Vicor PFC MicroS Power Factor Corrected AC-DC Switcher - Manual (Page 14)

PFC MicroS Design Guide
The Sense connector for a single and dual output board is a 3 pin connector providing the +Sense connection on
Pin 2 and the -Sense connection on Pin 3.
Remote Sense is not available for triple output configurations
External Trim (S1J2 and S1J1/J3 connectors)
The Trim pin (S1J2 connector for single outputs, and S1J1/J3 connector for dual outputs) is referenced to the -Sense pin
and can be used for external control of the output voltage. For triple output cards, the Trim pins are available on S1J1-14,
S1J1-6 and S1J1-3 for outputs 1, 2 and 3 respectively. A 10% increase to the trim pin voltage will result in a 10% increase
in output voltage. Reducing the trim pin voltage by 10% will result in a 10% decrease in output voltage.
OUTPUT MODULE V
ref
R
TH
VI-200/VI-J00
≥3.3V
2.5V 10k
Ω
VI-200/VI-J00 <3.3V 0.97V 3.88k
Ω
Maxi/Mini/Micro (Predefined) 1.23V 1k Ω
Maxi/Mini/Micro (Userdefined) 1.23V Consult Factory
Table 2: Module Internal Reference Voltages and Thevenin Resistances.
Example:
±10% trim adjust on a 12V nominal output.
Figure 6 shows a typical variable trim circuit. Using a 10k 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 2)
I
R5
= (2.75V - V
ref
)/R
TH
= (2.75V - 2.5V)/10k
Ω = 25μA
Given: R
TH
= 10k
Ω (see Table 2)
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 = 90kΩ
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
Load
+Out
+Sense
-Sense
-Out
Use 20-22 AWG
Twisted Pair Wires
Figure 5. Remote Sense
(Remote Sense)
Load
+P +Out
PIN 2 +Sense
PIN 3 -Sense
-P -Out
Use 20-22 AWG Twisted Pair Wires
2.5V
Ref.
PIN 1
R6
R8
R7
To Error
Amplifier
Use 20-22 AWG
Twisted Pair Wires
Figure 6. External Trim
Pg. 14 Vicor 800-735-6200 Westcor Division 408-522-5280 Applications Engineering 800-927-9474 03-000049 rev A