PICDEMPIC18 datasheet(19/38 Pages) MICROCHIP

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Getting Started

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

IADJ 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 k

Ω, and R26 = 330Ω. Without R102 and R101 being

inserted in parallel on the PIM board, VOUT =1.25V(1+ 1 k

Ω/330Ω) = 5.04V.

To calculate a desired VOUT:

Solve for R2, given R1 = R26 = 330

Ω.

Now knowing R2 and R25, solve for R102.

Determine the nearest available resistor value for R102 and recalculate the

resulting VDD 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 VDD values. The

table assumes that the PICDEM PIC18 Explorer Demonstration Board’s R25 and R26

resistors are left at their default values of 1K

Ω and 330Ω, respectively.

EQUATION 2-2:

CALCULATING OUTPUT VOLTAGE

TABLE 2-2:

CALCULATING R101, R102 VALUES FOR VDD OUTPUTS†

VDD

R101 Value

R102 Value

Open

3.6V

Open

1.62 k

3.3V

Open

1.18 k

3.0V

Open

866 R

† 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 330Ω,

respectively.

OUT

REF 1

-------

⎝⎠

⎛⎞

ADJ

⋅

OUT

1.25V 1

-------

⎝⎠

⎛⎞

R25

R102

R25 R102

⋅

()

R25

R102

()

----------------------------------

R26

R101

R26 R101

⋅

()

R26

R101

()

----------------------------------

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