User manual

Getting Started
© 2008 Microchip Technology Inc. DS51721B-page 15
2.3.3 Calculating Other VDD Values
Other VDD values can be produced by the LM317 adjustable voltage regulator by
populating the PIM board’s R101 and R102 with different value resistors.
A brief overview follows, on how to calculate alternate values for these resistors. For
detailed information, see the LM317 data sheet.
EQUATION 2-1: REGULATOR VOLTAGE OUTPUT
I
ADJ is minimized by the LM317, so it can be assumed to be zero, or very small. VREF
is the reference voltage developed by the LM317 between the output and adjustment
terminal and equals 1.25V.
That produces the equations shown in Equation 2-2.
As stated previously, R25 = 1 kΩ, and R26 = 330Ω. Without R102 and R101 being
inserted in parallel on the PIM board, V
OUT =1.25V(1+ 1 kΩ/330Ω) = 5.04V.
To calculate a desired V
OUT:
1. Solve for R2, given R1 = R26 = 330Ω.
2. Now knowing R2 and R25, solve for R102.
3. Determine the nearest available resistor value for R102 and recalculate the
resulting V
DD to make sure it does not exceed the maximum VDD for the part you
will be using.
Table 2-2 shows the R101 and R102 resistor values to use for different V
DD values. The
table assumes that the PICDEM PIC18 Explorer Demonstration Board’s R25 and R26
resistors are left at their default values of 1KΩ and 330Ω, respectively.
EQUATION 2-2: CALCULATING OUTPUT VOLTAGE
TABLE 2-2: CALCULATING R101, R102 VALUES FOR V
DD OUTPUTS
†
VDD R101 Value R102 Value
5V Open Open
3.6V Open 1.62 kΩ
3.3V Open 1.18 kΩ
3.0V Open 866 RΩ
† This table assumes that the PICDEM PIC18 Explorer Demonstration Board’s
R25 and R26 resistors are left at their default values of 1 kΩ and 330Ω,
respectively.
V
OUT
V
REF
1
R2
R1
-------+
⎝⎠
⎛⎞
I
ADJ
R2⋅+=
V
OUT
1.25V 1
R2
R1
-------+
⎝⎠
⎛⎞
=
R2 R25 R102
||
R25 R102⋅()
R25 R102+()
----------------------------------==
R1 R26 R101
||
R26 R101⋅()
R26 R101+()
----------------------------------==