Filter Control Register
151
SNIU028A – February 2016 – Revised April 2016
Copyright © 2016, Texas Instruments Incorporated
Filter
4.3.10 KComp as Output Multiplier
Using KComp as an output multiplier can be useful in places where the DPWM output is limited to less
than 100%, for example. This can permit using the whole dynamic range of the Filter for a more limited
range, and remove the need for output clamping. The scaling works exactly the same, except the 14 bit
KCOMP value is used instead of the 14 bit period value.
This option also makes it possible to change the gain of the Filter dynamically without change pole and
zero positions. It can also be used to increase the gain of the Filter beyond what the normal Filter
structure supports. It is also used to match up the PWM and Resonant mode waveforms in the LLC
configuration.
There are registers in the Loop Mux which contain KCOMP values and select which of 3 KCOMP values
is used for each filter.
Here is an example of a code which would provide a maximum output of 50%:
#define SWITCHING_PERIOD 2500 // 250 MHz clock divided by period of 100 KHZ
Dpwm0Regs.DPWMPRD.bit.PRD = SWITCHING_PERIOD;
LoopMuxRegs.FILTERKCOMPA.bit.KCOMP0 = SWITCHING_PERIOD >> 1;
//KCOMP = period/2
LoopMuxRegs.FILTERMUX.bit.FILTER0_KCOMP_SEL = 0;
//select KCOMP0 for filter 0
Filter1Regs.FILTERCTRL.bit.OUTPUT_MULT_SEL = 0;
//Multiply output by KCOMP
This approach yields 1 more bit of filter resolution than clamping the Filter output at 50%, which should
reduce the size of any limit cycling.
4.3.11 Feed Forward as Output Multiplier
When the feed forward is selected as the output multiplier, the multiplier value is the most significant 14
bits of the filter output from a different filter. The FILTERMUX register in the Loop Mux is also used to
select this filter. Configuring the link between the two filters is simple and is shown below. The actual filter
settings and operation for feed forward are available in the Hard Switching Full Bridge EVM
(UCD3138HSFBEVM-029) reference code from Texas Instruments.
Here is an example of link code:
LoopMuxRegs.FILTERMUX.bit.FILTER0_FFWD_SEL = 0; //select Filter1 as ffwd input
LoopMuxRegs.FILTERMUX.bit.FILTER0_FFWD_SEL = 0;
//select feed forward as output multiplier
See
in
for more information on these bit fields.
4.3.12 Period Multiplier Select
The Filter has a second output. In addition to Filter Duty, there is Filter Period.
The PERIOD_MULT_SEL bit is used to select the multiplier for Filter Period.
The only options are DPWM Period and KCOMP. Filter Period is used in LLC topologies so that the Filter
can control the switching frequency of the power supply
4.3.13 Ki Adder Mode
The KI_ADDER_MODE bit selects from two modes for calculating the input to Ki.
A 1 in KI_ADDER_MODE will select a trapezoidal mode which adds Xn and Xn-1. At the lowest AFE gain
setting, this may lead to saturation of the calculation.
With a 0 in the KI_ADDER_MODE bit, only Xn is used as an input to the integrator. This does not provide
trapezoidal calculations, but it does avoid any saturation issues. For a more complete description see
4.1.3 Integral Branch.