PIC16F716 datasheet(46/126 Pages) MICROCHIP

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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:

Reduce PWM for a PWM period before changing

directions.

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

Period(1)

SIGNAL

Note

The direction bit in the CCP1 Control Register (CCP1CON<7>) is written any time during the PWM cycle.

When changing directions, the P1A and P1C signals switch before the end of the current PWM cycle at intervals of

4 TOSC, 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)

Forward Period

Reverse Period

P1A

ton

toff

t = toff - ton

P1B

P1C

P1D

External Switch D

Potential

Shoot-Through

Current

Note

All signals are shown as active-high.

ton is the turn-on delay of power switch QC and its driver.

toff is the turn-off delay of power switch QD and its driver.

External Switch C

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