Datasheet

17
PICtail Daughter Board Power Interface
U6, D1 and R4 create a power switching circuit to allow both the PICDEM.net 2 and PIC18
Explorer motherboards to be automatically supported without using jumpers to select
between the two. The +5V net powers almost the entire board, including all bus switches,
and the 3.3V regulator for the Ethernet controller. Unfortunately, obtaining a 5V supply from
the PICtail Daughter Board connectors differs based on the motherboard in use.
For the PICDEM.net 2, 5V is supplied on the VPIC net (J4 Pin 26). However, on the PIC18
Explorer, 5V is always supplied on the 5VEXT net (J3 Pin 4), but only supplied sometimes
on VPIC. If a 3.3V PIM is used on the PIC18 Explorer (e.g., PIC18F87J11), then the VPIC
net is only powered to 3.3V. This potential voltage difference makes it illegal to directly
connect the 5VEXT and VPIC nets together.
When the PIC18 Explorer is used, the D1 shottkey diode provides a minimal voltage drop
between the 5VEXT net and the +5V net required to power the Ethernet circuit. The applica-
tion of voltage on 5VEXT also turns the U6 P-channel MOSFET off, so no current from the
+5V net flows back into the PIC18 Explorer through the VPIC net.
When the PICDEM.net 2 is used, the 5VEXT net is not connected. 5V power is supplied on
VPIC, which forward biases the body diode in U6, causing the +5V net to begin powering up.
Without being driven, the R4 pull-down resistor on the 5VEXT net drops the gate voltage on
U6 to GND. Partially powering +5V causes the U6 MOSFET to turn on and essentially short
the VPIC and +5V nets together.
The U6 MOSFET could be replaced with another schottky diode, like D1, but the use of a
MOSFET allows a wider motherboard voltage tolerance.
Other Information
To obtain the most recent and complete documentation for this demonstration board,
including:
- Information Sheet - Board Description - Board Schematics
- Source Code - Application Examples - Links to Web Seminars
please refer to the Microchip web site: www.microchip.com/ethernet