Vicor Half-chip PRM-RS Customer Board - Manual (Page 18)

UG:008 vicorpower.com Applications Engineering: 800 927.9474 Page 18
10.2 Current Limit Adjustment
The current limit set point is a function of the current sense amplifier gain, the shunt resistor, and
the internal current limit threshold of the PRM ® . The onboard current sense IC and shunt have a
typical combined gain of 400 mV/A. The gain of the current sense IC (Q 02 ) is fixed at 50 V/V, and
the IF current limit threshold is fixed at 2.0V. The current sense shunt (R 08 ) can be changed per
Equation 2 in order to adjust the current limit. Once the value has been calculated, obtain the
closest 1% standard value, size 2512 resistor and replace R 08 .
10.3 Trim Range Adjustment
The trim range is set by the reference voltage network shown below. The half-chip PRM-RS
Customer Board is nominally configured for a 20 V to 55 V trim range. Any operation outside of
the nominal trim range may require adjustments to the compensation.
Assuming local sense is implemented, the output voltage is defined by Equation 3. Referring to
Figure 17, the output of U 03 is 1.25 V. When R 22 is fully clockwise, the U 03 output voltage will be
applied directly to V REF resulting in the maximum reference voltage. This, in combination with the
voltage sense gain will determine the maximum output voltage. As R 22 is turned counter
clockwise, R 22 and R 42 form a divider which limits the minimum reference voltage.
In order to change the upper trim range, adjust the voltage sense gain (R 40 ) based on a maximum
reference voltage of 1.25 V. In order to change the lower trim range, use Equation 3 to determine
the minimum reference voltage required and adjust R 42 appropriately. Before making adjustments
to the trim range, refer to the data sheet to ensure that the device will operate outside of its
rated limits.
V
IF_IL
is the IF current limit threshold (2.0 V)
I LIMIT(SET) is the desired current limit set point
G CS is the current sense amplifier (Q O2 ) gain (50 V/V)
Equation 2
Equation 3
(local sense only)