Datasheet

2 1 2
EA O UT OFFSET
1 2 1 2
1 2 1 2
EA E A
R R R
V = V + I
R R R R
R + R + R + R +
R R
æ ö æ ö
ç ÷ ç ÷
è ø è ø
2
SW P
1
C =
2 0.35 F R´ ´ ´p
EA
OUT
V
K = VOUT_SCALE_LOOP
V
@
UCD9248
www.ti.com SLVSA33A –JANUARY 2010–REVISED AUGUST 2012
Where
(3)
It is recommended that a capacitor be placed across the lower resistor of the divider network. This acts as an
additional pole in the compensation and as an anti-alias filter for the EADC. To be effective as an anti-alias filter,
the corner frequency should be 35% to 40% of the switching frequency. Then the capacitor is calculated as:
(4)
To obtain the best possible accuracy, the input resistance and offset current on the device should be considered
when calculating the gain of a voltage divider between the output voltage and the EA sense inputs of the
UCD9224. The input resistance and input offset current are specified in the parametric tables in this datasheet.
V
EA
= V
EAP
– V
EAN
in the equation below.
(5)
The effect of the offset current can be reduced by making the resistance of the divider network low.
Digital Compensator
Each voltage rail controller in the UCD9248 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 UCD9248 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.
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