Texas Instruments TPS54973EVM-017, User Manual, Page: 13 / 34

Modifications
1-5 Introduction
Figure 1−1. Frequency Trimming Resistor Selection Graph
250
300
350
400
450
500
550
600
650
700
750
60 80 100 120 140 160 180
Switching Frequency − kHz
R − Resistance − k
Ω
1.3.3 Alternate Output Filters
The TPS54973EVM-017 EVM also supports alternate output filter
configurations by means of pads located on the back side of the PCB. The
positions for C15, C16, and C17 provide space for up to three electrolytic type
surface mount capacitors as an alternative to the ceramic types provided.
Pads for an alternate inductor at the L2 position are also included. Since
changes in the output filter affect the overall loop response, the user may find
it desirable to change the values used in the compensation network (R1, R3,
R5, C6, C7, and C8) the 0
Ω
resistor R7 in the feedback path is provided as
a convenient place to break the loop for testing any compensation value
changes. While the provided compensation network can provide a stable
output for a wide variety of output filter component values, it is always a good
idea to verify any changes to the output filter or compensation network.
1.3.4 Precharge Considerations
The primary intended usage for the TPS54973 device family is in applications
requiring a precharge condition on the output. These types of applications
include power supplies for DSPs and microprocessors where the I/O and core
voltages must track each other within a certain amount during startup. The
TPS54973 incorporates disable sink during startup to allow this type of
functionality in the SWIFT family of dc/dc converters. A typical design
approach is to tie the output of the core voltage to the output of the I/O voltage
with a number of series diodes so that the core voltage is at a level equal to
the I/O voltage minus the drop across the diodes during startup. The
TPS54973EVM−017 EVM provides four series diodes, D1 through D4, and
allows the user to precharge the output from either the EVM input voltage or
an external source. To use the input voltage as the precharge source, install
a jumper across the J3 header. To supply an external source, use the J4