piclist 2002\01\26\063440a >
Thread: Can you put multiple voltage regulators in parallel for more power?
picon face BY : Russell McMahon email (remove spam text)

> However, when it arrived and I started looking at the data sheet, it
> to be only a 150 ma regulator. What I'd like to know is, can I safely put
> two of these together, both outputs tied to the same positive (circuit)
> supply rail? Will I then be able to get 300 ma out of the two regulators?
> If I can't tie them to the same supply rail, can I run some of my
> off a separate rail? For example, can I have one regulator power the LCD
> my circuit, and the other regulator power the MCU, but still have output
> pins of the MCU connected to input pins of the LCD, and everyone sharing a
> common ground?

Two regulators will have slightly different Vouts - how different depend on
specified Vout tolerances.

You can definitely use separate regulators to each drive part of the load.
The small difference in voltages that result should generally be less than
you will get from other sources.

You can parallel two and expect reasonable results as long as the Vouts are
not vastly different and both have current limits.
The higher Vout regulator will take most of the load at low loads as it
keeps driving when the other turns off at set Vout. Once this has reached
its current limit it will start to sag and the other will then make up the
difference. This means one will TEND to run at near its max rated current.

You can put "spreading resistors" in the outputs to make Vout sag as current
is drawn but this destroys your regulation somewhat and is probably not
needed. At eg 100 mA a sag of 0.1 volt would be enough needing V/I = 1 ohm.
This may do nasty things to your regulator compensation and is probably best
avoided. Twill probably work well enough as is.

If you do NOT value the regulators over-current  protection and don't need
very low dropout you can try the olde days trick of a PNP transistor in
parallel with the regulator.
Vin feeds reg through a resistor Rb.
Emitter to Vin.
Base to Rb and regulator in.
Collector to Vout.

As long as drawn current through Rb is < Vbe turn on (about 0.6v) regulator
will be the sole source of Vout.
When Vrb exceeds Vbe the transistor turns on and Vout rises until the
regulator backs off to stop this thereby reducing Vrb.
Works well enough.
Short the output and transistor will still try to provide Vout.

Probably finding a single suitable SMD regulator would be better.

       Russell McMahon

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See also: www.piclist.com/techref/power/regulators.htm?key=power
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