Datasheet

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UCD9222

www.ti.com

SLVSAL7A –NOVEMBER 2010–REVISED FEBRUARY 2011

Digital Compensator

Each voltage rail controller in the UCD9222 includes a digital compensator. The compensator consists of a

nonlinear gain stage, followed by a digital filter consisting of a second order infinite impulse response (IIR) filter

section cascaded with a first order IIR filter section.

The Texas Instruments Fusion Digital Power™ Designer development tool can be used to assist in defining the

compensator coefficients. The design tool allows the compensator to be described in terms of the pole

frequencies, zero frequencies and gain desired for the control loop. In addition, the Fusion Digital Power™

Designer can be used to characterize the power stage so that the compensator coefficients can be chosen based

on the total loop gain for each feedback system. The coefficients of the filter sections are generated through

modeling the power stage and load.

Additionally, the UCD9222 has three banks of filter coefficients: Bank-0 is used during the soft start/stop ramp or

tracking; Bank-1 is used while in regulation mode; and Bank-2 is used when the measured output current is

below the configured light load threshold.

Figure 13. Digital Compensator

To calculate the values of the digital compensation filter continuous-time design parameters K

, F

ands Q

are

entered into the Fusion Digital Power Designer software (or it calculates them automatically). Where the

compensating filter transfer function is

(5)

There are approximate limits the design parameters K

, F

ands Q

. Though design parameters beyond these

upper a lower bounds can be used to calculate the discrete-time filter coefficients, there will be significant

round-off error when the continuous-time floating-point design parameters are converted to the discrete-time

fixed-point integer coefficients to be downloaded to the controller.

Product Folder Link(s) :UCD9222