PIC18F258-ESLASHSOSQTP datasheet(142/402 Pages) MICROCHIP

142 / 402 page

PIC18FXX8

DS41159E-page 140

16.5.4

PROGRAMMABLE DEAD-BAND

DELAY

In half-bridge or full-bridge applications, where all

power switches are modulated at the PWM frequency

at all times, the power switches normally require longer

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

will be on for a short period of time until one switch

completely turns off. During this time, a very high

current (

shoot-through current) flows through both

power switches, shorting the bridge supply. To avoid

this potentially destructive shoot-through current from

flowing during switching, turning on the power switch is

normally delayed to allow the other switch to

completely turn off.

In the Half-Bridge Output 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 16-3 for illustration.

The ECCP1DEL register (Register 16-2) sets the amount

of delay.

16.5.5

SYSTEM IMPLEMENTATION

When the ECCP module is used in the PWM mode, the

application hardware must use the proper external pull-

up and/or pull-down resistors on the PWM output pins.

When the microcontroller powers up, all of the I/O pins

are in the high-impedance state. The external pull-up

and pull-down resistors must keep the power switch

devices in the off state until the microcontroller drives

the I/O pins with the proper signal levels, or activates

the PWM output(s).

16.5.6

START-UP CONSIDERATIONS

Prior to enabling the PWM outputs, the P1A, P1B, P1C

and P1D latches may not be in the proper states.

Enabling the TRISD bits for output at the same time

with the ECCP1 module may cause damage to the

power switch devices. The ECCP1 module must be

enabled in the proper output mode with the TRISD bits

enabled as inputs. Once the ECCP1 completes a full

PWM cycle, the P1A, P1B, P1C and P1D output

latches are properly initialized. At this time, the TRISD

bits can be enabled for outputs to start driving the

power switch devices. The completion of a full PWM

cycle is indicated by the TMR2IF bit going from a ‘0’ to

a ‘1’.

16.5.7

OUTPUT POLARITY

CONFIGURATION

The ECCP1M<1:0> bits in the ECCP1CON register

allow user to choose the logic conventions (asserted

high/low) for each of the outputs.

The PWM output polarities must be selected before the

PWM outputs are enabled. Charging the polarity

configuration while the PWM outputs are active is not

recommended since it may result in unpredictable

operation.

ECCP1DEL: PWM DELAY REGISTER

R/W-0

EPDC7

EPDC6

EPDC5

EPDC4

EPDC3

EPDC2

EPDC1

EPDC0

bit 7

bit 0

bit 7-0

EPDC<7:0>: PWM Delay Count for Half-Bridge Output Mode bits

Number of FOSC/4 (TOSC * 4) cycles between the P1A transition and the P1B transition.

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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