SLUSAF8D – JUNE 2011 – REVISED JANUARY 2014
Input Voltage Feed-Forward
The TPS40322 uses input voltage feed-forward to maintain a constant power stage gain as the input voltage
varies and provides very good response to input voltage transient disturbances. The simple constant power
stage gain of the controller greatly simplifies feedback loop design because the loop characteristics remain
constant as the input voltage changes, unlike a typical 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 typically 8.5 V/V.
The TPS40322 uses a differential current sense design 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 shown in Figure 16, an R-C filter must be used to remove the large AC
component voltage across the inductor so that only the component of the voltage that remains is across the
resistance of the inductor. (See Figure 16)
The values of R1 and C1 for an ideal design can be calculated using Equation 2. The time constant of the R-C
filter should equal the time constant of the inductor itself. If the time constants are equal, the voltage across C1
equals the current in the inductor multiplied by the inductor resistance. The inductor ripple current is reflected in
the voltage across C1. Typically a capacitor with a value of 0.1-µF is recommended for C1.
Please refer to the LAYOUT CONSIDERATIONS section for proper placement of the sensing elements.
Figure 16. Inductor DCR Current Sensing
Copyright © 2011–2014, Texas Instruments Incorporated
Submit Documentation Feedback