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Old 24th March 2009, 01:44 AM   #1
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Default When to use 317 and 7805?

I need a regulated 5V 300mA supply. There are choices in using 7805 and 317. May I know what are the conditions/benefits in choosing between them?
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Old 24th March 2009, 07:57 AM   #2
winny is offline winny  Sweden
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7805 require fewer or none external components = cheap solution for applications with low requirements.

LM317 on the other hand do require external components for setting the output voltage but it is variable and the circuit is faster so the load step response looks better.
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Old 24th March 2009, 08:11 AM   #3
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LM317 has also better electrical performance.
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Old 24th March 2009, 02:46 PM   #4
okapi is offline okapi  United States
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the adjustable regulators enable bypassing of the adjust pin. I believe this is where most of the performance gains are obtained versus non adjustable regs. Bypassing the adjust pin with a capacitor to ground reduces output ripple, noise, and impedance. These parameters scale directly with output voltage if the adjustment pin is not bypassed. A bypass capacitor reduces ripple, noise and impedance to that of a regulator operating at its reference voltage.
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Old 24th March 2009, 04:34 PM   #5
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It looks like if we use a cap at the adjustment pin, 317 will perform better than 7805? Adding two resistors to the cost seems nothing at all but why does any manufacturer still produce 7805?
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Old 24th March 2009, 05:02 PM   #6
AndrewT is online now AndrewT  Scotland
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the 7805 and other voltages are quick to install and thus cheap.
The 317 gives similar performance, but by adding the extra resistors allows one stock item to cover many voltages. Inventory costs vs multiple component assembly costs may balance out in large scale production.

The improved performance only becomes available when the adj cap is added, further increasing cost.
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Old 30th March 2009, 02:33 PM   #7
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Default 7805, can't have enough of them

I like how the 7805 has a really good precise voltage of 5.01-5.03V loaded or not, on any I have measured. I always use good filter capacitors before and after the regulator, so I've never had issues with noise. Also no resistors to calculate or take up space, which makes a 7805 better for tight places or small circuits.

I just built an LED Li-Ion outhouse light, that runs off the 12VAC from the yard walkway lights, turns it to DC, and uses a 7805 to run a MAX1551 Li-Ion charger IC for the battery. The 7805 made things easy as everything was a tight squeeze, and I used few components. The LED light runs from the 3.7V Li-Ion battery, and the 7805 allows charging from the 12V source.

I like the 317, for being able to set any voltage, but you have to load them some, or the voltage goes up slightly when you remove the load. Also the voltage changes with temperature. I do like the adj pin, and always use a capacitor. For a current limited power supply or battery charger, it's easy to limit current with a transistor pulling down the adj pin.

They all have their places. I find I use a 78xx/79xx regulator chip more often than the 317, but I still use the 317 quite often. I'm designing a li-ion charger that uses 5 of the 317s.
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Old 6th April 2009, 02:55 PM   #8
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Quote:
I like the 317, for being able to set any voltage, but you have to load them some, or the voltage goes up slightly when you remove the load.
I've read that they need at least 10ma load to stay stable. It was suggested to decrease the 'standard' resistance between the output,and adjust pin to about half of what the datasheet suggests,thus keeping the regulator loaded. IIRC the usual suggestion is 220-240 ohms between adj,and output,so you would drop that to ~120ohms.
Basically just try to keep atleast 10ma flowing through the resistor sting.
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Old 6th April 2009, 06:47 PM   #9
AndrewT is online now AndrewT  Scotland
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yes, most 317 need >=5mA and most 337 need >=10mA.
The easy way is to use the adjust resistor to guarantee the minimum load current.
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