dsPIC30F Family Reference Manual
DS70062C-page 15-28 © 2004 Microchip Technology Inc.
Table 15-4 shows example dead time ranges as a function of the input clock prescaler selected
and the device operating frequency.
Table 15-4: Example Dead Time Ranges
15.6.4 Dead Time Distortion
For small PWM duty cycles, the ratio of dead time to the active PWM time may become large.
At the extreme case, when the duty cycle is less than or equal to the programmed duty cycle, no
PWM pulse will be generated. In these cases, the inserted dead time will introduce distortion
into waveforms produced by the PWM module. The user can ensure that dead time distortion is
minimized by keeping the PWM duty cycle at least three times larger than the dead time. Dead
time distortion can also be corrected by other techniques, such as closed loop current control.
A similar effect occurs for duty cycles near 100%. The maximum duty cycle used in the
application should be chosen such that the minimum inactive time of the PWM signal is at least
three times larger than the dead time.
15.7 Independent PWM Output Mode
An Independent PWM Output mode is useful for driving loads such as the one shown in
Figure 15-15. A particular PWM output pair is in the Independent Output mode when the
corresponding PMOD bit in the PWMCON1 register is set. The dead time generators are
disabled in the Independent mode and there are no restrictions on the state of the pins for a
given output pin pair.
Figure 15-15: Asymmetric Inverter
Figure 15-16: PWM Block Diagram for One Output Pin Pair, Independent Mode
T
CY (FCY) Prescaler Selection Resolution Dead Time Range
33 ns (30 MHz) 4 T
CY 130 ns 130 µs-9 usec
50 ns (20 MHz) 4 T
CY 200 ns 200 µs-12 usec
100 ns (10 MHz) 2 T
CY 200 ns 200 µs-12 usec
+V
1H
1L
PWM Generator
PWMxH
PWMxL
Override
and
Fault Logic
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