3

I need to create a negative voltage rail (from positive supply rail), say -5V to keep a P-MOS transistor in ON-state even when the voltage on its [S] source is 0V. So I found this schematic in an answer of another question here, but does this actually work?

enter image description here

Voltage Spike
  • 75,799
  • 36
  • 80
  • 208
velizarw
  • 53
  • 7

1 Answers1

11

Contrary to popular belief this does work, but the P7805 is a DC to DC converter and it can sink current. A regular 7805 voltage regulator requires a negative supply, this does not. Here is a excerpt from the P7805 datasheet:

enter image description here
Source: https://www.cui.com/product/resource/p7805-s.pdf

Voltage Spike
  • 75,799
  • 36
  • 80
  • 208
  • 6
    Uhhhh, let marketing decide the name of a component is harmful… +1 for pointing this out! – Janka Aug 26 '19 at 22:12
  • 6
    Well, they wanted it to be drop in comparable with a 7805... I agree bad idea – Voltage Spike Aug 26 '19 at 22:13
  • Wow, this is entirely different thing than one may think of...bad naming 100%. Thank you! – velizarw Aug 27 '19 at 09:32
  • For anyone else not spotting it immediately: the 2 and 3 pins stealthily switch places. – jpa Aug 27 '19 at 12:24
  • Also, note that in this circuit C1 is between pin 1 and 3 of the converter, while in OP's version it's between pins 1 and 2. – Nyos Aug 27 '19 at 12:36
  • 1
    The OP's schematic has the input capacitor C1 between pins 1 and 2, as opposed to between pins 1 and 3 as it's supposed to be. – joribama Sep 01 '19 at 02:19