Datasheet

PIC16F716
DS41206A-page 44 Preliminary  2003 Microchip Technology Inc.
Figure 7-14 shows an example where the PWM
direction changes from forward to reverse, at a near
100% duty cycle. At time t1, the output P1A and P1D
become inactive, while output P1C becomes active. In
this example, since the turn-off time of the power
devices is longer than the turn-on time, a shoot-through
current may flow through power devices QC and QD
(see Figure 7-12) for the duration of ‘t’. The same
phenomenon will occur to power devices QA and QB
for PWM direction change from reverse to forward.
If changing PWM direction at high duty cycle is required
for an application, one of the following requirements
must be met:
1. Reduce PWM for a PWM period before changing
directions.
2. Use switch drivers that can drive the switches off
faster than they can drive them on.
Other options to prevent shoot-through current may
exist.
FIGURE 7-13: PWM DIRECTION CHANGE
FIGURE 7-14: PWM DIRECTION CHANGE AT NEAR 100% DUTY CYCLE
DC
Period
(1)
SIGNAL
Note 1: The direction bit in the CCP1 Control Register (CCP1CON<7>) is written any time during the PWM cycle.
2: When changing directions, the P1A and P1C signals switch before the end of the current PWM cycle at intervals of
4 T
OSC, 16 TOSC or 64 TOSC, depending on the Timer2 prescaler value. The modulated P1B and P1D signals are
inactive at this time.
Period
(Note 2)
P1A (Active-High)
P1B (Active-High)
P1C (Active-High)
P1D (Active-High)
DC
Forward Period Reverse Period
P1A
t
on
t
off
t = t
off
- t
on
P1B
P1C
P1D
External Switch D
Potential
Shoot-Through
Current
Note 1: All signals are shown as active-high.
2: t
on
is the turn-on delay of power switch QC and its driver.
3: t
off
is the turn-off delay of power switch QD and its driver.
External Switch C
t1
DC
DC