ADMC328YN datasheet(14/32 Pages) AD

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ADMC328

–14–

REV. B

PWMCHA

PWMTM

PWMDT

PWMTM

PWMCHA

PWMTM

PWCHA

PWMDT

PWMTM

()

−

()

–

because for the completely general case in double update mode,

the switching period is given by:

TS = (PWMTM1 + PWMTM2) × tCK

Again, the values of TAH and TAL are constrained to lie between

zero and TS.

PWM signals similar to those illustrated in Figure 7 and Figure

8 can be produced on the BH, BL, CH, and CL outputs by pro-

gramming the PWMCHB and PWMCHC registers in a manner

identical to that described for PWMCHA.

The PWM controller does not produce any PWM outputs until

all of the PWMTM, PWMCHA, PWMCHB, and PWMCHC

registers have been written to at least once. After these registers

have been written, the counters in the three-phase timing unit

are enabled. Writing to these registers also starts the main PWM

timer. If during initialization, the PWMTM register is written

after the PWMCHA, PWMCHB, and PWMCHC registers,

then the first PWMSYNC pulse (and interrupt if enabled) will

be generated (1.5

× t

CK × PWMTM) seconds after the initial

write to the PWMTM register in single update mode. In double

update mode, the first PWMSYNC pulse will be generated

(tCK × PWMTM) seconds after the initial write to the PWMTM

Effective PWM Resolution

In single update mode, the same values of PWMCHA, PWMCHB

and PWMCHC are used to define the on-times in both half

cycles of the PWM period. As a result, the effective resolution of

the PWM generation process is 2 tCK (or 100 ns for a 20 MHz

CLKOUT) since incrementing one of the duty cycle registers by

one changes the resultant on-time of the associated PWM sig-

nals by tCK in each half period (or 2 tCK for the full period).

In double update mode, improved resolution is possible since

different values of the duty cycles registers are used to define the

on-times in both the first and second halves of the PWM period.

As a result, it is possible to adjust the on-time over the whole

period in increments of tCK. This corresponds to an effective

PWM resolution of tCK in double update mode (or 50 ns for a

20 MHz CLKOUT).

The achievable PWM switching frequency at a given PWM

resolution is tabulated in Table IV.

Table IV. Achievable PWM Resolution in Single and Double

Update Modes

Resolution

Single Update Mode

Double Update Mode

(Bit)

PWM Frequency (kHz)

39.1

78.1

19.5

39.1

9.8

19.5

4.9

9.8

2.4

4.9

Minimum Pulsewidth: PWMPD Register

In many power converter switching applications, it is desirable

to eliminate PWM switching pulses shorter than a certain width.

It takes a finite time to both turn on and turn off modern power

semiconductor devices. Therefore, if the width of any of the PWM

pulses is shorter than some minimum value, it may be desirable to

completely eliminate the PWM switching for that particular cycle.

The allowable minimum on-time for any of the six PWM out-

puts for half a PWM period that can be produced by the PWM

controller may be programmed using the PWMPD register. The

minimum on-time is programmed in increments of tCK so that

the minimum on-time that will be produced for any half PWM

period, TMIN, is related to the value in the PWMPD register by:

TMIN = PWMPD × tCK

A PWMPD value of 0x002 defines a permissible minimum

on-time of 100 ns for a 20 MHz CLKOUT.

In each half cycle of the PWM, the timing unit checks the on-

time of each of the six PWM signals. If any of the times is found

to be less than the value specified by the PWMPD register, the

corresponding PWM signal is turned OFF for the entire half

period, and its complementary signal is turned completely ON.

Consider the example where PWMTM = 200, PWMCHA = 5,

PWMDT = 3, and PWMPD = 10 with a CLKOUT of 20 MHz

while operating in single update mode. For this case, the PWM

switching frequency is 50 kHz and the dead time is 300 ns. The

minimum permissible on-time of any PWM signal over one-half

of any period is 500 ns. Clearly, for this example, the dead-time

adjusted on-time of the AH signal for one-half a PWM period is

(5–3)

× 50 ns = 100 ns. Because this is less than the minimum

permissible value, output AH of the timing unit will remain

OFF (0% duty cycle). Additionally, the AL signal will be turned

ON for the entire half period (100% duty cycle).

Output Control Unit: PWMSEG Register

The operation of the output control unit is managed by the 9-bit

read/write PWMSEG register. This register sets two distinct

features of the output control unit that are directly useful in the

control of ECM or BDCM.

The PWMSEG register contains three crossover bits, one for each

pair of PWM outputs. Setting Bit 8 of the PWMSEG register

enables the crossover mode for the AH/AL pair of PWM signals;

setting Bit 7 enables crossover on the BH/BL pair of PWM signals;

and setting Bit 6 enables crossover on the CH/CL pair of PWM

signals. If crossover mode is enabled for any pair of PWM signals,

the high-side PWM signal from the timing unit (for example

AH) is diverted to the associated low-side output of the output

control unit so that the signal will ultimately appear at the AL

pin. Of course, the corresponding low-side output of the timing

unit is also diverted to the complementary high-side output of

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