PIC18LF24K Datasheet, PDF (298/594 Pages) –

PIC18(L)F26/45/46K40
24.2.3.1 Direction Change in Full-Bridge
Mode
In Full-Bridge mode, changing MODE<2:0> controls
the forward/reverse direction. Changes to MODE<2:0>
change to the new direction on the next rising edge of
the modulated input.
A direction change is initiated in software by changing
the MODE<2:0> bits of the CWG1CON0 register. The
sequence is illustrated in Figure 24-8.
• The associated active output CWG1A and the
inactive output CWG1C are switched to drive in
the opposite direction.
• The previously modulated output CWG1D is
switched to the inactive state, and the previously
inactive output CWG1B begins to modulate.
• CWG modulation resumes after the direction-
switch dead band has elapsed.
24.2.3.2 Dead-Band Delay in Full-Bridge
Mode
Dead-band delay is important when either of the
following conditions is true:
1. The direction of the CWG output changes when
the duty cycle of the data input is at or near
100%, or
2. The turn-off time of the power switch, including
the power device and driver circuit, is greater
than the turn-on time.
The dead-band delay is inserted only when changing
directions, and only the modulated output is affected.
The statically-configured outputs (CWG1A and
CWG1C) are not afforded dead band, and switch
essentially simultaneously.
Figure 24-8 shows an example of the CWG outputs
changing directions from forward to reverse, at near
100% duty cycle. In this example, at time t1, the output
of CWG1A and CWG1D become inactive, while output
CWG1C becomes active. Since the turn-off time of the
power devices is longer than the turn-on time, a shoot-
through current will flow through power devices QC and
QD for the duration of ‘t’. The same phenomenon will
occur to power devices QA and QB for the CWG
direction change from reverse to forward.
When changing the CWG direction at high duty cycle is
required for an application, two possible solutions for
eliminating the shoot-through current are:
1. Reduce the CWG duty cycle for one period
before changing directions.
2. Use switch drivers that can drive the switches off
faster than they can drive them on.
FIGURE 24-8:
EXAMPLE OF PWM DIRECTION CHANGE AT NEAR 100% DUTY CYCLE
t1
Forward Period
Reverse Period
CWG1A
CWG1B
CWG1C
CWG1D
External Switch C
External Switch D
Potential Shoot-
Through Current
Pulse Width
Pulse Width
T_ON
T_OFF
T = T_OFF - T_ON
 2016 Microchip Technology Inc.
Preliminary
DS40001816C-page 298