PIC18F4520ESLASHMLQTP datasheet(158/390 Pages) MICROCHIP

158 / 390 page

PIC18F2420/2520/4420/4520

DS39631A-page 156

Preliminary

 2004 Microchip Technology Inc.

16.4.6

PROGRAMMABLE DEAD-BAND

DELAY

In half-bridge applications where all power switches are

modulated at the PWM frequency at all times, 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) may flow through both power

switches, shorting the bridge supply. To avoid this

potentially destructive shoot-through current from flow-

ing during switching, turning on either of the power

switches is normally delayed to allow the other switch

to completely turn off.

In the Half-Bridge Output mode, a digitally programma-

ble 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 nonactive

state to the active state. See Figure 16-4 for illustration.

Bits

PDC6:PDC0

the

PWM1CON

(Register 16-2) set the delay period in terms of micro-

controller instruction cycles (TCY or 4 TOSC). These bits

are not available on 28-pin devices as the standard

CCP module does not support half-bridge operation.

16.4.7

ENHANCED PWM AUTO-SHUTDOWN

When the CCP1 is programmed for any of the enhanced

PWM modes, the active output pins may be configured

for auto-shutdown. Auto-shutdown immediately places

the enhanced PWM output pins into a defined shutdown

state when a shutdown event occurs.

A shutdown event can be caused by either of the

comparator modules, a low level on the Fault input pin

(FLT0) or any combination of these three sources. The

comparators may be used to monitor a voltage input

proportional to a current being monitored in the bridge

circuit. If the voltage exceeds a threshold, the

comparator switches state and triggers a shutdown.

Alternatively, a low digital signal on FLT0 can also trigger

a shutdown. The auto-shutdown feature can be disabled

by not selecting any auto-shutdown sources. The auto-

shutdown sources to be used are selected using the

ECCPAS2:ECCPAS0 bits (bits<6:4> of the ECCP1AS

When a shutdown occurs, the output pins are asyn-

chronously placed in their shutdown states, specified

by the PSSAC1:PSSAC0 and PSSBD1:PSSBD0 bits

(ECCPAS2:ECCPAS0). Each pin pair (P1A/P1C and

P1B/P1D) may be set to drive high, drive low or be tri-

stated (not driving). The ECCPASE bit (ECCP1AS<7>)

is also set to hold the enhanced PWM outputs in their

shutdown states.

The ECCPASE bit is set by hardware when a shutdown

event occurs. If automatic restarts are not enabled, the

ECCPASE bit is cleared by firmware when the cause of

the shutdown clears. If automatic restarts are enabled,

the ECCPASE bit is automatically cleared when the

cause of the auto-shutdown has cleared.

If the ECCPASE bit is set when a PWM period begins,

the PWM outputs remain in their shutdown state for that

entire PWM period. When the ECCPASE bit is cleared,

the PWM outputs will return to normal operation at the

beginning of the next PWM period.

PWM1CON: PWM CONFIGURATION REGISTER

Note:

Programmable dead-band delay is not

implemented

28-pin

devices

with

standard CCP modules.

Note:

Writing to the ECCPASE bit is disabled

while a shutdown condition is active.

R/W-0

PRSEN

PDC6(1)

PDC5(1)

PDC4(1)

PDC3(1)

PDC2(1)

PDC1(1)

PDC0(1)

bit 7

bit 0

bit 7

PRSEN: PWM Restart Enable bit

= Upon auto-shutdown, the ECCPASE bit clears automatically once the shutdown event

goes away; the PWM restarts automatically

= Upon auto-shutdown, ECCPASE must be cleared in software to restart the PWM

bit 6-0

PDC6:PDC0: PWM Delay Count bits(1)

Delay time, in number of FOSC/4 (4 * TOSC) cycles, between the scheduled and actual time for

a PWM signal to transition to active.

Note 1: Reserved on 28-pin devices; maintain these bits clear.

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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