TPS40400 datasheet(21/72 Pages) TI

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100

110

120

125

Junction Temperature (°C)

105

145

OUT

CAP

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 (VISNS+ – VISNS– < 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

•

ICAP is the startup charging current of the output capacitance in A

•

VOUT is the output voltage of the converter in V

•

COUT is the total output capacitance in F

•

tSS 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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