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Old 25th July 2012, 07:21 AM   #1
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Default LM1084 5.0 need 6v

any way to trick the Lm1084 5.0 into giving 6v. I believe that the 5.0 version fixed to 5v and i need regulated heaters.A shame to have to buy another adjustable one. Any thoughts?
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Old 25th July 2012, 11:17 AM   #2
Elvee is offline Elvee  Belgium
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From the datasheet, I gather that you just need to insert a suitable resistor in the GND terminal. See p10 of the data sheet: http://www.ti.com/lit/ds/symlink/lm1084.pdf

The typical value should be 200Ω for a 1V increase, but you could make it adjustable for more flexibility
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Old 25th July 2012, 04:52 PM   #3
Nisbeth is offline Nisbeth  Denmark
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A couple of diodes in series in the GND terminal works as well (but you may not hit 6.0 spot on then)

/U.
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Old 25th July 2012, 07:34 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Nisbeth View Post
A couple of diodes in series in the GND terminal works as well (but you may not hit 6.0 spot on then)

/U.
It will also confer a ~ -4mV/°C tempco to the output voltage. Might be desirable or not.
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Old 26th July 2012, 06:18 AM   #5
RJM1 is offline RJM1  United States
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You could use an ADR510 voltage reference from the ground pin of the LM1084 to ground and a 4.7K resistor from the output of the LM1084 to the ground pin of the LM1084 to supply sufficient current for the ADR510.
This way you would not reduce the power supply rejection ratio nearly as much as the previous two examples.
Data sheet.
http://www.analog.com/static/importe...ets/ADR510.pdf
DigiKey.
ADR510ARTZ-REEL7 Analog Devices Inc | ADR510ARTZ-REEL7CT-ND | DigiKey

Last edited by RJM1; 26th July 2012 at 06:36 AM.
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Old 26th July 2012, 07:27 AM   #6
RJM1 is offline RJM1  United States
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If you were to use a resistor in series with the ground pin of the Lm1084 the value of the resistor for a typical ground pin current of 55μA would be 18.181K. From the datasheet page 5. The example shown on page 10 was probably copied from the 7805 regulator which has a 5mA ground pin current.
http://www.ti.com/lit/ds/symlink/lm1084.pdf

Last edited by RJM1; 26th July 2012 at 07:36 AM.
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Old 26th July 2012, 08:55 AM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by RJM1 View Post
If you were to use a resistor in series with the ground pin of the Lm1084 the value of the resistor for a typical ground pin current of 55μA would be 18.181K. From the datasheet page 5. The example shown on page 10 was probably copied from the 7805 regulator which has a 5mA ground pin current.
You confuse the LM1084-adj with the LM1084-5. The latter includes a resistive divider passing 5mA
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Old 26th July 2012, 09:05 AM   #8
RJM1 is offline RJM1  United States
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I guess the adjustment pin current is not the ground pin current. I found another data sheet with different values 7 to 10 mA here. and 3 μA on the adjustment pin.
http://www.htckorea.co.kr/Datasheet/LDO/LM1084-R1.3.pdf
I wonder what the actual current is in the LM1084 as I don't see it on the datasheet?
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Old 26th July 2012, 09:18 AM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by RJM1 View Post
I guess the adjustment pin current is not the ground pin current.
Since it is a 3-pin device and it has to comply with Kirchhoff's law, Iadj necessarily equals Ignd.
Iadj applies to the adjustable version and Ignd to the fixed ones, but it is purely a terminology issue.
Anyway, an adjustable resistor of 220 ohm in the ground terminal will cover all the range of possible currents for the fixed type, and will not appreciably degrade any of the characteristics
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