Vicor Westcor PFC Mini Power Factor Corrected AC-DC Switcher - Manual (Page 16)

UG:115 vicorpower.com Applications Engineering: 800 927.9474 Page 16
Example:
±10% Trim adjust on a 12 V nominal output.
Figure 7 shows a typical variable trim circuit. Using a 10 k trimpot (R7), the resistor
values for R6 and R8 can be calculated as follows:
V1 = V
ref
+ 10% = 2.75 V Given: V
ref
= 2.5 V (see Table 1)
I
R5
= (2.75 V - V
ref
)/R
TH
= (2.75 V - 2.5 V)/10 kΩ = 25 µA Given: R
TH
= 10 kΩ (see Table 1)
Setting the bottom limit:
V
R6
= 2.5 V - 10% = 2.25 V
And since I
R5
= I
R6
= 25 µA,
R6 = V
R6
/I
R6
= 2.25 V/25 µA = 90 kΩ
V
2
= V
1
+ V
R6
= 2.75 V + 2.25 V = 5 V
I
R7
= V
2
/R7 = 5 V/10 kΩ = 500 µA
I
R8
= I
R7
+ I
R6
= 525 µA
V
R8
= (V
nom
+10%) - V
2
= 13.2 V - 5 V = 8.2 V Given: V
nom
= 12 V
R8 = V
R8
/I
R8
= 8.2 V/525 µA = 15.62 kΩ
CONSULT APPLICATIONS ENGINEERING WHEN TRIMMING OUTPUTS BELOW 5 V.
Signal Ground (J3-4)
Signal Ground on J3-4 is an isolated secondary ground reference for all J3 interfacing
signals. This is not the same as Earth Ground on input power connector MBJ1.
Enable/Disable (J3-1 to J3-3)
The Enable/Disable control pins allow outputs to be sequenced either on or off. J3-1
through J3-3 are the control pins for output cards 1 through 3, respectively. For 2-output
cards, both outputs are enabled or disabled with a single control. The Enable/Disable
pins should be pulled low to less than 0.7 V with respect to Signal Ground to disable
the outputs. They will source 4 mA maximum. These pins should be open circuited or
allowed to exceed 4.5 V when enabled. Do not apply more than 5 V to these inputs.
PFC Mini
4
7
Signal Ground
General Shutdown
Enable/Disable Output 1
J3
1
0
TTL "1" (OFF)
TTL "0" (ON)
A TTL "1" applied to the base of the transistor turns
output OFF. Pin 1 (or Pin 7 for GSD) is pulled Low
with respect to Signal Ground.
5
Vcc
1
Enable/Disable Control
Figure 8.
Enable/Disable and
General Shutdown