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Powering PIC24F using 3.3 V

Discussions pertaining to the electrical issues (e.g. circuitry, power, etc.) of Pumpkin's CubeSat kit.

Powering PIC24F using 3.3 V

Postby jepost » Thu Feb 06, 2014 11:37 am

Question #1. On the CSK PSPM D drawings only VCC is shown, but not VCC_SYS. On the CSK DB drawings only VCC_SYS is shown (pin 28), but not VCC. Are VCC and VCC_SYS the same for the PSPM D (PIC24F)? If not, is it possible to connect to VCC on the PSPM?

Question #2. My application only requires 3.3 V so I want to run the PIC24F from 3.3 V instead of 5V. On the voltage regulator drawing of the CSK PSPM D VCC appears at the output (pin 2) of the MCP1700 LDO Regulator. In reviewing the data sheet for the MCP1700 it appears that if +5V_SYS is left floating the pass transistor in the LDO is turned off. Thus, it is not necessary to cut the trace in order to disconnnect the MCP1700 from VCC. Any comments on this approach? Has anyone else tried operating the PIC24F from 3.3 V?


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Re: Powering PIC24F using 3.3 V

Postby aek » Thu Feb 06, 2014 1:12 pm

Thank you for your good questions.

To be clear, all PIC24Fs on PPM D1 "operate ... from 3.3 V" ... but not from VCC_SYS.

VCC is:
    Generated on the PPM.
    (Usually) the voltage that the uC on the PPM operates at.
    The high range of the I/O of the uC on the PPM (and thereby, the entire system).
    The voltage that various components on the MB (like the RTC) operate at.

Note also that another voltage (VCC_SD) is required for the SD Card to work properly ... see schematics.

If I understand you correctly, what you want to do is run a CubeSat Kit's MB+PPM D1 in a system that provides +3.3V via VCC_SYS, but doesn't provide a +5V via +5V_SYS.

You can do that by fitting the following:
DB: JP16 (connects VCC to VCC_SYS)
MB: R68

This will connect VCC_SYS (from an EPS) to VCC on the PPM and on the MB. VCC_SD will also go live, since it is connected to VCC on PPM D1.

All of this assumes that you will not have any voltage on +5V_SYS in your system.

Also, on the DB, you may want to remove jumpers JP1, JP20 and JP6, unless you want +5V_SYS to still be present in the system.

As to your question re "back-driving" an LDO like the MCP1700 ... well, you just have to try it. Some LDOs will see large currents flow -- or even be damaged .. when Vout > Vin and there is a current path to ground from Vout through Vin.
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