SPRUHM9H October 2014 – May 2024 TMS320F28075 , TMS320F28075-Q1 , TMS320F28076
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 1.25MHz. In conventional PWM generation with a system clock of 100MHz, the duty cycle choices are in the vicinity of 40.5%. As shown in Figure 14-85, 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 320ns instead of the desired 324ns. This data is shown in Table 14-15.
By utilizing the MEP, an edge position much closer to the desired point of 324ns can be achieved. Table 14-15 shows that in addition to the CMPA value, 22 steps of the MEP (CMPAHR register) positions the edge at 323.96ns, resulting in almost zero error. In this example, assume that the MEP has a step resolution of 180ps.
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 |