Microchip Technology AC164337 Data Sheet
2006-2014 Microchip Technology Inc.
DS70000178D-page 123
dsPIC30F1010/202X
12.4.7
CONSTANT OFF-TIME PWM
Constant Off-Time mode is shown in Figure 12-9.
Constant Off-Time PWM is a variable-frequency mode
where the actual PWM period is less than or equal to
the specified period value. The PWM time base is
externally reset some time after the PWM signal duty
cycle value has been reached, and the PWM signal has
been deasserted. This mode is implemented by
enabling the On-Time PWM mode (Current Reset
mode) and using the complementary output.
Constant Off-Time PWM is a variable-frequency mode
where the actual PWM period is less than or equal to
the specified period value. The PWM time base is
externally reset some time after the PWM signal duty
cycle value has been reached, and the PWM signal has
been deasserted. This mode is implemented by
enabling the On-Time PWM mode (Current Reset
mode) and using the complementary output.
FIGURE 12-9:
CONSTANT OFF-TIME
PWM
PWM
12.4.8
CURRENT RESET PWM MODE
Current Reset PWM is shown in Figure 12-10. Current
Reset PWM uses a Variable-Frequency mode where
the actual PWM period is less than or equal to the spec-
ified period value. The PWM time base is externally
reset some time after the PWM signal duty cycle value
has been reached and the PWM signal has been deas-
serted. Current Reset PWM is a constant on-time PWM
mode.
Reset PWM uses a Variable-Frequency mode where
the actual PWM period is less than or equal to the spec-
ified period value. The PWM time base is externally
reset some time after the PWM signal duty cycle value
has been reached and the PWM signal has been deas-
serted. Current Reset PWM is a constant on-time PWM
mode.
FIGURE 12-10:
CURRENT RESET PWM
Typically, in the converter application, an energy stor-
age inductor is charged with current while the PWM
signal is asserted, and the inductor current is dis-
charged by the load when the PWM signal is deas-
serted. In this application of current reset PWM, an
external current measurement circuit determines when
the inductor is discharged, and then generates a signal
that the PWM module uses to reset the time base
counter. In Current Reset mode, complementary
outputs are available.
age inductor is charged with current while the PWM
signal is asserted, and the inductor current is dis-
charged by the load when the PWM signal is deas-
serted. In this application of current reset PWM, an
external current measurement circuit determines when
the inductor is discharged, and then generates a signal
that the PWM module uses to reset the time base
counter. In Current Reset mode, complementary
outputs are available.
12.4.9
INDEPENDENT TIME BASE PWM
Independent Time Base PWM, as shown in
Figure 12-11, is often used when the dsPIC DSC is
controlling different power converter subcircuits such
as the Power Factor Correction circuit, which may use
100 kHz PWM, and the full-bridge forward converter
section may use 250 kHz PWM.
Figure 12-11, is often used when the dsPIC DSC is
controlling different power converter subcircuits such
as the Power Factor Correction circuit, which may use
100 kHz PWM, and the full-bridge forward converter
section may use 250 kHz PWM.
FIGURE 12-11:
INDEPENDENT TIME
BASE PWM
BASE PWM
Duty Cycle
0
Period
Timer
Value
Value
Programmed Period
PWML
Value
External Timer Reset
Duty Cycle
Actual Period
External Timer Reset
Note: Duty Cycle represents off time
Duty Cycle
0
Period
Timer
Value
Value
Programmed Period
PWMH
Value
External Timer Reset
Duty Cycle
Actual Period
External Timer Reset
Programmed Period
Duty Cycle
PWM2H
Duty Cycle
PWM4H
Duty Cycle
PWM3H
Duty Cycle
PWM1H
Period 4
Period 2
Period 3
Period 1
Note:
With independent time bases,
PWM signals are no longer
phase related to each other.
PWM signals are no longer
phase related to each other.