Operator`s manual

Rev. 1/99 Vicor 800-735-6200 Westcor Division 408-522-5280 Applications Engineering 978-749-3524 or -3525 Pg. 15
3 Phase MegaPAC Owner’s Manual
Example:
±±
10% Trim adjust on a 12V nominal output.
Figure 17 shows a typical variable Trim circuit. Using a 10k Trimpot (R7), the resistor values for R6 and R8 can be calcu-
lated as follows:
V
1
= V
ref
+ 10% = 2.75V Given: V
ref
= 2.5V (see Table 2)
I
R5
= (2.75V - V
ref
)/R
TH
= (2.75V - 2.5V)/10kΩ = 25µA
Setting the bottom limit:
V
R6
= 2.5V - 10% = 2.25V
And since I
R5
= I
R6
= 25µA,
R6 = V
R6
/I
R6
= 2.25V/25µA = 90kΩ
V
2
= V
1
+ V
R6
= 2.75V + 2.25V = 5V
I
R7
= V
2
/R7 = 5V/10kΩ = 500µA
I
R8
= I
R7
+ I
R6
= 525µA
V
R8
= (V
nom
+10%) - V
2
= 13.2V - 5V = 8.2V Given: V
nom
= 12V
R8 = V
R8
/I
R8
= 8.2V/525µA = 15.62kΩ
Using the above resistor combination, a 12V output can be trimmed externally up to 13.2V and down to 10.8V. For further
information on external trimming, refer to Chapter 5 of the Applications Manual or consult the factory for assistance.
CONSULT APPLICATIONS ENGINEERING WHEN TRIMMING OUTPUTS BELOW 5V.