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I have a quick question... I have purchased a 5VDC fixed voltage LDO. P/N: MC7805CT-BP I was wondering if I can still use a voltage divider to set the output to a higher voltage.

schematic diagram

If yes... does this mean that virtually ALL LDOs are adjustable?

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  • \$\begingroup\$ Actually, what manufacturer is your part? \$\endgroup\$ – Passerby Feb 6 '13 at 5:42
  • \$\begingroup\$ Micro Commercial Co \$\endgroup\$ – hassan789 Feb 15 '13 at 1:23
  • \$\begingroup\$ The datasheet from MCC is very very lacking. But since it's a generic part, the information is still valid. Sorry about the slight offputing tone. \$\endgroup\$ – Passerby Feb 15 '13 at 6:13
  • \$\begingroup\$ No worries. Thanks for taking out the effort and responding. \$\endgroup\$ – hassan789 Feb 15 '13 at 18:37
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Actually, yes, the MC7805 is adjustable. Your part has a very sparse datasheet, but that is okay since it's a very generic part. It is part of the 7800 series linear regulators, which is duplicated by many manufacturers. OnSemi's version of the standard 7805 Linear Regulator has a very detailed datasheet. It's schematic diagram even has the RSense pin internally, though they do not show how it is connected. It varies somewhat from Micro Commercial's schematic, but in practical application, they are the same.

First, it is not a LDO. It requires at least 2 volts to produce the correct output voltage. OnSemi does not consider it a LDO. Most LDOs would have a 1v or smaller dropout.

Second, the identical part from OnSemi states (in the datasheet and product page):

Although designed primarily as a fixed voltage regulator, these devices can be used with external components to obtain adjustable voltages and currents.

Mainly Figure 10- Adjustable Output Regulator, which has a diagram of how to connect a op amp to make it Adjustable:

The addition of an operational amplifier allows adjustment to higher or intermediate values while retaining regulation characteristics. The minimum voltage obtainable with this arrangement is 2.0 V greater than the regulator voltage.

These instructions are also seen in other manufacturer's datasheets.

Finally, not all Linear Regulators or LDOs are adjustable, but those based on the 7800 series are. Since most manufacturers enjoy producing the same part with no major change in it's schematic, that gives a wide variety of sources to choose from.

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Oli answered the first part of the question pretty well. Coming to your second question

If yes... does this mean that virtually ALL LDOs are adjustable?

All LDOs may not be adjustable. Read the datasheet of your LDO to verify if that one is adjustable.

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  • \$\begingroup\$ The first thing I checked was the datasheet before asking this question. The datasheet does not mention anything about the LDO being adjustable. It just says the output is always 5VDC \$\endgroup\$ – hassan789 Feb 6 '13 at 4:35
  • \$\begingroup\$ @hassan789 Therefore "All LDOs may not be adjustable". \$\endgroup\$ – Chetan Bhargava Feb 6 '13 at 4:45
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You can adjust a fixed regulator, but only higher than it's nominal voltage. For example if you have a 5V regulator, you can only adjust it to > 5V (assuming no negative voltage used)

A fixed regulator is just like an adjustable one but with the divider resistors internal to set the voltage.

The LM7805 datasheet has an example of raising the output voltage:

7805 example

Any standard linear regulator (by this I mean those that use the most common 78xx type topology - there are others that don't, and have e.g. voltage set using only 1 resistor) is adjustable by altering the reference pin voltage level in some way (yours included - it's just a "copy" of the LM7805, so you can use these kind of circuits - see figure 12 in the MC7805 datasheet). For example, you can also drive it directly with an opamp:

7805 opamp

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