Vicor VI-J00 Family DC-DC Converters and Configurable Power Supplies - Manual (Page 9)

Design Guide & Applications Manual
For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies
VI-200 and VI-J00 Family Design Guide Rev 3.5 vicorpower.com
Page 8 of 98
Apps. Eng. 800 927.9474 800 735.6200
OVERVIEW
Specifications such as efficiency, ripple and input voltage
range are a function of output voltage settings. As the
output voltage is trimmed down, efficiency goes down;
ripple as a percent of Vout goes up and the input voltage
range widens since input voltage dropout (loss of regulation)
moves down. As the units are trimmed up, the reverse of
the above effects occurs.
All converters have a fixed current limit. The overvoltage
protection setpoint is also fixed; trimming the output
voltage does not alter its setting. As the output voltage is
trimmed down, the current limit setpoint remains constant.
Therefore, in terms of output power, if the unit is trimmed
down, available output power drops accordingly.
The output voltage of most Vicor converters can be
trimmed +10%, –50%. Certain modules have restricted
trim ranges. Consult the latest datasheet for details.
Do not attempt to trim the module output voltage more
than +10%, as overvoltage shut down may occur. Do not
exceed maximum rated output power when the module is
trimmed up.
CAUTION: When trimming up VI- / MI-J00 converter
modules, additional care should be taken as an
improper component selection could result in module
failure. Improper connection of the sense leads on
VI- / MI-J00 converter modules can also result in an
excessive overvoltage condition and module failure.
The following procedures describe methods for output
voltage adjustment (–10 to +10% of nominal) of the
VI-/ MI-200, VI-/ MI-J00, ComPAC / MI-ComPAC, FlatPAC
and MegaMod / MI-MegaMod Families.
Modules with nominal 3.3 V outputs and above have
the 2.5 V precision reference and 10 k internal resistor.
For trim resistor calculations on modules with 2.0 V
outputs use 0.97 V in place of the 2.5 V reference
and substitute 3.88 k
Ω for the internal 10 kΩ resistor.
Resistors are 0.25 W. When trimming down any module,
always maintain a minimum preload of at least 1% of
rated output power and in some cases up to 10% may be
required. For more specific information on trimming down
a specific module, please consult Vicor’s Applications
Engineering Department at (800) 927-9474.
RESISTIVE ADJUSTMENT PROCEDURE
To achieve a variable trim range, an external resistor
network must be added. (Figure 5–1)
Example 1. For trimming –10% to +10% with a standard
off-the-shelf 10 k
Ω potentiometer (R7), values for resistors
R6 and R8 need to be calculated.
Resistor R6 limits the trim down range. For a given
percentage, its value is independent of output voltage.
Refer to
Table 5–1 , for limiting resistor values.
TRIMMING DOWN –10%
A 10% drop of the 2.5 V reference at the TRIM pin is
needed to effect a 10% drop in the output voltage.
(Figure 5–2)
V 1 = 2.5 V – 10% = 2.25 V
Therefore:
I
R5
=
(2.5 V – 2.25 V)
= 25 µA
10 k
Ω
Since I
R5
= I
R6
= 25 µA:
R6 =
2.25 V
= 90 k
Ω
25 µA
This value will limit the trim down range to –10% of
nominal output voltage.
5. Output Voltage Trimming
Figure 5–1 — External resistive network for variable trimming
+OUT
+SENSE
–OUT
R3
–
+
C1
Load
[a]
For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V.
Error Amp
R1 47 Ω Typ.
R4 27 Ω Typ.
R2
R5 10 kΩ[a]
TRIM R6
–SENSE
R8
R7
2.5 V[a]
R6 TRIM
–SENSE
–OUT
R7 10 kΩ POT
R5 10 kΩ[a]
(internal)
V1
R8
IR6
2.5 V[a]
reference
(internal)
[a]
For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V.
+OUT
+SENSE
–SENSE
Figure 5–2 — Circuit diagram “Trim Down”