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Example:
±10% Trim adjust on a 12V nominal output.
Figure 11 shows a typical variable Trim circuit. Using a 10kΩ trimpot (R7), the resistor values for R6 and
R8 can be calculated as follows:
V
1
= V
REF
+ 10% = 2.75V
Given: V
REF
= 2.5V (see Table 1)
I
R5
= (2.75V – V
REF
)/R
TH
= (2.75V – 2.5V)/10kΩ = 25mA
Setting the bottom limit:
V
R6
= 2.5V – 10% = 2.25V
And since I
R5
= I
R6
= 25mA,
R6
= V
R6
/I
R6
= 2.25V/25mA = 90kΩ
V
2
= V
1
+ V
R6
= 2.75V + 2.25V = 5V
I
R7
= V
2
/R7 = 5V/10kΩ = 500mA
I
R8
= I
R7
+ I
R6
= 525mA
V
R8
= (V
NOM
+ 10%) – V
2
= 13.2V – 5V = 8.2V
Given: V
NOM
= 12V
R8
= V
R8
/I
R8
= 8.2V/525mA = 15.62kΩ
Using the previous 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.
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