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Fasttrack
05-08-2013, 10:46 AM
My question is regarding charge pumps. Is it possible to take one of those cheap, "prepackaged" charge pumps that go from 3.3V to 5V or similar and float them so the "ground" is actually tied to 45V and the input is 48V? (For example)

For instance, this little guy is a whopping ~$0.40 and very simple. I may be overlooking something stupid, but I don't see any reason why it wouldn't work if I connect the ground to 45V and made sure that the logic signals for shutdown are kept between 45V and 48V. Presumably, the output would be +5V with respect to its "ground" pin or 50V with respect to the actual circuit ground, right?

http://www.mouser.com/ds/2/408/TCA62753FUG_E_Ver04-53117.pdf

MaxHeadRoom
05-08-2013, 11:08 AM
Not to sure of what configuration you are looking for but for one, no pin can be at anything over 5v WRT pin 5 common.
And any other input or output has to be referenced to pin 5.
The IC is only rated for 100Ma +6vdc output.
Max.

ed_h
05-08-2013, 11:54 AM
Should work fine.

Paul Alciatore
05-08-2013, 01:36 PM
Yes, it should work as long as your Input Voltage (48V in your example) is between 2.7 and 5.5 Volts greater than your "ground" Voltage (45V in your example).

But you do not say where the 48V input is going to come from. DO be sure that the absolute limits on the Input Voltage relationship is maintained during power up and power down sequences or the chip may blow when you turn the circuit on or off. Failing to take these transient situations into account is a common error to make in power supply design. It may be possible to add diodes (rectifier and Zener) on the input pin to ensure that this condition is always true. An example of how this could be a problem is if the 48V is the unregulated supply which will come up to value quickly when power is turned on. Now, if the 45V is a regulated Voltage and there is a delay while some capacitors charge up, then there could be a large difference between them during this process. And the smoke escapes from the chip. Likewise on power off, one may discharge before the other and again, POOF!

The sequencing of the various Voltages in a multiple Voltage power supply must always be considered in view of the power supply itself and of the circuit(s) that it powers.

darryl
05-08-2013, 08:24 PM
It might be possible to modify the 'typical' circuit for that chip to enable the chip to be 0 volts referenced (ground) , and powered by +5v also referenced to ground, yet add the 5 volt output to any other higher voltage- in your case it sounds like it would be 45 volts.

A 555 timer chip will do it for sure, but it is a bit more complicated than the chip you show.