Digilent MX3cK - Manual (Page 5)

Cerebot MX3cK Reference Manual
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Board Hardware Description
The following describes the various features of
the Cerebot MX3cK hardware and the
PIC32XM320F128H microcontroller.
Power Supply
The Cerebot MX3cK is designed to be
powered either from USB or from an external
power supply. There is an automatic
switchover circuit that causes the external
supply to be used if both supplies are present.
The power supply section uses two voltage
regulators. The first regulates the external
voltage to 5V to power the VCC5V0 bus. The
second regulates the VCC5V0 bus to 3.3V to
provide power to the VCC3V3 bus that powers
the PIC32 microcontroller.
The 5V voltage regulator, IC2, is normally an
NCP1117. The board is designed to be able to
also use an LM1117, but the NCP1117 is the
part normally used. The NCP1117 is rated for
an output current of 1A (the LM1117 is rated
for 800mA). The dropout voltage of the
NCP1117 is a maximum of 1.2V at the rated
output current (1.3V for the LM1117). There is
a reverse polarity protection diode in the
external power supply circuit. Considering the
diode drop plus the forward drop across the
regulator, the minimum input voltage to the
regulator should be 7V to produce a reliable 5V
output. The absolute maximum input voltage of
both the NCP1117 and the LM1117 is 20V.
The recommended maximum operating
voltage is 15V. For input voltages above 9V,
the regulator will get extremely hot when
drawing high currents. Both the NCP1117 and
the LM1117 have output short circuit protection
and internal thermal protection and will shut
down automatically to prevent damage.
The 3.3V regulator, IC3, is a Microchip
MCP1725. This regulator is rated for a
maximum output current of 500mA. The
absolute maximum input voltage for the
MCP1725 is 6V. This regulator has internal
short circuit protection and thermal protection.
It will get noticeably warm when the current
consumed by the VCC3V3 bus is close to the
500mA maximum.
The 5V power bus, VCC5V0 can be powered
from one of three sources: 1) The USB5V0 bus
when the board is operating under USB power;
2) The output of the on-board 5V regulator
when operating from an external 7V–15V
supply; or 3) Directly from the external supply
when operating from a regulated 5V external
supply and jumper JP2 is in the BYP position.
Switchover from USB power to external power
is done automatically and the external supply
will be used if both are present.
Jumper JP2 is used to route the external
power supply voltage through the on-board 5V
regulator or directly to the VCC5V0 bus,
bypassing the on-board 5V regulator.
Normally, JP2 should be in the REG position.
This routes the external supply through the 5V
regulator. Operation from an externally
regulated 5V supply is accomplished by
placing the jumper in the BYP position.
The forward drop across the MCP1725 is
typically 210mV (350mV max) at 500mA
output. With JP2 in the BYP position, this will
allow correct operation of the 3.3V power
supply from an input voltage down to 3.5V.
This allows powering the board from batteries
and other lower voltage power sources. In this
case, the VCC5V0 power bus will not be
powered at 5V.
NOTE: It is extremely important to observe the
MCP1725 maximum input voltage rating of 6V
when JP2 is in the BYP position. Applying
more than 6V to the external power input with
the jumper in the BYP position can damage the
3.3V regulator and possibly the PIC32
microcontroller as well.
The PIC32 microcontroller is rated to use a
maximum of 75mA of current when operating
at 80Mhz. This allows up to 425mA from the
VCC3V3 bus and up to 925mA from the
VCC5V0 bus to power external devices. The