PIC16C770-04SLASHSS datasheet(65/200 Pages) MICROCHIP

65 / 200 page

PIC16C717/770/771

Advanced Information

DS41120A-page 65

Note that in the Full-Bridge output mode, the ECCP

module does not provide any deadband delay. In gen-

eral, since only one output is modulated at all time,

deadband delay is not required. However, there is a sit-

uation where a deadband delay might be required. This

situation occurs when all of the following conditions are

true:

The direction of the PWM output changes when

the duty cycle of the output is at or near 100%.

The turn off time of the power switch, including

the power device and driver circuit, is greater

than turn on time.

Figure 8-11 shows an example, where the PWM direc-

tion 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

flows through the power devices, QB and QD, for the

duration of t= toff-ton. The same phenomenon will occur

to power devices, QC and QB, for PWM direction

change from reverse to forward.

If changing PWM direction at high duty cycle is required

for the user’s application, one of the following require-

ments must be met:

Avoid changing PWM output direction at or near

100% duty cycle.

Use switch drivers that compensate the slow

turn off of the power devices. The total turn off

time (toff) of the power device and the driver

must be less than the turn on time (ton).

FIGURE 8-11: PWM DIRECTION CHANGE AT NEAR 100% DUTY CYCLE

FORWARD PERIOD

REVERSE PERIOD

(PWM)

P1A 1

(PWM)

ton

toff

t = toff - ton

P1B

P1C

P1D

External Switch D

Potential

Shoot Through

Current

Note 1: All signals are shown as active high.

2: ton is the turn on delay of power switch and driver.

3: toff is the turn off delay of power switch and driver.

External Switch C

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