circuit. This approach can help isolate noisy digital switching circuits from noise-sensitive analog circuits, and helps spread heat dissipation more evenly.

\$\endgroup\$
  • \$\begingroup\$ The question was intended to understand the outcome of combining regulators. I can create different banks as suggested by you but that would answer "how to" and not "why not". Thanks for your reply. \$\endgroup\$ – Anuj Purohit Apr 29 '15 at 6:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.