Datasheet

( )
( )
FB
V VOUT _ MARGIN _ LOW VOUT _ TRIM VOUT _ SCALE _ LOOP= + ´
( )
OUT FB
R2 R1
V V
R2
æ ö
+
= ´
ç ÷
ç ÷
è ø
OC
ISNS
1.875mV
I
R
D
=
TPS40400
www.ti.com SLUS930B –APRIL 2011– REVISED OCTOBER 2011
Margin low state:
where
• V
FB
is the FB pin voltage
• VOUT_TRIM is the offset voltage in volts to be applied to the output voltage
• VOUT_SCALE_LOOP is the output voltage divider scale parameter
• VOUT_MARGIN_HIGH is the requested margin high voltage
• VOUT_MARGIN_LOW is the requested margin low voltage (9)
For these conditions, the output voltage is shown in Equation 10.
where
• V
FB
is the pin voltage calculated in Equation 7
• R2 and R1 are in consistent units from Figure 17
• VOUT is the output voltage (10)
NOTE
The sum of the margin and trim voltages cannot be more that ±25% from the nominal
output voltage. The FB pin voltage can deviate no more that this from the nominal 600
mV.
When using the margin commands, the transition rate between any two of the three states (margin high, no
margin and margin low) is determined by the soft start time (set by TON_RISE and the output voltage information
available to the controller using the VOUT_SCALE_LOOP command. The result is that the transition rate
between margin states is the same volts per second as the soft start ramp – assuming that the user has input the
correct value for VOUT_SCALE_LOOP.
Overcurrent thresholds. PMBus provides for adjustable overcurrent in the TPS40400. To function properly, the
TPS40400 must be given the current sensing element resistance value. This is accomplished by issuing the
IOUT_CAL_GAIN command with the argument set to the resistance of the sense element (see the
IOUT_CAL_GAIN command description). The resolution of this command is 30.5 µΩ and the range is 0 to
15.6 mΩ.
Another command, IOUT_CAL_OFFSET (see the command description) can be used to trim out offset errors in
the READ_IOUT command results, overcurrent warning and fault level thresholds. The resolution of this
command is 62.5 mA Offsets cannot be trimmed closer than half of this amount. The range for this command is
-4 A to 3.937 A. Calibrating offsets to a level greater than this is not possible.
Once these two parameters have been set the IOUT_OC_WARN_LIMIT and IOUT_OC_FAULT_LIMIT limit
commands can be used to set the overcurrent warning and fault thresholds for the converter. There are two
resolution limiting factors in setting the overcurrent thresholds. The first is the resolution available in the
IOUT_OC_WARN_LIMIT and IOUT_OC_FAULT_LIMIT commands. The resolution available here is 500 mA.
This is the absolute minimum adjustment that can be made to these thresholds. The other potential limit is the
resolution of the overcurrent DAC and can result in lower resolution. The overcurrent detection is done using a
DAC to set the threshold and a comparator to sense when the actual current level is above that threshold. The
resolution of the DAC is 1.875 mV. The resistance of the current sense element and this resolution determine the
minimum adjustment that can be made to the overcurrent warning and fault thresholds. That minimum
adjustment is given in Equation 11.
where
• I
ΔOC
is the minimum change that can be made in the overcurrent warning or fault threshold
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