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.
If yes... does this mean that virtually ALL LDOs are adjustable?
Answer
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.
No comments:
Post a Comment