Pwm Global Period Register (Pwmglbprd); Sync Control (Syncctrl); 5.10 Light Load (Burst) Mode; 5.11 Constant Current / Constant Power - Texas Instruments UCD3138 Technical Reference Manual

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5.8

PWM Global Period Register (PWMGLBPRD)

If the Global Period Enable (GLOBAL_PERIOD_EN) bit is set in DPWMCTRL1 of a DPWM module, it will
use the value in PWMGLBPRD for its period. This can be used to change the periods of multiple DPWMs
with one C statement. This is useful for frequency dithering. Note that the period change takes effect at
the end of the previous period. If DPWMs are out of phase, the frequency change will take place at a
different time for each DPWM.
Also note that the if the Filter is using the Period from the DPWM for calculations, it will still use the
DPWM Period Register even if the Global Period is enabled. So to use the Global Period, it is necessary
to use the KCOMP register as a multiplier and to change both the Global Period Register and the KCOMP
register at the same time. In fact, a careful sequence should be followed:
If the period is increasing – first change the Global Period Register, then wait 1 period, then change the
KCOMP. This sequence guards against dead time violations.
For the same reason, when decreasing the period, change the KCOMP first.
For any frequency change, the order of changes should be carefully designed based on the actual
topology and IC configuration.
5.9

Sync Control (SYNCCTRL)

SYNCTRL controls the Sync pin. The Sync pin can be an input or an output. It can output the Sync pulse
from any DPWM, or it can be used as a general purpose output. It can also output some internal
processor clocks for debugging purposes.
As an input, it can be used as a sync input to the DPWMs, and as a general purpose input. To use it as a
sync input to the DPWMs, set the EXT_SYNC_EN bit in DPWMCTRL1.

5.10 Light Load (Burst) Mode

There are several registers in the Loop Mux related to Burst mode/Light Load Mode. This mode works
based on the output of a selected filter. In its simplest form, it disables DPWM output pulses when the
filter output goes below a threshold – LoopMuxRegs.LLDISTHRESH. It reenables them when the filter
goes above another threshold – LoopMuxRegs.LLENTHRESH.bit.TURN_ON_THRESH. There are bits in
the LLCTRL register to enable Light Load Mode, and to select which filter output is used to drive the Light
Load control.
To enable the single Light Load Module to control a DPWM module, the BURST_EN bit in DPWMCTRL1
must be set.
In normal mode, if PWM pulses get very short, they will stop altogether. The output voltage will drop by a
small amount, and the filter output will rise again, and it will start up again. There are added features
which permit output of fixed size pulses in burst mode. This function is very application specific. Consult
the reference firmware code provided with the UCD3138 EVM for the desired topology for further
information.
It may be necessary to adjust the thresholds as a function of Vin, for example.

5.11 Constant Current / Constant Power

Constant Current/Constant Power is very dependent on topology for configuration and utilization. Consult
the appropriate EVM firmware for the appropriate Constant Current/Constant Power setup.
SNIU028A – February 2016 – Revised April 2016
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PWM Global Period Register (PWMGLBPRD)
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Loop Mux