SPRUJ17H March 2022 – October 2024 AM2631 , AM2631-Q1 , AM2632 , AM2632-Q1 , AM2634 , AM2634-Q1
In a typical power control loop, a digital controller issues a duty command, usually expressed in a per unit or percentage terms. Assume that for a particular operating point, the demanded duty cycle is 0.405 or 40.5% on time and the required converter PWM frequency is 1.25 MHz. In conventional PWM generation with a system clock of 100 MHz, the duty cycle choices are in the vicinity of 40.5%. As shown in Figure 7-239, a compare value of 32 counts (duty = 40%) is the closest to 40.5% that can be attained. This is equivalent to an edge position of 320 ns instead of the desired 324 ns. This data is shown in Table 7-168.
By utilizing the MEP, an edge position much closer to the desired point of 324 ns can be achieved. Table 7-168 shows that in addition to the CMPA value, 22 steps of the MEP (CMPAHR register) positions the edge at 323.96 ns, resulting in almost zero error. In this example, it is assumed that the MEP has a step resolution of 180 ps.
CMPA (count)(1)(2)(3) |
Duty (%) |
High Time (ns) |
CMPA (count) |
CMPAHR (count) |
Duty (%) |
High Time (ns) |
---|---|---|---|---|---|---|
28 | 35.0 | 280 | 32 | 18 | 40.405 | 323.24 |
29 | 36.3 | 290 | 32 | 19 | 40.428 | 323.42 |
30 | 37.5 | 300 | 32 | 20 | 40.450 | 323.60 |
31 | 38.8 | 310 | 32 | 21 | 40.473 | 323.78 |
32 | 40.0 | 320 | 32 | 22 | 40.495 | 323.96 |
33 | 41.3 | 330 | 32 | 23 | 40.518 | 324.14 |
34 | 42.5 | 340 | 32 | 24 | 40.540 | 324.32 |
32 | 25 | 40.563 | 324.50 | |||
Required | 32 | 26 | 40.585 | 324.68 | ||
32.40 | 40.5 | 324 | 32 | 27 | 40.608 | 324.86 |