Datasheet

TPS40400
SLUS930B –APRIL 2011– REVISED OCTOBER 2011 www.ti.com
The FB pin is used to sense the output voltage for the purposes of power good detection. Because of this there
is the inherent filtering action provided by the compensation network connected from COMP to FB. As the output
voltage rises or falls below the nominal value, the error amplifier attempts to force FB to match its reference
voltage. When the error amplifier is no longer able to do this, the FB pin begins to drift and trip the power good
threshold. For this reason the network from COMP to FB should have no purely resistive path.
Power good de-asserts during all startups, after any fault condition is detected or whenever the device is turned
off or in a disabled state (OPERATION command or CNTL pin put the device into a disabled or off state). The
PGOOD pin acts like a diode to GND when the device has no power applied to the VDD pin.
Undervoltage lockout. The TPS40400 provides flexible user adjustment of the undervoltage lockout threshold
and the hysteresis. Two PMBus commands VIN_ON and VIN_OFF allow the user to set these input voltage turn
on and turn off thresholds independently, with a 500-mV resolution from a minimum of 2.5-V turn off to a
maximum 18-V turn on. See the command descriptions for more details.
Output overvoltage and undervoltage thresholds. The TPS40400 has output overvoltage protection and
undervoltage protection capability. The comparators that look at the overvoltage conditions and undervoltage
conditions use the FB pin as the output sensing point so the filtering effect of the compensation network
connected from COMP to FB has an effect on the speed of detection. As the output voltage rises or falls below
the nominal value, the error amplifier attempts to force FB to match its reference voltage. When the error
amplifier is no longer able to do this, the FB pin begins to drift and trip the overvoltage threshold or the
undervoltage threshold. For this reason the network from COMP to FB should have no purely resistive path.
The VOUT_OV_FAULT_LIMIT and VOUT_UV_FAULT_LIMIT commands are used to set the output overvoltage
and undervoltage thresholds. There are four possible thresholds that can be set with the undervoltage and
overvoltage commands. See the command descriptions for complete details.
Programmable fault responses. For the various fault conditions, the TPS40400 allows the user to select the
fault response. The faults that have programmable responses with the TPS40400 are overcurrent (see the
IOUT_OC_FAULT_RESPONSE command description), overtemperature, (see the OT_FAULT_RESPONSE
command description), output overvoltage, (see the VOUT_OV_FAULT_RESPONSE command description) and
output undervoltage, (see the VOUT_UV_FAULT_RESPONSE command description). These commands
program the TPS40400 response to the corresponding fault condition. Possible responses include ignoring the
fault, latching off and requiring a reset (either VDD power cycle or a toggling of the CNTL pin and/or
OPERATION command status) for the converter to restart. See the individual fault response command
descriptions for details on what is available for the specific command.
User data and adjustable anti-cross conduction delay. The TPS 40400 provides a command,
MFR_SPECIFIC_00, which can be used as a scratchpad to store 14 bits of arbitrary data. These bits can
represent anything that the user desires and can be stored in EEPROM for non-volatility. Bit 0 of this command
is used to select between two dead time settings for the controller. The particular setting required for a given
application depends upon several things, including total FET gate charge, FET gate resistance, PCB layout
quality, temperature, etc. It is not possible to give a hard and fast rule as to when to use which setting, but
generally, for FETs above 25 nC gate charge, the longer dead time setting should be looked at. The shorter dead
time setting allows higher efficiency in applications where the FETs are generally small and switch very quickly,
while may lead to minimum amounts of cross conduction in applications with larger, slower switching FETs.
Conversely, using the longer dead time setting with smaller, faster switching FETs leads to excessive body diode
conduction in the low-side FET, leading to a drop in converter efficiency. Bit 1 of this command permanently
locks certain parameters from being changed when set to 1. Use with caution. For more detail, see the
MFR_SPECIFIC_00 command description.
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