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Getting Started
2.3.3
Calculating Other V DD Values
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.
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
V OUT = V REF ? 1 + ------- ? + I ADJ ? R2
R2
? R1 ?
I ADJ is minimized by the LM317, so it can be assumed to be zero, or very small. V REF
is the reference voltage developed by the LM317 between the output and adjustment
terminal and equals 1.25V.
EQUATION 2-2:
CALCULATING OUTPUT VOLTAGE
V OUT = 1.25V 1 + ------- ?
R2 = R25 || R102 = ----------------------------------
R1 = R26 || R101 = ----------------------------------
? R2
? R1 ?
( R25 ? R102 )
( R25 + R102 )
( R26 ? R101 )
( R26 + R101 )
As stated previously, R25 = 1 k Ω , and R26 = 330 Ω . Without R102 and R101 being
inserted in parallel on the PIM board, V OUT =1.25V(1+ 1 k Ω /330 Ω ) = 5.04V.
To calculate a desired V OUT :
1. Solve for R2, given R1 = R26 = 330 Ω .
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 V DD for the part you
will be using.
table assumes that the PICDEM PIC18 Explorer Demonstration Board’s R25 and R26
resistors are left at their default values of 1K Ω and 330 Ω , respectively.
TABLE 2-2:
CALCULATING R101, R102 VALUES FOR V DD OUTPUTS ?
V DD
5V
3.6V
3.3V
3.0V
R101 Value
Open
Open
Open
Open
R102 Value
Open
1.62 k Ω
1.18 k Ω
866 R Ω
? This table assumes that the PICDEM PIC18 Explorer Demonstration Board ’s
R25 and R26 resistors are left at their default values of 1 k Ω and 330 Ω ,
respectively.
? 2008 Microchip Technology Inc.
DS51721B-page 15
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