Datasheet

60
70
80
90
100
110
120
25 45 85 125
Junction Temperature (°C)
Internal Overcurrent Threshold (mV)
65 105 145
OUT OUT
CAP
SS
V C
I
t
´
=
TPS40400
www.ti.com SLUS930B –APRIL 2011– REVISED OCTOBER 2011
Figure 20. Internal Overcurrent Threshold Variation
Reading the output current. The average output current for the converter is readable using the READ_IOUT
command. The results of this command support only positive or current sourced from the converter. If the
converter is sinking current the result of this command is a reading of 0 A. Another consideration is the amount
of ripple voltage applied to the ISNS pins when the DC voltage level is low – i.e., low or no output current.
Because the TPS40400 averages out the ripple voltage when reporting the output current using the READ_IOUT
command. Excessive negative ripple voltage (V
ISNS+
– V
ISNS–
< 0) at the ISNS pins causes an error in the
reported output current. To ensure accurate readings the differential voltage at these pins should not be allowed
to exceed –45 mV.
Soft-start time. The TPS40400 supports several soft-start times from 600 μs to 9 ms selected by the TON_RISE
PMBus command. See the command description for full details on the levels and implementation. When
selecting the soft-start time, care must be taken to ensure that the charging current for the output capacitors is
considered. In some applications (e.g., those with large amounts of output capacitance) this current can lead to
problems with nuisance tripping of the overcurrent protection circuitry. To ensure that this does not happen, the
output capacitor charging current should be included when considering where to set the overcurrent threshold.
The output capacitor charging current can be found using Equation 14:
where
• I
CAP
is the startup charging current of the output capacitance in A
• V
OUT
is the output voltage of the converter in V
• C
OUT
is the total output capacitance in F
• t
SS
is the selected soft-start time in seconds (14)
With the charging current calculated, the overcurrent threshold can then be calibrated to the sum of the
maximum load current and the output capacitor charging current plus some margin. The amount of margin
required depends on the individual application, but 25% is a suggested starting point. More or less may be
required.
Power good. The TPS40400 has user selectable power good thresholds. These thresholds determine at what
voltage the PGOOD pin is allowed to go high and the associated PMBus flags are cleared. There are three
possible settings that can be had. See the POWER_GOOD_ON and POWER_GOOD_OFF command
descriptions for complete details. Note that these commands establish symmetrical values above and below the
nominal voltage. Values entered for each threshold should be the voltages corresponding to the threshold below
the nominal output voltage. For instance, if the nominal output voltage is 3.3 V, and the desired power good on
thresholds are ±5%, the POWER_GOOD_ON command is issued with 2.85 V as the desired threshold. The
POWER_GOOD_OFF command must be set to a lower value (higher percentage) than the POWER_GOOD_ON
command as well. The VOUT_SCALE_LOOP command must be set to approximately 0.1818 for these examples
to work correctly.
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