´

=

L1

R1 C1

DCR

R1

C1

L

DCR

+

–

CS1–

CS1+

L1

UDG-11112

TPS40322

www.ti.com

SLUSAF8

– JUNE 2011

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.

Current Sensing

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.

(2)

Figure 16. Inductor DCR Current Sensing

Copyright

© 2011, Texas Instruments Incorporated

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