PIC16F684-I/SLG Microchip, PIC16F684-I/SLG Datasheet - Page 85

• Manufacturer Part Number: PIC16F684-I/SLG
• Description: pic, flash, Microcontrollers, Microprocessors, Semiconductors and Actives, ic, mcu
• Manufacturer: Microchip

FIGURE 11-9:
11.3.5.1
In the Full-bridge Output mode, the P1M1 bit
(CCP1CON<7>)
Forward/Reverse direction. When the application firm-
ware changes this direction control bit, the module will
assume the new direction on the next PWM cycle.
Just before the end of the current PWM period, the
modulated outputs (P1B and P1D) are placed in their
inactive state, while the unmodulated outputs (P1A and
P1C) are switched to drive in the opposite direction. This
occurs in a time interval of (4 T
value)) before the next PWM period begins. The Timer2
prescaler will be either 1, 4 or 16, depending on the
value of the T2CKPS<1:0> bits (T2CON<1:0>). During
the interval from the switch of the unmodulated outputs
to the beginning of the next period, the modulated out-
puts (P1B and P1D) remain inactive. This relationship is
shown in Figure 11-10.
Note that in the Full-bridge Output mode, the ECCP
module does not provide any dead band delay. In
general, since only one output is modulated at all times,
dead band delay is not required. However, there is a
situation where a dead band delay might be required.
This situation occurs when both of the following
conditions are true:
1.
2.
 2004 Microchip Technology Inc.
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 the turn on time.
PIC16F684
Direction Change in Full-Bridge
Mode
allows
P1A
P1B
P1C
P1D
EXAMPLE OF FULL-BRIDGE APPLICATION
user
OSC
to
*(Timer2 Prescale
control
FET
Driver
FET
Driver
Preliminary
the
QA
QB
Figure 11-11 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 11-9) 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.
2.
Other options to prevent shoot-through current may
exist.
Reduce PWM duty cycle for one PWM period
before changing directions.
Use switch drivers that can drive the switches off
faster than they can drive them on.
Load
V+
V-
QC
QD
PIC16F684
FET
Driver
FET
Driver
DS41202C-page 83