UG:003 vicorpower.com Applications Engineering: 800 927.9474 Page 6
6.1.3 Adjust R08 such that the meter reads ~ 200 Ω. This value added to the fixed resistor
R07 should total ~ 2.3 kΩ. Note: R07 is 1.7 kΩ on the boards with the
MIL-COTS PRM.
6.1.4 One could also probe between SG and the ON side of switch OSV (#5) and set the
total resistance shown to ~ 2.3 kΩ.
6.1.5 Move switch OSF (#6) to the OFF position.
6.1.6 Move switch OSV (#5) to the ON position.
6.1.7 The unit is now ready to provide the desired output voltage set point.
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.1.8 Powering the device on should yield a no-load output voltage of the nominal specified
value.
6.1.9 The output voltage can now be set to the desired value by varying trim pot OS (R08).
6.1.10 IT MAY BE POSSIBLE TO INADVERTENTLY TRIM THE OUTPUT TOO HIGH ENABLING
THE OVERVOLTAGE PROTECTION CIRCUIT. If this happens, reduce the impedance of
R08. The initial conditions of steps 6.1.2 and 6.1.3 can be used to recover.
6.1.11 The desired output voltage set point can also be achieved by using the equations in
the data sheet and setting the total OS resistance (R07+R08) to that value and then
configuring the switches as above.
6.1.12 Be certain to adhere to the power vs. output voltage curve in the data sheet to avoid
over powering the device!
6.2 Adjusting the Current Limit
6.2.1 Increasing the load beyond the rated maximum may activate the PRM’s internal
current limit (see data sheet for values) feature. Certain applications may require a
lower limit and for those situations we offer the adjustment feature.
6.2.2 Refer to the data sheet for the resistance vs. limit curve for the desired limit value.
Note: the IL values of resistors installed on this board (R01+R02: 2 kΩ+100 kΩ) may
not cover the entire range of the curve.
6.2.3 The adjustable IL mode is engaged by repositioning the IL switch (#2) from OFF to ON.
(This may be done while the unit is powered.)
6.2.4 Adjusting the IL trim pot will reduce the current limit from the factory preset limits
specified in the data sheet. It is not possible to increase this limit beyond the factory
settings.
6.2.5 To reinstate the factory limit return the IL switch to the OFF position.
6.3 Trimming the output voltage using SC
6.3.1 Once the output voltage has been determined and set using the OS resistors, it is still
possible to trim the output down to 26 V using the SC control. Adjusting the output
in this manner rather than merely adjusting the output via the OS values provides
improved regulation.
6.3.2 Depending upon the chosen initial output voltage determined by the OS resistors, the
amount of available adjustment might be very limited. Enabling SC adjustment by
toggling the SC switch (#3) to the ON position may result in shut down.
6.3.3 To avoid the situation described in 6.3.2, set the SC trim pot R04 (100 kΩ) to its
maximum value prior to enabling the switch.
6.3.4 The data sheet has an equation for calculating the appropriate resistor value for a
trimmed output voltage. This value would be the sum of fixed resistor R03 (5.11 kΩ)
and the trim pot R04. R04 can be set using the test points provided locally, or the
sum set using the SC and SG test points (Items 3 and 6) when the SC switch is ON.
6.4 Activating Adaptive Loop Regulation and Interconnect Compensation CD.
6.4.1 The CD switch (#4) and the associated fixed resistor R05 (20 Ω) and trim pot R06
(20 Ω) enable the PRM Adaptive Loop regulation feature, which compensates for
the VTM output resistance induced voltage drop as the load current from the VTM is
increased.
6.4.2 This feature is intended to be used with mating VTM-CB discussed in detail in the
next section.
