PIC16F631/677/685/687/689/690
11.4.6
PROGRAMMABLE DEAD-BAND
DELAY MODE
FIGURE 11-16:
EXAMPLE OF
HALF-BRIDGE PWM
OUTPUT
In Half-Bridge applications where all power switches
are modulated at the PWM frequency, the power
switches normally require more time to turn off than to
turn on. If both the upper and lower power switches are
switched at the same time (one turned on, and the
other turned off), both switches may be on for a short
period of time until one switch completely turns off.
Period
Period
Pulse Width
(2)
(2)
P1A
td
td
During this brief interval,
a very high current
P1B
(shoot-through current) will flow through both power
switches, shorting the bridge supply. To avoid this
potentially destructive shoot-through current from
flowing during switching, turning on either of the power
switches is normally delayed to allow the other switch
to completely turn off.
(1)
(1)
(1)
td = Dead-Band Delay
Note 1: At this time, the TMR2 register is equal to the
PR2 register.
In Half-Bridge mode,
a
digitally programmable
2: Output signals are shown as active-high.
dead-band delay is available to avoid shoot-through
current from destroying the bridge power switches. The
delay occurs at the signal transition from the non-active
state to the active state. See Figure 11-8 for illustration.
The lower seven bits of the associated PWM1CON
register (Register 11-3) sets the delay period in terms
of microcontroller instruction cycles (TCY or 4 TOSC).
FIGURE 11-17:
EXAMPLE OF HALF-BRIDGE APPLICATIONS
V+
Standard Half-Bridge Circuit (“Push-Pull”)
FET
Driver
+
V
-
P1A
Load
FET
Driver
+
V
-
P1B
V-
© 2007 Microchip Technology Inc.
DS41262D-page 143
MICROCHIP [ MICROCHIP ]
