User manual

Getting Started
 2015 Microchip Technology Inc. DS40001812A-page 27
2.6 SELECTING VDD VALUES
The Explorer 8 Development Board is capable of supplying 5V, 3.3V and variable
supply voltages between 1.2 and 5V through dedicated on-board regulators. The
variable supply voltage, called V_VAR (also equal to V
DD), is used to power the device
and the on-board components.
2.6.1 Varying the Device Voltage
Figure 2-8 shows the jumper configuration for the three voltage settings.
FIGURE 2-8: JUMPER CONFIGURATION FOR DIFFERENT DEVICE
VOLTAGES
2.6.2 Calculating other VDD Values
For voltages other than 5V and 3.3V, jumpers J14 and J30 must be configured for
variable supply as shown in Figure 2-8. Other V
DD values can be produced by the
LM317 adjustable voltage regulator by populating the PIM board’s R101 and R102 with
different value resistors. This section discusses how to calculate alternate values for
these resistors. For detailed information, see the LM317 data sheet.
EQUATION 2-1: LM317 REGULATOR VOLTAGE OUTPUT
IADJ is minimized by the LM317 and can be neglected or assumed to be zero. VREF is
the reference voltage developed by the LM317 between the output and adjustment
terminal, and is typically equal to 1.25V.
Therefore, the equation can be rewritten as shown in Equation 2-2.
EQUATION 2-2: CALCULATING THE LM317 OUTPUT VOLTAGE
J14
J30
+5V
+3.3V
ADJ+V
J14
J30
+5V
+3.3V
ADJ+V
J14
J30
+5V
+3.3V
ADJ+V
V_VAR = 5V
(Fixed)
V_VAR = 3.3V
(Fixed)
V_VAR = ADJ + V REF
(Variable)
Note: R101 and R102 are named R1 and R2, respectively, in other PIM boards.
These must not be confused with the R1 and R2 values discussed in this
section.
V
OUT
V
REF
1
R2
R1
-------+


I
ADJ
R2+=
V
OUT
1.25V 1
R2
R1
-------+


=
Where:
R2 R20 R102

R20 R 102
R20 R102+
----------------------------------==
R1 R19 R101

R19 R101
R19 R101+
----------------------------------==