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The 9V regulator in this datasheet has a 2V dropout voltage. However, in the electrical characteristics on Page 9, the datasheet says that you need 15V input to get a  9V output. Does that mean I can't get 9V out with a 12V input? Or am I interpreting the datasheet wrong?

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    \$\begingroup\$ No, it does not say that, it gives the conditions where other parameters were tested. What is your maximum current need? What is the nature of the load, and of the 12v source? \$\endgroup\$ – Chris Stratton Aug 3 '19 at 15:12
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    \$\begingroup\$ dropout is 2V, details in figure 29. dropout is across base emitter junction of Q16 and Q17 and very low value sense resistor R11 in figure 4 schematic \$\endgroup\$ – Indraneel Aug 3 '19 at 15:19
  • \$\begingroup\$ I plan on using it to drive an amplifier. The amplifier's purpose is to negate the dropout voltage of some diodes for rectification of an audio signal, so not too much current is needed (I think). \$\endgroup\$ – Sal M Aug 3 '19 at 15:22
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Yes, this regulator will give you 9V from a 12V input. The dropout voltage is indeed 2V.

The heading on page 9 is just telling you the test conditions that were used to characterize the part and get the data shown in the table. It's not practical to specify a device at every possible operating point, so the manufacturer chooses some typical conditions and measures the device parameters under those conditions.

If you look at the quiescent current specification, for example, you will see a maximum value of 6mA. The section on quiescent current change gives you an idea of how the quiescent current will vary for a few different cases: varying input with a smaller load, varying input but held at room temperature, varying the load current with constant 15V input.

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  • \$\begingroup\$ If the datasheet is mentioning the test conditions, then why is the output not 9V on pages 6-8? I know you can make the regulator a variable one using some external circuitry, but you'd think the datasheet would mention this in the test conditions if they used it.... \$\endgroup\$ – Sal M Aug 3 '19 at 15:24
  • \$\begingroup\$ ^Edited the above comment to say pages 6-8. If the regulator is not variable, I thought the output should always be 9V if used in the simplest configuration \$\endgroup\$ – Sal M Aug 3 '19 at 15:27
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    \$\begingroup\$ This regulator is available with different fixed output voltages...you are interested in the 9V version (L7809) but there are also 5V (L7805), 6V (L7806), and 8V (L7808) versions available. It looks like the manufacturer always tests the parts with an input voltage 6V higher than the output. If you want to use the 9V part then you should ignore pages 6-8 and 10-12. Likewise, pages 13-22 are for the 'C' version of the device. \$\endgroup\$ – Elliot Alderson Aug 3 '19 at 15:45
  • \$\begingroup\$ Ahhhh gotcha, thanks! Feel like an idiot for not seeing that lol \$\endgroup\$ – Sal M Aug 3 '19 at 16:33

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