Datasheet

 2010 Microchip Technology Inc. Preliminary DS41412C-page 197
PIC18(L)F2X/4XK22
14.4.5 PROGRAMMABLE DEAD-BAND
DELAY MODE
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.
During this brief interval, a very high current (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.
In Half-Bridge mode, a digitally programmable 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 14-16 for illustration.
The lower seven bits of the associated PWMxCON
register (Register 14-6) sets the delay period in terms
of microcontroller instruction cycles (T
CY or 4 TOSC).
FIGURE 14-16: EXAMPLE OF HALF-
BRIDGE PWM OUTPUT
FIGURE 14-17: EXAMPLE OF HALF-BRIDGE APPLICATIONS
Period
Pulse Width
td
td
(1)
PxA
(2)
PxB
(2)
td = Dead-Band Delay
Period
(1) (1)
Note 1: At this time, the TMRx register is equal to the
PRx register.
2: Output signals are shown as active-high.
PxA
PxB
FET
Driver
FET
Driver
V+
V-
Load
+
V
-
+
V
-
Standard Half-Bridge Circuit (“Push-Pull”)