Texas Instruments TPS54673EVM-225, Руководство пользователя, Страница: 15 / 42

Modifications
1-5 Introduction
The TPS54x73EVM–225 EVM also supports alternate output filter configura-
tions 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, while the position for L2 accommodates popular inductors
such as Vishay IHLP-5050 series with a 0.5 in.
×
0.5 in. package. 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, R2,
R3, C1, C2, and C3). The 0-
Ω
resistor R8 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 out-
put 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.
The primary intended usage for the TPS54x73 device family is in applications
requiring a precharge condition on the output. These types of applications in-
clude power supplies for DSPs and microprocessors where the I/O and core
voltages must track each other within a certain amount during start-up. The
TPS54673 and TPS 54873 incorporate disable sink during start-up (DSDS) to
allow this type of functionality in the SWIFT

family of dc/dc converters. A typi-
cal 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 are at a level
equal to the I/O voltage minus the drop across the diodes during start-up. The
TPS54x73EVM-225 EVM provides four series diodes, D1 through D4, and al-
lows 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 connec-
tor terminals, while leaving J3 open. Headers J5 through J8 are provided to
select the number of series diodes; install a jumper across the header to by-
pass the adjacent diode. Care must be taken to use the correct number of
diodes for the application. Under no circumstances can the output voltage be
allowed to precharge to a level higher than the preset output voltage. If this
condition occurs during start-up, the TPS54x73 device does not begin switch-
ing. If a voltage transient on the precharge voltage source causes the series
diodes to conduct, current may be sunk through the low side FET in the device,
possibly damaging the device. The actual voltage drop across the diodes dur-
ing start-up depends on the initial load condition of the circuit as well as the
ambient temperature.