Datasheet
6
7
COMP
FB
R1
R2
C1
C2R4
C3
R3
8
DIFFO
+
9
10
X1
VSNS+
VSNS–
To load supply
connections
UDG-09075
( )
FB
OUT FB
R1
R2 V
V V
æ ö
= ´
ç ÷
ç ÷
-
è ø
TPS40400
www.ti.com
SLUS930B –APRIL 2011– REVISED OCTOBER 2011
Figure 17. Setting the Output Voltage
The components in Figure 17 that determine the nominal output voltage are R1 and R2. R1 is normally chosen to
make the feedback compensation values (R3, R4, C1, C2 andC3) come close to readily available standard
values. R2 is then calculated in Equation 1.
where
• V
FB
is the feedback voltage
• V
OUT
is the desired output voltage
• R1 and R2 are in the same units (1)
The feedback voltage can be changed ±25% from the nominal 600mV using PMBus commands. This allows the
output voltage to vary by the same percentage. See the PMBus Functionality and Additional Setup section for
further details. Once the output voltage is set and the values of R1 and R2 are known, the VOUT_SCALE LOOP
parameter can be calculated. This parameter is required for the PMBus interface to function properly when
making output voltage adjustments.
Voltage feed forward. The TPS40400 has input voltage feed forward that maintains a constant power stage
gain as input voltage varies and provides for very good response to input voltage transient disturbances. The
simple constant power stage gain of the controller greatly simplifies feedback loop design because loop
characteristics remains constant as the input voltage changes, unlike a buck converter without voltage feed
forward. For modeling purposes, the gain from the COMP pin to the average voltage at the input of the L-C filter
is 6V/V.
Output current limit and warning. The TPS40400 uses a differential current sense scheme to sense the output
current. The sense element can be either the series resistance of the power stage filter inductor or a separate
current sense resistor. When using the inductor series resistance as in Figure 18, a filter must be used to remove
the large AC component of voltage across the inductor and leave only the component of the voltage that appears
across the resistance of the inductor. The values of R5 and C4 for the ideal case can be found by Equation 2.
The time constant of the R-C filter should be equal to or greater than the time constant of the inductor itself. If the
time constants are equal, the voltage appearing across C4 is be the current in the inductor multiplied the inductor
resistance. The inductor ripple current is reflected in the voltage across C4 perfectly in this case and there is no
reason to have a shorter R-C time constant. The time constant of the R-C filter can be made longer than the
inductor time constant because this is a voltage mode controller and the current sensing is done for overcurrent
detection and output current reporting only. Extending the R-C filter time constant beyond the inductor time
constant lowers the AC ripple component of voltage present at the ISNS pins of the TPS40400 but leaves the
correct DC current information intact. This also delays slightly the response to an overcurrent event, but reduces
noise in the system leading to cleaner overcurrent performance and current reporting data over the PMBus
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