Microchip Technology DV164136 User Manual

© 2008 Microchip Technology Inc.

DS51721B-page 15

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.

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.

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, V

OUT

 =1.25V(1+ 1 k

Ω/330Ω) = 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 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 1K

Ω and 330Ω, respectively. 

5V

Open

Open

3.6V

Open

1.62 k

Ω

3.3V

Open 1.18 

k

Ω

3.0V

Open

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 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

+

(

)

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

=

=