External Adjustable Trim Features: SP/SPW Series

APPLICATION NOTES
SP/SPW Series Trim Function
The following information is provided to allow quick calculation of the trim resistor value for a desired output voltage. The general
procedure for calculating a trim resistor is as follows:
1.
2.
3.
4.
Determine the desired output voltage (Vo)
Select Equation. (Trim Low/Trim High)
Use the data in Table 1 to complete the equation.
Evaluate.
In order to trim low use Equation 1 and Table 1 to calculate resistor RTL for the desired output voltage.
Equation 1: Trim Low


Vo  VREF
RTL   V
  RLIM
1
REF
 ( RL )  ( RH  (Vo  VREF )) 
Vo - Desired output voltage.
All resistor values in K ohms.
Schematic 1: Trim Low
In order to trim high use Equation 2 and Table 1 to calculate resistor RTH for the desired output voltage.
Equation 2: Trim High

RTH  



VREF
Vo VREF
RH
 
VREF
RL

  R LIM


Vo - Desired output voltage.
All resistor values in K ohms.
Schematic 2: Trim High
MODEL
(Output Voltage)
RH
RLIM
RL
VREF
(K OHMS)
(K OHMS)
(K OHMS)
(VOLTS)
3.3V
5.0V
8.0V
9.0V
12.0V
15.0V
24.0V
26.0V
32.0V
0.750
2.49
5.49
6.49
9.53
12.4
21.5
17.6
23.7
0.499
10.0
10.0
10.0
13.7
13.7
15.4
15.4
12.7
2.32
2.49
2.49
2.49
2.49
2.49
2.49
1.87
2.00
2.495
2.495
2.495
2.495
2.495
2.495
2.495
2.495
2.495
Table 1 : Trim Low/High Data Table.
Note: Output trim +/- 10% max.
Wall Industries, Inc. 5 Watson Brook Road Exeter, NH 03833
603-778-2300 www.wallindustries.com Fax 603-778-9797