Vicor DC MegaPAC DC-DC Switcher - Manual (Page 12)

Pg. 12 Vicor 800-735-6200 Mission Power Solutions (760) 631-6846 (phone) or (760) 631-6972 (fax) Rev. 3/04
DC MegaPAC Operator’s Manual
Note: Converters are sometimes pretrimmed at the factory if a nonstandard output voltage is requested. Standard voltages
include 2V, 3.3V, 5V, 12V, 15V, 24V, 28V, and 48V. If using a nonstandard voltage, or if a ConverterPAC is ordered with a
Trim option, the resistor calculations will differ from those below. Please consult the factory for assistance.
OUTPUT MODULE V
ref
R
TH
VI-200/VI-J00 ≥3.3V 2.50V 10.0 k Ω
VI-200/VI-J00 <3.3V 0.97V 3.88k Ω
Table 3. Module Internal Reference Voltages and Thevenin Resistances.
Example:
±±10% Trim adjust on a 12V nominal output.
Figure 13 shows a typical variable Trim circuit. Using a 10k trimpot (R7), the resistor values for R6 and R8 can be calculat-
ed as follows:
V
1
= V
ref
+ 10% = 2.75V Given: V
ref
= 2.5V (see Table 3)
I
R5
= (2.75V - V
ref
)/R
TH
= (2.75V - 2.5V)/10k Ω = 25µA
Setting the bottom limit:
V
R6
= 2.5V - 10% = 2.25V
And since I
R5
= I
R6
= 25 µA,
R6 = V
R6
/I
R6
= 2.25V/25 µA = 90kΩσ
V
2
= V
1
+ V
R6
= 2.75V + 2.25V = 5V
I
R7
= V
2
/R7 = 5V/10k Ω = 500µA
I
R8
= I
R7
+ I
R6
= 525 µA
V
R8
= (V
nom
+10%) - V
2
= 13.2V - 5V = 8.2V Given: V
nom
= 12V
R8 = V
R8
/I
R8
= 8.2V/525 µA = 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.
(Remote Sense)
Load
+P +Out
J2-2 +Sense
J2-3 -Sense
-P -Out
Use 22-24 AWG Twisted Pair Wires
R5
10KW
2.5V
Bandgap
J2-1
R6 = 40k
R8
R7 = 10k
To Error
Amplifier
R4
20-200W
R3
R2
R1
10-300W
Figure 13. External Trim