14
Bits [17:4]
Filter Period
18
24
15
38
18
KCompx
DPWMx Period
Loop_VFF
Filter YN (Duty %)
Filter Duty
S0.23
14.0
14.0
14.0
14.0
S14.23
Resonant Duty
14.0
Round to
18 bits,
Clamp to
Positive
Clamp
Filter Output
Clamp High
Filter Output
Clamp Low
X
14.4
24
15
38
18
KCompx
DPWMx Period
Filter YN
S0.23
14.0
14.0
14.0
S14.23
Round to
18 bits,
Clamp to
Positive
Truncate
low 4 bits
X
14.0
14.4
PERIOD_MULT_SEL
14.4
OUTPUT_MULT_SEL
Filter Math Details
147
SNIU028A – February 2016 – Revised April 2016
Copyright © 2016, Texas Instruments Incorporated
Filter
4.1.6 Filter Output Stage
Here is the final stage of the filter:
Figure 4-3.
This stage converts the filter output to match the input requirements of the DPWM. Two different
calculations are performed sequentially, using the same multiplier, but with different settings. One
calculation provides a DPWM duty value, and the other provides a DPWM period value. In many
topologies, only the duty value is used. In LLC, both values are used.
At the start of this stage, the PID output is multiplied by one of several 14 bit unsigned numbers, giving a
38 bit output. For the Filter Duty calculation, there are 4 numbers which can be used. See
for a discussion of these numbers. For the Filter Period calculation, only 2 numbers can be selected.
After the multiplication, there is a 38 bit signed result. Negative values are clamped to zero and the sign
bit is removed. This gives a 37 bit positive result. This number is rounded to the 18 most significant bits.
This section is the same for both calculations.
After rounding and clamping, the two outputs are handled differently. The Filter Duty value is clamped
using the values in the Filter Output Clamp High Register (FILTEROCLPHI) and the Filter Output Clamp
Low Register (FILTEROCLPLO). After that, the Filter Duty value is used by the DPWM as a time value. It
is considered a 14.4 bit value, with the low 4 bits going to the high resolution section of the DPWM. So the
resolution of the Filter Duty value is 250 psec.
The Filter Period value gets no clamp. The DPWM Period does not support high resolution, so the low 4
bits from the 18 bit result are truncated. The Filter Period only presents the high 14 bits to the DPWM,
giving a resolution of a nominal 4 nanoseconds.